diff --git a/src/applications/diffusion/xaction/DiffusionCommitAuditorsTransaction.php b/src/applications/diffusion/xaction/DiffusionCommitAuditorsTransaction.php
index a4b1d817b4..a02a34590d 100644
--- a/src/applications/diffusion/xaction/DiffusionCommitAuditorsTransaction.php
+++ b/src/applications/diffusion/xaction/DiffusionCommitAuditorsTransaction.php
@@ -1,216 +1,241 @@
 <?php
 
 final class DiffusionCommitAuditorsTransaction
   extends DiffusionCommitTransactionType {
 
   const TRANSACTIONTYPE = 'diffusion.commit.auditors';
 
   public function generateOldValue($object) {
     $auditors = $object->getAudits();
     return mpull($auditors, 'getAuditStatus', 'getAuditorPHID');
   }
 
   public function generateNewValue($object, $value) {
     $actor = $this->getActor();
 
     $auditors = $this->generateOldValue($object);
     $old_auditors = $auditors;
 
     $request_status = PhabricatorAuditStatusConstants::AUDIT_REQUESTED;
 
     $rem = idx($value, '-', array());
     foreach ($rem as $phid) {
       unset($auditors[$phid]);
     }
 
     $add = idx($value, '+', array());
     $add_map = array();
     foreach ($add as $phid) {
       $add_map[$phid] = $request_status;
     }
 
     $set = idx($value, '=', null);
     if ($set !== null) {
       foreach ($set as $phid) {
         $add_map[$phid] = $request_status;
       }
 
       $auditors = array();
     }
 
     foreach ($add_map as $phid => $new_status) {
       $old_status = idx($old_auditors, $phid);
 
       if ($old_status) {
         $auditors[$phid] = $old_status;
         continue;
       }
 
       $auditors[$phid] = $new_status;
     }
 
     return $auditors;
   }
 
   public function getTransactionHasEffect($object, $old, $new) {
     ksort($old);
     ksort($new);
     return ($old !== $new);
   }
 
+  public function mergeTransactions(
+    $object,
+    PhabricatorApplicationTransaction $u,
+    PhabricatorApplicationTransaction $v) {
+
+    $u_new = $u->getNewValue();
+    $v_new = $v->getNewValue();
+
+    $result = $v_new;
+    foreach (array('-', '+') as $key) {
+      $u_value = idx($u_new, $key, array());
+      $v_value = idx($v_new, $key, array());
+
+      $merged = $u_value + $v_value;
+
+      if ($merged) {
+        $result[$key] = $merged;
+      }
+    }
+
+    $u->setNewValue($result);
+
+    return $u;
+  }
+
   public function applyExternalEffects($object, $value) {
     $src_phid = $object->getPHID();
 
     $old = $this->generateOldValue($object);
     $new = $value;
 
     $auditors = $object->getAudits();
     $auditors = mpull($auditors, null, 'getAuditorPHID');
 
     $rem = array_diff_key($old, $new);
     foreach ($rem as $phid => $status) {
       $auditor = idx($auditors, $phid);
       if ($auditor) {
         $auditor->delete();
       }
     }
 
     $this->updateAudits($object, $new);
   }
 
   public function getTitle() {
     $old = $this->getOldValue();
     $new = $this->getNewValue();
 
     $rem = array_diff_key($old, $new);
     $add = array_diff_key($new, $old);
     $rem_phids = array_keys($rem);
     $add_phids = array_keys($add);
     $total_count = count($rem) + count($add);
 
     if ($rem && $add) {
       return pht(
         '%s edited %s auditor(s), removed %s: %s; added %s: %s.',
         $this->renderAuthor(),
         new PhutilNumber($total_count),
         phutil_count($rem_phids),
         $this->renderHandleList($rem_phids),
         phutil_count($add_phids),
         $this->renderHandleList($add_phids));
     } else if ($add) {
       return pht(
         '%s added %s auditor(s): %s.',
         $this->renderAuthor(),
         phutil_count($add_phids),
         $this->renderHandleList($add_phids));
     } else {
       return pht(
         '%s removed %s auditor(s): %s.',
         $this->renderAuthor(),
         phutil_count($rem_phids),
         $this->renderHandleList($rem_phids));
     }
   }
 
   public function getTitleForFeed() {
     $old = $this->getOldValue();
     $new = $this->getNewValue();
 
     $rem = array_diff_key($old, $new);
     $add = array_diff_key($new, $old);
     $rem_phids = array_keys($rem);
     $add_phids = array_keys($add);
     $total_count = count($rem) + count($add);
 
     if ($rem && $add) {
       return pht(
         '%s edited %s auditor(s) for %s, removed %s: %s; added %s: %s.',
         $this->renderAuthor(),
         new PhutilNumber($total_count),
         $this->renderObject(),
         phutil_count($rem_phids),
         $this->renderHandleList($rem_phids),
         phutil_count($add_phids),
         $this->renderHandleList($add_phids));
     } else if ($add) {
       return pht(
         '%s added %s auditor(s) for %s: %s.',
         $this->renderAuthor(),
         phutil_count($add_phids),
         $this->renderObject(),
         $this->renderHandleList($add_phids));
     } else {
       return pht(
         '%s removed %s auditor(s) for %s: %s.',
         $this->renderAuthor(),
         phutil_count($rem_phids),
         $this->renderObject(),
         $this->renderHandleList($rem_phids));
     }
   }
 
   public function validateTransactions($object, array $xactions) {
     $actor = $this->getActor();
     $errors = array();
 
     if (!$xactions) {
       return $errors;
     }
 
     $author_phid = $object->getAuthorPHID();
     $can_author_close_key = 'audit.can-author-close-audit';
     $can_author_close = PhabricatorEnv::getEnvConfig($can_author_close_key);
 
     $old = $this->generateOldValue($object);
     foreach ($xactions as $xaction) {
       $new = $this->generateNewValue($object, $xaction->getNewValue());
 
       $add = array_diff_key($new, $old);
       if (!$add) {
         continue;
       }
 
       $objects = id(new PhabricatorObjectQuery())
         ->setViewer($actor)
         ->withPHIDs(array_keys($add))
         ->execute();
       $objects = mpull($objects, null, 'getPHID');
 
       foreach ($add as $phid => $status) {
         if (!isset($objects[$phid])) {
           $errors[] = $this->newInvalidError(
             pht(
               'Auditor "%s" is not a valid object.',
               $phid),
             $xaction);
           continue;
         }
 
         switch (phid_get_type($phid)) {
           case PhabricatorPeopleUserPHIDType::TYPECONST:
           case PhabricatorOwnersPackagePHIDType::TYPECONST:
           case PhabricatorProjectProjectPHIDType::TYPECONST:
             break;
           default:
             $errors[] = $this->newInvalidError(
               pht(
                 'Auditor "%s" must be a user, a package, or a project.',
                 $phid),
               $xaction);
             continue 2;
         }
 
         $is_self = ($phid === $author_phid);
         if ($is_self && !$can_author_close) {
           $errors[] = $this->newInvalidError(
             pht('The author of a commit can not be an auditor.'),
             $xaction);
           continue;
         }
       }
     }
 
     return $errors;
   }
 
 }
diff --git a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
index 5851e0186c..60dcc1d513 100644
--- a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
+++ b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
@@ -1,3830 +1,3836 @@
 <?php
 
 /**
  *
  * Publishing and Managing State
  * ======
  *
  * After applying changes, the Editor queues a worker to publish mail, feed,
  * and notifications, and to perform other background work like updating search
  * indexes. This allows it to do this work without impacting performance for
  * users.
  *
  * When work is moved to the daemons, the Editor state is serialized by
  * @{method:getWorkerState}, then reloaded in a daemon process by
  * @{method:loadWorkerState}. **This is fragile.**
  *
  * State is not persisted into the daemons by default, because we can not send
  * arbitrary objects into the queue. This means the default behavior of any
  * state properties is to reset to their defaults without warning prior to
  * publishing.
  *
  * The easiest way to avoid this is to keep Editors stateless: the overwhelming
  * majority of Editors can be written statelessly. If you need to maintain
  * state, you can either:
  *
  *   - not require state to exist during publishing; or
  *   - pass state to the daemons by implementing @{method:getCustomWorkerState}
  *     and @{method:loadCustomWorkerState}.
  *
  * This architecture isn't ideal, and we may eventually split this class into
  * "Editor" and "Publisher" parts to make it more robust. See T6367 for some
  * discussion and context.
  *
  * @task mail Sending Mail
  * @task feed Publishing Feed Stories
  * @task search Search Index
  * @task files Integration with Files
  * @task workers Managing Workers
  */
 abstract class PhabricatorApplicationTransactionEditor
   extends PhabricatorEditor {
 
   private $contentSource;
   private $object;
   private $xactions;
 
   private $isNewObject;
   private $mentionedPHIDs;
   private $continueOnNoEffect;
   private $continueOnMissingFields;
   private $parentMessageID;
   private $heraldAdapter;
   private $heraldTranscript;
   private $subscribers;
   private $unmentionablePHIDMap = array();
   private $applicationEmail;
 
   private $isPreview;
   private $isHeraldEditor;
   private $isInverseEdgeEditor;
   private $actingAsPHID;
   private $disableEmail;
 
   private $heraldEmailPHIDs = array();
   private $heraldForcedEmailPHIDs = array();
   private $heraldHeader;
   private $mailToPHIDs = array();
   private $mailCCPHIDs = array();
   private $feedNotifyPHIDs = array();
   private $feedRelatedPHIDs = array();
   private $modularTypes;
 
   const STORAGE_ENCODING_BINARY = 'binary';
 
   /**
    * Get the class name for the application this editor is a part of.
    *
    * Uninstalling the application will disable the editor.
    *
    * @return string Editor's application class name.
    */
   abstract public function getEditorApplicationClass();
 
 
   /**
    * Get a description of the objects this editor edits, like "Differential
    * Revisions".
    *
    * @return string Human readable description of edited objects.
    */
   abstract public function getEditorObjectsDescription();
 
 
   public function setActingAsPHID($acting_as_phid) {
     $this->actingAsPHID = $acting_as_phid;
     return $this;
   }
 
   public function getActingAsPHID() {
     if ($this->actingAsPHID) {
       return $this->actingAsPHID;
     }
     return $this->getActor()->getPHID();
   }
 
 
   /**
    * When the editor tries to apply transactions that have no effect, should
    * it raise an exception (default) or drop them and continue?
    *
    * Generally, you will set this flag for edits coming from "Edit" interfaces,
    * and leave it cleared for edits coming from "Comment" interfaces, so the
    * user will get a useful error if they try to submit a comment that does
    * nothing (e.g., empty comment with a status change that has already been
    * performed by another user).
    *
    * @param bool  True to drop transactions without effect and continue.
    * @return this
    */
   public function setContinueOnNoEffect($continue) {
     $this->continueOnNoEffect = $continue;
     return $this;
   }
 
   public function getContinueOnNoEffect() {
     return $this->continueOnNoEffect;
   }
 
 
   /**
    * When the editor tries to apply transactions which don't populate all of
    * an object's required fields, should it raise an exception (default) or
    * drop them and continue?
    *
    * For example, if a user adds a new required custom field (like "Severity")
    * to a task, all existing tasks won't have it populated. When users
    * manually edit existing tasks, it's usually desirable to have them provide
    * a severity. However, other operations (like batch editing just the
    * owner of a task) will fail by default.
    *
    * By setting this flag for edit operations which apply to specific fields
    * (like the priority, batch, and merge editors in Maniphest), these
    * operations can continue to function even if an object is outdated.
    *
    * @param bool  True to continue when transactions don't completely satisfy
    *              all required fields.
    * @return this
    */
   public function setContinueOnMissingFields($continue_on_missing_fields) {
     $this->continueOnMissingFields = $continue_on_missing_fields;
     return $this;
   }
 
   public function getContinueOnMissingFields() {
     return $this->continueOnMissingFields;
   }
 
 
   /**
    * Not strictly necessary, but reply handlers ideally set this value to
    * make email threading work better.
    */
   public function setParentMessageID($parent_message_id) {
     $this->parentMessageID = $parent_message_id;
     return $this;
   }
   public function getParentMessageID() {
     return $this->parentMessageID;
   }
 
   public function getIsNewObject() {
     return $this->isNewObject;
   }
 
   protected function getMentionedPHIDs() {
     return $this->mentionedPHIDs;
   }
 
   public function setIsPreview($is_preview) {
     $this->isPreview = $is_preview;
     return $this;
   }
 
   public function getIsPreview() {
     return $this->isPreview;
   }
 
   public function setIsInverseEdgeEditor($is_inverse_edge_editor) {
     $this->isInverseEdgeEditor = $is_inverse_edge_editor;
     return $this;
   }
 
   public function getIsInverseEdgeEditor() {
     return $this->isInverseEdgeEditor;
   }
 
   public function setIsHeraldEditor($is_herald_editor) {
     $this->isHeraldEditor = $is_herald_editor;
     return $this;
   }
 
   public function getIsHeraldEditor() {
     return $this->isHeraldEditor;
   }
 
   /**
    * Prevent this editor from generating email when applying transactions.
    *
    * @param bool  True to disable email.
    * @return this
    */
   public function setDisableEmail($disable_email) {
     $this->disableEmail = $disable_email;
     return $this;
   }
 
   public function getDisableEmail() {
     return $this->disableEmail;
   }
 
   public function setUnmentionablePHIDMap(array $map) {
     $this->unmentionablePHIDMap = $map;
     return $this;
   }
 
   public function getUnmentionablePHIDMap() {
     return $this->unmentionablePHIDMap;
   }
 
   protected function shouldEnableMentions(
     PhabricatorLiskDAO $object,
     array $xactions) {
     return true;
   }
 
   public function setApplicationEmail(
     PhabricatorMetaMTAApplicationEmail $email) {
     $this->applicationEmail = $email;
     return $this;
   }
 
   public function getApplicationEmail() {
     return $this->applicationEmail;
   }
 
   public function getTransactionTypesForObject($object) {
     $old = $this->object;
     try {
       $this->object = $object;
       $result = $this->getTransactionTypes();
       $this->object = $old;
     } catch (Exception $ex) {
       $this->object = $old;
       throw $ex;
     }
     return $result;
   }
 
   public function getTransactionTypes() {
     $types = array();
 
     $types[] = PhabricatorTransactions::TYPE_CREATE;
 
     if ($this->object instanceof PhabricatorSubscribableInterface) {
       $types[] = PhabricatorTransactions::TYPE_SUBSCRIBERS;
     }
 
     if ($this->object instanceof PhabricatorCustomFieldInterface) {
       $types[] = PhabricatorTransactions::TYPE_CUSTOMFIELD;
     }
 
     if ($this->object instanceof HarbormasterBuildableInterface) {
       $types[] = PhabricatorTransactions::TYPE_BUILDABLE;
     }
 
     if ($this->object instanceof PhabricatorTokenReceiverInterface) {
       $types[] = PhabricatorTransactions::TYPE_TOKEN;
     }
 
     if ($this->object instanceof PhabricatorProjectInterface ||
         $this->object instanceof PhabricatorMentionableInterface) {
       $types[] = PhabricatorTransactions::TYPE_EDGE;
     }
 
     if ($this->object instanceof PhabricatorSpacesInterface) {
       $types[] = PhabricatorTransactions::TYPE_SPACE;
     }
 
     $template = $this->object->getApplicationTransactionTemplate();
     if ($template instanceof PhabricatorModularTransaction) {
       $xtypes = $template->newModularTransactionTypes();
       foreach ($xtypes as $xtype) {
         $types[] = $xtype->getTransactionTypeConstant();
       }
     }
 
     return $types;
   }
 
   private function adjustTransactionValues(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     if ($xaction->shouldGenerateOldValue()) {
       $old = $this->getTransactionOldValue($object, $xaction);
       $xaction->setOldValue($old);
     }
 
     $new = $this->getTransactionNewValue($object, $xaction);
     $xaction->setNewValue($new);
   }
 
   private function getTransactionOldValue(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     $type = $xaction->getTransactionType();
 
     $xtype = $this->getModularTransactionType($type);
     if ($xtype) {
       return $xtype->generateOldValue($object);
     }
 
     switch ($type) {
       case PhabricatorTransactions::TYPE_CREATE:
         return null;
       case PhabricatorTransactions::TYPE_SUBSCRIBERS:
         return array_values($this->subscribers);
       case PhabricatorTransactions::TYPE_VIEW_POLICY:
         if ($this->getIsNewObject()) {
           return null;
         }
         return $object->getViewPolicy();
       case PhabricatorTransactions::TYPE_EDIT_POLICY:
         if ($this->getIsNewObject()) {
           return null;
         }
         return $object->getEditPolicy();
       case PhabricatorTransactions::TYPE_JOIN_POLICY:
         if ($this->getIsNewObject()) {
           return null;
         }
         return $object->getJoinPolicy();
       case PhabricatorTransactions::TYPE_SPACE:
         if ($this->getIsNewObject()) {
           return null;
         }
 
         $space_phid = $object->getSpacePHID();
         if ($space_phid === null) {
           $default_space = PhabricatorSpacesNamespaceQuery::getDefaultSpace();
           if ($default_space) {
             $space_phid = $default_space->getPHID();
           }
         }
 
         return $space_phid;
       case PhabricatorTransactions::TYPE_EDGE:
         $edge_type = $xaction->getMetadataValue('edge:type');
         if (!$edge_type) {
           throw new Exception(
             pht(
               "Edge transaction has no '%s'!",
               'edge:type'));
         }
 
         $old_edges = array();
         if ($object->getPHID()) {
           $edge_src = $object->getPHID();
 
           $old_edges = id(new PhabricatorEdgeQuery())
             ->withSourcePHIDs(array($edge_src))
             ->withEdgeTypes(array($edge_type))
             ->needEdgeData(true)
             ->execute();
 
           $old_edges = $old_edges[$edge_src][$edge_type];
         }
         return $old_edges;
       case PhabricatorTransactions::TYPE_CUSTOMFIELD:
         // NOTE: Custom fields have their old value pre-populated when they are
         // built by PhabricatorCustomFieldList.
         return $xaction->getOldValue();
       case PhabricatorTransactions::TYPE_COMMENT:
         return null;
       default:
         return $this->getCustomTransactionOldValue($object, $xaction);
     }
   }
 
   private function getTransactionNewValue(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     $type = $xaction->getTransactionType();
 
     $xtype = $this->getModularTransactionType($type);
     if ($xtype) {
       return $xtype->generateNewValue($object, $xaction->getNewValue());
     }
 
     switch ($type) {
       case PhabricatorTransactions::TYPE_CREATE:
         return null;
       case PhabricatorTransactions::TYPE_SUBSCRIBERS:
         return $this->getPHIDTransactionNewValue($xaction);
       case PhabricatorTransactions::TYPE_VIEW_POLICY:
       case PhabricatorTransactions::TYPE_EDIT_POLICY:
       case PhabricatorTransactions::TYPE_JOIN_POLICY:
       case PhabricatorTransactions::TYPE_BUILDABLE:
       case PhabricatorTransactions::TYPE_TOKEN:
       case PhabricatorTransactions::TYPE_INLINESTATE:
         return $xaction->getNewValue();
       case PhabricatorTransactions::TYPE_SPACE:
         $space_phid = $xaction->getNewValue();
         if (!strlen($space_phid)) {
           // If an install has no Spaces or the Spaces controls are not visible
           // to the viewer, we might end up with the empty string here instead
           // of a strict `null`, because some controller just used `getStr()`
           // to read the space PHID from the request.
           // Just make this work like callers might reasonably expect so we
           // don't need to handle this specially in every EditController.
           return $this->getActor()->getDefaultSpacePHID();
         } else {
           return $space_phid;
         }
       case PhabricatorTransactions::TYPE_EDGE:
         $new_value = $this->getEdgeTransactionNewValue($xaction);
 
         $edge_type = $xaction->getMetadataValue('edge:type');
         $type_project = PhabricatorProjectObjectHasProjectEdgeType::EDGECONST;
         if ($edge_type == $type_project) {
           $new_value = $this->applyProjectConflictRules($new_value);
         }
 
         return $new_value;
       case PhabricatorTransactions::TYPE_CUSTOMFIELD:
         $field = $this->getCustomFieldForTransaction($object, $xaction);
         return $field->getNewValueFromApplicationTransactions($xaction);
       case PhabricatorTransactions::TYPE_COMMENT:
         return null;
       default:
         return $this->getCustomTransactionNewValue($object, $xaction);
     }
   }
 
   protected function getCustomTransactionOldValue(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
     throw new Exception(pht('Capability not supported!'));
   }
 
   protected function getCustomTransactionNewValue(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
     throw new Exception(pht('Capability not supported!'));
   }
 
   protected function transactionHasEffect(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     switch ($xaction->getTransactionType()) {
       case PhabricatorTransactions::TYPE_CREATE:
         return true;
       case PhabricatorTransactions::TYPE_COMMENT:
         return $xaction->hasComment();
       case PhabricatorTransactions::TYPE_CUSTOMFIELD:
         $field = $this->getCustomFieldForTransaction($object, $xaction);
         return $field->getApplicationTransactionHasEffect($xaction);
       case PhabricatorTransactions::TYPE_EDGE:
         // A straight value comparison here doesn't always get the right
         // result, because newly added edges aren't fully populated. Instead,
         // compare the changes in a more granular way.
         $old = $xaction->getOldValue();
         $new = $xaction->getNewValue();
 
         $old_dst = array_keys($old);
         $new_dst = array_keys($new);
 
         // NOTE: For now, we don't consider edge reordering to be a change.
         // We have very few order-dependent edges and effectively no order
         // oriented UI. This might change in the future.
         sort($old_dst);
         sort($new_dst);
 
         if ($old_dst !== $new_dst) {
           // We've added or removed edges, so this transaction definitely
           // has an effect.
           return true;
         }
 
         // We haven't added or removed edges, but we might have changed
         // edge data.
         foreach ($old as $key => $old_value) {
           $new_value = $new[$key];
           if ($old_value['data'] !== $new_value['data']) {
             return true;
           }
         }
 
         return false;
     }
 
     $type = $xaction->getTransactionType();
     $xtype = $this->getModularTransactionType($type);
     if ($xtype) {
       return $xtype->getTransactionHasEffect(
         $object,
         $xaction->getOldValue(),
         $xaction->getNewValue());
     }
 
     return ($xaction->getOldValue() !== $xaction->getNewValue());
   }
 
   protected function shouldApplyInitialEffects(
     PhabricatorLiskDAO $object,
     array $xactions) {
     return false;
   }
 
   protected function applyInitialEffects(
     PhabricatorLiskDAO $object,
     array $xactions) {
     throw new PhutilMethodNotImplementedException();
   }
 
   private function applyInternalEffects(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     $type = $xaction->getTransactionType();
 
     $xtype = $this->getModularTransactionType($type);
     if ($xtype) {
       return $xtype->applyInternalEffects($object, $xaction->getNewValue());
     }
 
     switch ($type) {
       case PhabricatorTransactions::TYPE_CUSTOMFIELD:
         $field = $this->getCustomFieldForTransaction($object, $xaction);
         return $field->applyApplicationTransactionInternalEffects($xaction);
       case PhabricatorTransactions::TYPE_CREATE:
       case PhabricatorTransactions::TYPE_BUILDABLE:
       case PhabricatorTransactions::TYPE_TOKEN:
       case PhabricatorTransactions::TYPE_VIEW_POLICY:
       case PhabricatorTransactions::TYPE_EDIT_POLICY:
       case PhabricatorTransactions::TYPE_JOIN_POLICY:
       case PhabricatorTransactions::TYPE_SUBSCRIBERS:
       case PhabricatorTransactions::TYPE_INLINESTATE:
       case PhabricatorTransactions::TYPE_EDGE:
       case PhabricatorTransactions::TYPE_SPACE:
       case PhabricatorTransactions::TYPE_COMMENT:
         return $this->applyBuiltinInternalTransaction($object, $xaction);
     }
 
     return $this->applyCustomInternalTransaction($object, $xaction);
   }
 
   private function applyExternalEffects(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     $type = $xaction->getTransactionType();
 
     $xtype = $this->getModularTransactionType($type);
     if ($xtype) {
       return $xtype->applyExternalEffects($object, $xaction->getNewValue());
     }
 
     switch ($type) {
       case PhabricatorTransactions::TYPE_SUBSCRIBERS:
         $subeditor = id(new PhabricatorSubscriptionsEditor())
           ->setObject($object)
           ->setActor($this->requireActor());
 
         $old_map = array_fuse($xaction->getOldValue());
         $new_map = array_fuse($xaction->getNewValue());
 
         $subeditor->unsubscribe(
           array_keys(
             array_diff_key($old_map, $new_map)));
 
         $subeditor->subscribeExplicit(
           array_keys(
             array_diff_key($new_map, $old_map)));
 
         $subeditor->save();
 
         // for the rest of these edits, subscribers should include those just
         // added as well as those just removed.
         $subscribers = array_unique(array_merge(
           $this->subscribers,
           $xaction->getOldValue(),
           $xaction->getNewValue()));
         $this->subscribers = $subscribers;
         return $this->applyBuiltinExternalTransaction($object, $xaction);
 
       case PhabricatorTransactions::TYPE_CUSTOMFIELD:
         $field = $this->getCustomFieldForTransaction($object, $xaction);
         return $field->applyApplicationTransactionExternalEffects($xaction);
       case PhabricatorTransactions::TYPE_CREATE:
       case PhabricatorTransactions::TYPE_EDGE:
       case PhabricatorTransactions::TYPE_BUILDABLE:
       case PhabricatorTransactions::TYPE_TOKEN:
       case PhabricatorTransactions::TYPE_VIEW_POLICY:
       case PhabricatorTransactions::TYPE_EDIT_POLICY:
       case PhabricatorTransactions::TYPE_JOIN_POLICY:
       case PhabricatorTransactions::TYPE_INLINESTATE:
       case PhabricatorTransactions::TYPE_SPACE:
       case PhabricatorTransactions::TYPE_COMMENT:
         return $this->applyBuiltinExternalTransaction($object, $xaction);
     }
 
     return $this->applyCustomExternalTransaction($object, $xaction);
   }
 
   protected function applyCustomInternalTransaction(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
     $type = $xaction->getTransactionType();
     throw new Exception(
       pht(
         "Transaction type '%s' is missing an internal apply implementation!",
         $type));
   }
 
   protected function applyCustomExternalTransaction(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
     $type = $xaction->getTransactionType();
     throw new Exception(
       pht(
         "Transaction type '%s' is missing an external apply implementation!",
         $type));
   }
 
   /**
    * @{class:PhabricatorTransactions} provides many built-in transactions
    * which should not require much - if any - code in specific applications.
    *
    * This method is a hook for the exceedingly-rare cases where you may need
    * to do **additional** work for built-in transactions. Developers should
    * extend this method, making sure to return the parent implementation
    * regardless of handling any transactions.
    *
    * See also @{method:applyBuiltinExternalTransaction}.
    */
   protected function applyBuiltinInternalTransaction(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     switch ($xaction->getTransactionType()) {
       case PhabricatorTransactions::TYPE_VIEW_POLICY:
         $object->setViewPolicy($xaction->getNewValue());
         break;
       case PhabricatorTransactions::TYPE_EDIT_POLICY:
         $object->setEditPolicy($xaction->getNewValue());
         break;
       case PhabricatorTransactions::TYPE_JOIN_POLICY:
         $object->setJoinPolicy($xaction->getNewValue());
         break;
       case PhabricatorTransactions::TYPE_SPACE:
         $object->setSpacePHID($xaction->getNewValue());
         break;
     }
   }
 
   /**
    * See @{method::applyBuiltinInternalTransaction}.
    */
   protected function applyBuiltinExternalTransaction(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     switch ($xaction->getTransactionType()) {
       case PhabricatorTransactions::TYPE_EDGE:
         if ($this->getIsInverseEdgeEditor()) {
           // If we're writing an inverse edge transaction, don't actually
           // do anything. The initiating editor on the other side of the
           // transaction will take care of the edge writes.
           break;
         }
 
         $old = $xaction->getOldValue();
         $new = $xaction->getNewValue();
         $src = $object->getPHID();
         $const = $xaction->getMetadataValue('edge:type');
 
         $type = PhabricatorEdgeType::getByConstant($const);
         if ($type->shouldWriteInverseTransactions()) {
           $this->applyInverseEdgeTransactions(
             $object,
             $xaction,
             $type->getInverseEdgeConstant());
         }
 
         foreach ($new as $dst_phid => $edge) {
           $new[$dst_phid]['src'] = $src;
         }
 
         $editor = new PhabricatorEdgeEditor();
 
         foreach ($old as $dst_phid => $edge) {
           if (!empty($new[$dst_phid])) {
             if ($old[$dst_phid]['data'] === $new[$dst_phid]['data']) {
               continue;
             }
           }
           $editor->removeEdge($src, $const, $dst_phid);
         }
 
         foreach ($new as $dst_phid => $edge) {
           if (!empty($old[$dst_phid])) {
             if ($old[$dst_phid]['data'] === $new[$dst_phid]['data']) {
               continue;
             }
           }
 
           $data = array(
             'data' => $edge['data'],
           );
 
           $editor->addEdge($src, $const, $dst_phid, $data);
         }
 
         $editor->save();
 
         $this->updateWorkboardColumns($object, $const, $old, $new);
         break;
       case PhabricatorTransactions::TYPE_VIEW_POLICY:
       case PhabricatorTransactions::TYPE_SPACE:
         $this->scrambleFileSecrets($object);
         break;
     }
   }
 
   /**
    * Fill in a transaction's common values, like author and content source.
    */
   protected function populateTransaction(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     $actor = $this->getActor();
 
     // TODO: This needs to be more sophisticated once we have meta-policies.
     $xaction->setViewPolicy(PhabricatorPolicies::POLICY_PUBLIC);
 
     if ($actor->isOmnipotent()) {
       $xaction->setEditPolicy(PhabricatorPolicies::POLICY_NOONE);
     } else {
       $xaction->setEditPolicy($this->getActingAsPHID());
     }
 
     // If the transaction already has an explicit author PHID, allow it to
     // stand. This is used by applications like Owners that hook into the
     // post-apply change pipeline.
     if (!$xaction->getAuthorPHID()) {
       $xaction->setAuthorPHID($this->getActingAsPHID());
     }
 
     $xaction->setContentSource($this->getContentSource());
     $xaction->attachViewer($actor);
     $xaction->attachObject($object);
 
     if ($object->getPHID()) {
       $xaction->setObjectPHID($object->getPHID());
     }
 
     return $xaction;
   }
 
   protected function didApplyInternalEffects(
     PhabricatorLiskDAO $object,
     array $xactions) {
     return $xactions;
   }
 
   protected function applyFinalEffects(
     PhabricatorLiskDAO $object,
     array $xactions) {
     return $xactions;
   }
 
   public function setContentSource(PhabricatorContentSource $content_source) {
     $this->contentSource = $content_source;
     return $this;
   }
 
   public function setContentSourceFromRequest(AphrontRequest $request) {
     return $this->setContentSource(
       PhabricatorContentSource::newFromRequest($request));
   }
 
   public function getContentSource() {
     return $this->contentSource;
   }
 
   final public function applyTransactions(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $this->object = $object;
     $this->xactions = $xactions;
     $this->isNewObject = ($object->getPHID() === null);
 
     $this->validateEditParameters($object, $xactions);
 
     $actor = $this->requireActor();
 
     // NOTE: Some transaction expansion requires that the edited object be
     // attached.
     foreach ($xactions as $xaction) {
       $xaction->attachObject($object);
       $xaction->attachViewer($actor);
     }
 
     $xactions = $this->expandTransactions($object, $xactions);
     $xactions = $this->expandSupportTransactions($object, $xactions);
     $xactions = $this->combineTransactions($xactions);
 
     foreach ($xactions as $xaction) {
       $xaction = $this->populateTransaction($object, $xaction);
     }
 
     $is_preview = $this->getIsPreview();
     $read_locking = false;
     $transaction_open = false;
 
     if (!$is_preview) {
       $errors = array();
       $type_map = mgroup($xactions, 'getTransactionType');
       foreach ($this->getTransactionTypes() as $type) {
         $type_xactions = idx($type_map, $type, array());
         $errors[] = $this->validateTransaction($object, $type, $type_xactions);
       }
 
       $errors[] = $this->validateAllTransactions($object, $xactions);
       $errors = array_mergev($errors);
 
       $continue_on_missing = $this->getContinueOnMissingFields();
       foreach ($errors as $key => $error) {
         if ($continue_on_missing && $error->getIsMissingFieldError()) {
           unset($errors[$key]);
         }
       }
 
       if ($errors) {
         throw new PhabricatorApplicationTransactionValidationException($errors);
       }
 
       $this->willApplyTransactions($object, $xactions);
 
       if ($object->getID()) {
         foreach ($xactions as $xaction) {
 
           // If any of the transactions require a read lock, hold one and
           // reload the object. We need to do this fairly early so that the
           // call to `adjustTransactionValues()` (which populates old values)
           // is based on the synchronized state of the object, which may differ
           // from the state when it was originally loaded.
 
           if ($this->shouldReadLock($object, $xaction)) {
             $object->openTransaction();
             $object->beginReadLocking();
             $transaction_open = true;
             $read_locking = true;
             $object->reload();
             break;
           }
         }
       }
 
       if ($this->shouldApplyInitialEffects($object, $xactions)) {
         if (!$transaction_open) {
           $object->openTransaction();
           $transaction_open = true;
         }
       }
     }
 
     if ($this->shouldApplyInitialEffects($object, $xactions)) {
       $this->applyInitialEffects($object, $xactions);
     }
 
     foreach ($xactions as $xaction) {
       $this->adjustTransactionValues($object, $xaction);
     }
 
     try {
       $xactions = $this->filterTransactions($object, $xactions);
     } catch (Exception $ex) {
       if ($read_locking) {
         $object->endReadLocking();
       }
       if ($transaction_open) {
         $object->killTransaction();
       }
       throw $ex;
     }
 
     // TODO: Once everything is on EditEngine, just use getIsNewObject() to
     // figure this out instead.
     $mark_as_create = false;
     $create_type = PhabricatorTransactions::TYPE_CREATE;
     foreach ($xactions as $xaction) {
       if ($xaction->getTransactionType() == $create_type) {
         $mark_as_create = true;
       }
     }
 
     if ($mark_as_create) {
       foreach ($xactions as $xaction) {
         $xaction->setIsCreateTransaction(true);
       }
     }
 
     // Now that we've merged, filtered, and combined transactions, check for
     // required capabilities.
     foreach ($xactions as $xaction) {
       $this->requireCapabilities($object, $xaction);
     }
 
     $xactions = $this->sortTransactions($xactions);
     $file_phids = $this->extractFilePHIDs($object, $xactions);
 
     if ($is_preview) {
       $this->loadHandles($xactions);
       return $xactions;
     }
 
     $comment_editor = id(new PhabricatorApplicationTransactionCommentEditor())
       ->setActor($actor)
       ->setActingAsPHID($this->getActingAsPHID())
       ->setContentSource($this->getContentSource());
 
     if (!$transaction_open) {
       $object->openTransaction();
     }
 
     try {
       foreach ($xactions as $xaction) {
         $this->applyInternalEffects($object, $xaction);
       }
 
       $xactions = $this->didApplyInternalEffects($object, $xactions);
 
       try {
         $object->save();
       } catch (AphrontDuplicateKeyQueryException $ex) {
         // This callback has an opportunity to throw a better exception,
         // so execution may end here.
         $this->didCatchDuplicateKeyException($object, $xactions, $ex);
 
         throw $ex;
       }
 
       foreach ($xactions as $xaction) {
         $xaction->setObjectPHID($object->getPHID());
         if ($xaction->getComment()) {
           $xaction->setPHID($xaction->generatePHID());
           $comment_editor->applyEdit($xaction, $xaction->getComment());
         } else {
           $xaction->save();
         }
       }
 
       if ($file_phids) {
         $this->attachFiles($object, $file_phids);
       }
 
       foreach ($xactions as $xaction) {
         $this->applyExternalEffects($object, $xaction);
       }
 
       $xactions = $this->applyFinalEffects($object, $xactions);
       if ($read_locking) {
         $object->endReadLocking();
         $read_locking = false;
       }
 
       $object->saveTransaction();
     } catch (Exception $ex) {
       $object->killTransaction();
       throw $ex;
     }
 
     // If we need to perform cache engine updates, execute them now.
     id(new PhabricatorCacheEngine())
       ->updateObject($object);
 
     // Now that we've completely applied the core transaction set, try to apply
     // Herald rules. Herald rules are allowed to either take direct actions on
     // the database (like writing flags), or take indirect actions (like saving
     // some targets for CC when we generate mail a little later), or return
     // transactions which we'll apply normally using another Editor.
 
     // First, check if *this* is a sub-editor which is itself applying Herald
     // rules: if it is, stop working and return so we don't descend into
     // madness.
 
     // Otherwise, we're not a Herald editor, so process Herald rules (possibly
     // using a Herald editor to apply resulting transactions) and then send out
     // mail, notifications, and feed updates about everything.
 
     if ($this->getIsHeraldEditor()) {
       // We are the Herald editor, so stop work here and return the updated
       // transactions.
       return $xactions;
     } else if ($this->getIsInverseEdgeEditor()) {
       // If we're applying inverse edge transactions, don't trigger Herald.
       // From a product perspective, the current set of inverse edges (most
       // often, mentions) aren't things users would expect to trigger Herald.
       // From a technical perspective, objects loaded by the inverse editor may
       // not have enough data to execute rules. At least for now, just stop
       // Herald from executing when applying inverse edges.
     } else if ($this->shouldApplyHeraldRules($object, $xactions)) {
       // We are not the Herald editor, so try to apply Herald rules.
       $herald_xactions = $this->applyHeraldRules($object, $xactions);
 
       if ($herald_xactions) {
         $xscript_id = $this->getHeraldTranscript()->getID();
         foreach ($herald_xactions as $herald_xaction) {
           // Don't set a transcript ID if this is a transaction from another
           // application or source, like Owners.
           if ($herald_xaction->getAuthorPHID()) {
             continue;
           }
 
           $herald_xaction->setMetadataValue('herald:transcriptID', $xscript_id);
         }
 
         // NOTE: We're acting as the omnipotent user because rules deal with
         // their own policy issues. We use a synthetic author PHID (the
         // Herald application) as the author of record, so that transactions
         // will render in a reasonable way ("Herald assigned this task ...").
         $herald_actor = PhabricatorUser::getOmnipotentUser();
         $herald_phid = id(new PhabricatorHeraldApplication())->getPHID();
 
         // TODO: It would be nice to give transactions a more specific source
         // which points at the rule which generated them. You can figure this
         // out from transcripts, but it would be cleaner if you didn't have to.
 
         $herald_source = PhabricatorContentSource::newForSource(
           PhabricatorHeraldContentSource::SOURCECONST);
 
         $herald_editor = newv(get_class($this), array())
           ->setContinueOnNoEffect(true)
           ->setContinueOnMissingFields(true)
           ->setParentMessageID($this->getParentMessageID())
           ->setIsHeraldEditor(true)
           ->setActor($herald_actor)
           ->setActingAsPHID($herald_phid)
           ->setContentSource($herald_source);
 
         $herald_xactions = $herald_editor->applyTransactions(
           $object,
           $herald_xactions);
 
         // Merge the new transactions into the transaction list: we want to
         // send email and publish feed stories about them, too.
         $xactions = array_merge($xactions, $herald_xactions);
       }
 
       // If Herald did not generate transactions, we may still need to handle
       // "Send an Email" rules.
       $adapter = $this->getHeraldAdapter();
       $this->heraldEmailPHIDs = $adapter->getEmailPHIDs();
       $this->heraldForcedEmailPHIDs = $adapter->getForcedEmailPHIDs();
     }
 
     $this->didApplyTransactions($xactions);
 
     if ($object instanceof PhabricatorCustomFieldInterface) {
       // Maybe this makes more sense to move into the search index itself? For
       // now I'm putting it here since I think we might end up with things that
       // need it to be up to date once the next page loads, but if we don't go
       // there we could move it into search once search moves to the daemons.
 
       // It now happens in the search indexer as well, but the search indexer is
       // always daemonized, so the logic above still potentially holds. We could
       // possibly get rid of this. The major motivation for putting it in the
       // indexer was to enable reindexing to work.
 
       $fields = PhabricatorCustomField::getObjectFields(
         $object,
         PhabricatorCustomField::ROLE_APPLICATIONSEARCH);
       $fields->readFieldsFromStorage($object);
       $fields->rebuildIndexes($object);
     }
 
     $herald_xscript = $this->getHeraldTranscript();
     if ($herald_xscript) {
       $herald_header = $herald_xscript->getXHeraldRulesHeader();
       $herald_header = HeraldTranscript::saveXHeraldRulesHeader(
         $object->getPHID(),
         $herald_header);
     } else {
       $herald_header = HeraldTranscript::loadXHeraldRulesHeader(
         $object->getPHID());
     }
     $this->heraldHeader = $herald_header;
 
     // We're going to compute some of the data we'll use to publish these
     // transactions here, before queueing a worker.
     //
     // Primarily, this is more correct: we want to publish the object as it
     // exists right now. The worker may not execute for some time, and we want
     // to use the current To/CC list, not respect any changes which may occur
     // between now and when the worker executes.
     //
     // As a secondary benefit, this tends to reduce the amount of state that
     // Editors need to pass into workers.
     $object = $this->willPublish($object, $xactions);
 
     if (!$this->getDisableEmail()) {
       if ($this->shouldSendMail($object, $xactions)) {
         $this->mailToPHIDs = $this->getMailTo($object);
         $this->mailCCPHIDs = $this->getMailCC($object);
       }
     }
 
     if ($this->shouldPublishFeedStory($object, $xactions)) {
       $this->feedRelatedPHIDs = $this->getFeedRelatedPHIDs($object, $xactions);
       $this->feedNotifyPHIDs = $this->getFeedNotifyPHIDs($object, $xactions);
     }
 
     PhabricatorWorker::scheduleTask(
       'PhabricatorApplicationTransactionPublishWorker',
       array(
         'objectPHID' => $object->getPHID(),
         'actorPHID' => $this->getActingAsPHID(),
         'xactionPHIDs' => mpull($xactions, 'getPHID'),
         'state' => $this->getWorkerState(),
       ),
       array(
         'objectPHID' => $object->getPHID(),
         'priority' => PhabricatorWorker::PRIORITY_ALERTS,
       ));
 
     return $xactions;
   }
 
   protected function didCatchDuplicateKeyException(
     PhabricatorLiskDAO $object,
     array $xactions,
     Exception $ex) {
     return;
   }
 
   public function publishTransactions(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $this->object = $object;
     $this->xactions = $xactions;
 
     // Hook for edges or other properties that may need (re-)loading
     $object = $this->willPublish($object, $xactions);
 
     // The object might have changed, so reassign it.
     $this->object = $object;
 
     $messages = array();
     if (!$this->getDisableEmail()) {
       if ($this->shouldSendMail($object, $xactions)) {
         $messages = $this->buildMail($object, $xactions);
       }
     }
 
     if ($this->supportsSearch()) {
       PhabricatorSearchWorker::queueDocumentForIndexing(
         $object->getPHID(),
         array(
           'transactionPHIDs' => mpull($xactions, 'getPHID'),
         ));
     }
 
     if ($this->shouldPublishFeedStory($object, $xactions)) {
 
       $mailed = array();
       foreach ($messages as $mail) {
         foreach ($mail->buildRecipientList() as $phid) {
           $mailed[$phid] = $phid;
         }
       }
 
       $this->publishFeedStory($object, $xactions, $mailed);
     }
 
     // NOTE: This actually sends the mail. We do this last to reduce the chance
     // that we send some mail, hit an exception, then send the mail again when
     // retrying.
     foreach ($messages as $mail) {
       $mail->save();
     }
 
     return $xactions;
   }
 
   protected function didApplyTransactions(array $xactions) {
     // Hook for subclasses.
     return;
   }
 
 
   /**
    * Determine if the editor should hold a read lock on the object while
    * applying a transaction.
    *
    * If the editor does not hold a lock, two editors may read an object at the
    * same time, then apply their changes without any synchronization. For most
    * transactions, this does not matter much. However, it is important for some
    * transactions. For example, if an object has a transaction count on it, both
    * editors may read the object with `count = 23`, then independently update it
    * and save the object with `count = 24` twice. This will produce the wrong
    * state: the object really has 25 transactions, but the count is only 24.
    *
    * Generally, transactions fall into one of four buckets:
    *
    *   - Append operations: Actions like adding a comment to an object purely
    *     add information to its state, and do not depend on the current object
    *     state in any way. These transactions never need to hold locks.
    *   - Overwrite operations: Actions like changing the title or description
    *     of an object replace the current value with a new value, so the end
    *     state is consistent without a lock. We currently do not lock these
    *     transactions, although we may in the future.
    *   - Edge operations: Edge and subscription operations have internal
    *     synchronization which limits the damage race conditions can cause.
    *     We do not currently lock these transactions, although we may in the
    *     future.
    *   - Update operations: Actions like incrementing a count on an object.
    *     These operations generally should use locks, unless it is not
    *     important that the state remain consistent in the presence of races.
    *
    * @param   PhabricatorLiskDAO  Object being updated.
    * @param   PhabricatorApplicationTransaction Transaction being applied.
    * @return  bool                True to synchronize the edit with a lock.
    */
   protected function shouldReadLock(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
     return false;
   }
 
   private function loadHandles(array $xactions) {
     $phids = array();
     foreach ($xactions as $key => $xaction) {
       $phids[$key] = $xaction->getRequiredHandlePHIDs();
     }
     $handles = array();
     $merged = array_mergev($phids);
     if ($merged) {
       $handles = id(new PhabricatorHandleQuery())
         ->setViewer($this->requireActor())
         ->withPHIDs($merged)
         ->execute();
     }
     foreach ($xactions as $key => $xaction) {
       $xaction->setHandles(array_select_keys($handles, $phids[$key]));
     }
   }
 
   private function loadSubscribers(PhabricatorLiskDAO $object) {
     if ($object->getPHID() &&
         ($object instanceof PhabricatorSubscribableInterface)) {
       $subs = PhabricatorSubscribersQuery::loadSubscribersForPHID(
         $object->getPHID());
       $this->subscribers = array_fuse($subs);
     } else {
       $this->subscribers = array();
     }
   }
 
   private function validateEditParameters(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     if (!$this->getContentSource()) {
       throw new PhutilInvalidStateException('setContentSource');
     }
 
     // Do a bunch of sanity checks that the incoming transactions are fresh.
     // They should be unsaved and have only "transactionType" and "newValue"
     // set.
 
     $types = array_fill_keys($this->getTransactionTypes(), true);
 
     assert_instances_of($xactions, 'PhabricatorApplicationTransaction');
     foreach ($xactions as $xaction) {
       if ($xaction->getPHID() || $xaction->getID()) {
         throw new PhabricatorApplicationTransactionStructureException(
           $xaction,
           pht('You can not apply transactions which already have IDs/PHIDs!'));
       }
 
       if ($xaction->getObjectPHID()) {
         throw new PhabricatorApplicationTransactionStructureException(
           $xaction,
           pht(
             'You can not apply transactions which already have %s!',
             'objectPHIDs'));
       }
 
       if ($xaction->getCommentPHID()) {
         throw new PhabricatorApplicationTransactionStructureException(
           $xaction,
           pht(
             'You can not apply transactions which already have %s!',
             'commentPHIDs'));
       }
 
       if ($xaction->getCommentVersion() !== 0) {
         throw new PhabricatorApplicationTransactionStructureException(
           $xaction,
           pht(
             'You can not apply transactions which already have '.
             'commentVersions!'));
       }
 
       $expect_value = !$xaction->shouldGenerateOldValue();
       $has_value = $xaction->hasOldValue();
 
       if ($expect_value && !$has_value) {
         throw new PhabricatorApplicationTransactionStructureException(
           $xaction,
           pht(
             'This transaction is supposed to have an %s set, but it does not!',
             'oldValue'));
       }
 
       if ($has_value && !$expect_value) {
         throw new PhabricatorApplicationTransactionStructureException(
           $xaction,
           pht(
             'This transaction should generate its %s automatically, '.
             'but has already had one set!',
             'oldValue'));
       }
 
       $type = $xaction->getTransactionType();
       if (empty($types[$type])) {
         throw new PhabricatorApplicationTransactionStructureException(
           $xaction,
           pht(
             'Transaction has type "%s", but that transaction type is not '.
             'supported by this editor (%s).',
             $type,
             get_class($this)));
       }
     }
   }
 
   protected function requireCapabilities(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     if ($this->getIsNewObject()) {
       return;
     }
 
     $actor = $this->requireActor();
     switch ($xaction->getTransactionType()) {
       case PhabricatorTransactions::TYPE_COMMENT:
         PhabricatorPolicyFilter::requireCapability(
           $actor,
           $object,
           PhabricatorPolicyCapability::CAN_VIEW);
         break;
       case PhabricatorTransactions::TYPE_VIEW_POLICY:
       case PhabricatorTransactions::TYPE_EDIT_POLICY:
       case PhabricatorTransactions::TYPE_JOIN_POLICY:
       case PhabricatorTransactions::TYPE_SPACE:
         PhabricatorPolicyFilter::requireCapability(
           $actor,
           $object,
           PhabricatorPolicyCapability::CAN_EDIT);
         break;
     }
   }
 
   private function buildSubscribeTransaction(
     PhabricatorLiskDAO $object,
     array $xactions,
     array $changes) {
 
     if (!($object instanceof PhabricatorSubscribableInterface)) {
       return null;
     }
 
     if ($this->shouldEnableMentions($object, $xactions)) {
       // Identify newly mentioned users. We ignore users who were previously
       // mentioned so that we don't re-subscribe users after an edit of text
       // which mentions them.
       $old_texts = mpull($changes, 'getOldValue');
       $new_texts = mpull($changes, 'getNewValue');
 
       $old_phids = PhabricatorMarkupEngine::extractPHIDsFromMentions(
         $this->getActor(),
         $old_texts);
 
       $new_phids = PhabricatorMarkupEngine::extractPHIDsFromMentions(
         $this->getActor(),
         $new_texts);
 
       $phids = array_diff($new_phids, $old_phids);
     } else {
       $phids = array();
     }
 
     $this->mentionedPHIDs = $phids;
 
     if ($object->getPHID()) {
       // Don't try to subscribe already-subscribed mentions: we want to generate
       // a dialog about an action having no effect if the user explicitly adds
       // existing CCs, but not if they merely mention existing subscribers.
       $phids = array_diff($phids, $this->subscribers);
     }
 
     if ($phids) {
       $users = id(new PhabricatorPeopleQuery())
         ->setViewer($this->getActor())
         ->withPHIDs($phids)
         ->execute();
       $users = mpull($users, null, 'getPHID');
 
       foreach ($phids as $key => $phid) {
         // Do not subscribe mentioned users
         // who do not have VIEW Permissions
         if ($object instanceof PhabricatorPolicyInterface
           && !PhabricatorPolicyFilter::hasCapability(
           $users[$phid],
           $object,
           PhabricatorPolicyCapability::CAN_VIEW)
         ) {
           unset($phids[$key]);
         } else {
           if ($object->isAutomaticallySubscribed($phid)) {
             unset($phids[$key]);
           }
         }
       }
       $phids = array_values($phids);
     }
     // No else here to properly return null should we unset all subscriber
     if (!$phids) {
       return null;
     }
 
     $xaction = newv(get_class(head($xactions)), array());
     $xaction->setTransactionType(PhabricatorTransactions::TYPE_SUBSCRIBERS);
     $xaction->setNewValue(array('+' => $phids));
 
     return $xaction;
   }
 
   protected function mergeTransactions(
     PhabricatorApplicationTransaction $u,
     PhabricatorApplicationTransaction $v) {
 
     $type = $u->getTransactionType();
 
+    $xtype = $this->getModularTransactionType($type);
+    if ($xtype) {
+      $object = $this->object;
+      return $xtype->mergeTransactions($object, $u, $v);
+    }
+
     switch ($type) {
       case PhabricatorTransactions::TYPE_SUBSCRIBERS:
         return $this->mergePHIDOrEdgeTransactions($u, $v);
       case PhabricatorTransactions::TYPE_EDGE:
         $u_type = $u->getMetadataValue('edge:type');
         $v_type = $v->getMetadataValue('edge:type');
         if ($u_type == $v_type) {
           return $this->mergePHIDOrEdgeTransactions($u, $v);
         }
         return null;
     }
 
     // By default, do not merge the transactions.
     return null;
   }
 
   /**
    * Optionally expand transactions which imply other effects. For example,
    * resigning from a revision in Differential implies removing yourself as
    * a reviewer.
    */
   protected function expandTransactions(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $results = array();
     foreach ($xactions as $xaction) {
       foreach ($this->expandTransaction($object, $xaction) as $expanded) {
         $results[] = $expanded;
       }
     }
 
     return $results;
   }
 
   protected function expandTransaction(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
     return array($xaction);
   }
 
 
   public function getExpandedSupportTransactions(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     $xactions = array($xaction);
     $xactions = $this->expandSupportTransactions(
       $object,
       $xactions);
 
     if (count($xactions) == 1) {
       return array();
     }
 
     foreach ($xactions as $index => $cxaction) {
       if ($cxaction === $xaction) {
         unset($xactions[$index]);
         break;
       }
     }
 
     return $xactions;
   }
 
   private function expandSupportTransactions(
     PhabricatorLiskDAO $object,
     array $xactions) {
     $this->loadSubscribers($object);
 
     $xactions = $this->applyImplicitCC($object, $xactions);
 
     $changes = $this->getRemarkupChanges($xactions);
 
     $subscribe_xaction = $this->buildSubscribeTransaction(
       $object,
       $xactions,
       $changes);
     if ($subscribe_xaction) {
       $xactions[] = $subscribe_xaction;
     }
 
     // TODO: For now, this is just a placeholder.
     $engine = PhabricatorMarkupEngine::getEngine('extract');
     $engine->setConfig('viewer', $this->requireActor());
 
     $block_xactions = $this->expandRemarkupBlockTransactions(
       $object,
       $xactions,
       $changes,
       $engine);
 
     foreach ($block_xactions as $xaction) {
       $xactions[] = $xaction;
     }
 
     return $xactions;
   }
 
   private function getRemarkupChanges(array $xactions) {
     $changes = array();
 
     foreach ($xactions as $key => $xaction) {
       foreach ($this->getRemarkupChangesFromTransaction($xaction) as $change) {
         $changes[] = $change;
       }
     }
 
     return $changes;
   }
 
   private function getRemarkupChangesFromTransaction(
     PhabricatorApplicationTransaction $transaction) {
     return $transaction->getRemarkupChanges();
   }
 
   private function expandRemarkupBlockTransactions(
     PhabricatorLiskDAO $object,
     array $xactions,
     array $changes,
     PhutilMarkupEngine $engine) {
 
     $block_xactions = $this->expandCustomRemarkupBlockTransactions(
       $object,
       $xactions,
       $changes,
       $engine);
 
     $mentioned_phids = array();
     if ($this->shouldEnableMentions($object, $xactions)) {
       foreach ($changes as $change) {
         // Here, we don't care about processing only new mentions after an edit
         // because there is no way for an object to ever "unmention" itself on
         // another object, so we can ignore the old value.
         $engine->markupText($change->getNewValue());
 
         $mentioned_phids += $engine->getTextMetadata(
           PhabricatorObjectRemarkupRule::KEY_MENTIONED_OBJECTS,
           array());
       }
     }
 
     if (!$mentioned_phids) {
       return $block_xactions;
     }
 
     $mentioned_objects = id(new PhabricatorObjectQuery())
       ->setViewer($this->getActor())
       ->withPHIDs($mentioned_phids)
       ->execute();
 
     $mentionable_phids = array();
     if ($this->shouldEnableMentions($object, $xactions)) {
       foreach ($mentioned_objects as $mentioned_object) {
         if ($mentioned_object instanceof PhabricatorMentionableInterface) {
           $mentioned_phid = $mentioned_object->getPHID();
           if (idx($this->getUnmentionablePHIDMap(), $mentioned_phid)) {
             continue;
           }
           // don't let objects mention themselves
           if ($object->getPHID() && $mentioned_phid == $object->getPHID()) {
             continue;
           }
           $mentionable_phids[$mentioned_phid] = $mentioned_phid;
         }
       }
     }
 
     if ($mentionable_phids) {
       $edge_type = PhabricatorObjectMentionsObjectEdgeType::EDGECONST;
       $block_xactions[] = newv(get_class(head($xactions)), array())
         ->setIgnoreOnNoEffect(true)
         ->setTransactionType(PhabricatorTransactions::TYPE_EDGE)
         ->setMetadataValue('edge:type', $edge_type)
         ->setNewValue(array('+' => $mentionable_phids));
     }
 
     return $block_xactions;
   }
 
   protected function expandCustomRemarkupBlockTransactions(
     PhabricatorLiskDAO $object,
     array $xactions,
     array $changes,
     PhutilMarkupEngine $engine) {
     return array();
   }
 
 
   /**
    * Attempt to combine similar transactions into a smaller number of total
    * transactions. For example, two transactions which edit the title of an
    * object can be merged into a single edit.
    */
   private function combineTransactions(array $xactions) {
     $stray_comments = array();
 
     $result = array();
     $types = array();
     foreach ($xactions as $key => $xaction) {
       $type = $xaction->getTransactionType();
       if (isset($types[$type])) {
         foreach ($types[$type] as $other_key) {
           $other_xaction = $result[$other_key];
 
           // Don't merge transactions with different authors. For example,
           // don't merge Herald transactions and owners transactions.
           if ($other_xaction->getAuthorPHID() != $xaction->getAuthorPHID()) {
             continue;
           }
 
           $merged = $this->mergeTransactions($result[$other_key], $xaction);
           if ($merged) {
             $result[$other_key] = $merged;
 
             if ($xaction->getComment() &&
                 ($xaction->getComment() !== $merged->getComment())) {
               $stray_comments[] = $xaction->getComment();
             }
 
             if ($result[$other_key]->getComment() &&
                 ($result[$other_key]->getComment() !== $merged->getComment())) {
               $stray_comments[] = $result[$other_key]->getComment();
             }
 
             // Move on to the next transaction.
             continue 2;
           }
         }
       }
       $result[$key] = $xaction;
       $types[$type][] = $key;
     }
 
     // If we merged any comments away, restore them.
     foreach ($stray_comments as $comment) {
       $xaction = newv(get_class(head($result)), array());
       $xaction->setTransactionType(PhabricatorTransactions::TYPE_COMMENT);
       $xaction->setComment($comment);
       $result[] = $xaction;
     }
 
     return array_values($result);
   }
 
   protected function mergePHIDOrEdgeTransactions(
     PhabricatorApplicationTransaction $u,
     PhabricatorApplicationTransaction $v) {
 
     $result = $u->getNewValue();
     foreach ($v->getNewValue() as $key => $value) {
       if ($u->getTransactionType() == PhabricatorTransactions::TYPE_EDGE) {
         if (empty($result[$key])) {
           $result[$key] = $value;
         } else {
           // We're merging two lists of edge adds, sets, or removes. Merge
           // them by merging individual PHIDs within them.
           $merged = $result[$key];
 
           foreach ($value as $dst => $v_spec) {
             if (empty($merged[$dst])) {
               $merged[$dst] = $v_spec;
             } else {
               // Two transactions are trying to perform the same operation on
               // the same edge. Normalize the edge data and then merge it. This
               // allows transactions to specify how data merges execute in a
               // precise way.
 
               $u_spec = $merged[$dst];
 
               if (!is_array($u_spec)) {
                 $u_spec = array('dst' => $u_spec);
               }
               if (!is_array($v_spec)) {
                 $v_spec = array('dst' => $v_spec);
               }
 
               $ux_data = idx($u_spec, 'data', array());
               $vx_data = idx($v_spec, 'data', array());
 
               $merged_data = $this->mergeEdgeData(
                 $u->getMetadataValue('edge:type'),
                 $ux_data,
                 $vx_data);
 
               $u_spec['data'] = $merged_data;
               $merged[$dst] = $u_spec;
             }
           }
 
           $result[$key] = $merged;
         }
       } else {
         $result[$key] = array_merge($value, idx($result, $key, array()));
       }
     }
     $u->setNewValue($result);
 
     // When combining an "ignore" transaction with a normal transaction, make
     // sure we don't propagate the "ignore" flag.
     if (!$v->getIgnoreOnNoEffect()) {
       $u->setIgnoreOnNoEffect(false);
     }
 
     return $u;
   }
 
   protected function mergeEdgeData($type, array $u, array $v) {
     return $v + $u;
   }
 
   protected function getPHIDTransactionNewValue(
     PhabricatorApplicationTransaction $xaction,
     $old = null) {
 
     if ($old !== null) {
       $old = array_fuse($old);
     } else {
       $old = array_fuse($xaction->getOldValue());
     }
 
     $new = $xaction->getNewValue();
     $new_add = idx($new, '+', array());
     unset($new['+']);
     $new_rem = idx($new, '-', array());
     unset($new['-']);
     $new_set = idx($new, '=', null);
     if ($new_set !== null) {
       $new_set = array_fuse($new_set);
     }
     unset($new['=']);
 
     if ($new) {
       throw new Exception(
         pht(
           "Invalid '%s' value for PHID transaction. Value should contain only ".
           "keys '%s' (add PHIDs), '%s' (remove PHIDs) and '%s' (set PHIDS).",
           'new',
           '+',
           '-',
           '='));
     }
 
     $result = array();
 
     foreach ($old as $phid) {
       if ($new_set !== null && empty($new_set[$phid])) {
         continue;
       }
       $result[$phid] = $phid;
     }
 
     if ($new_set !== null) {
       foreach ($new_set as $phid) {
         $result[$phid] = $phid;
       }
     }
 
     foreach ($new_add as $phid) {
       $result[$phid] = $phid;
     }
 
     foreach ($new_rem as $phid) {
       unset($result[$phid]);
     }
 
     return array_values($result);
   }
 
   protected function getEdgeTransactionNewValue(
     PhabricatorApplicationTransaction $xaction) {
 
     $new = $xaction->getNewValue();
     $new_add = idx($new, '+', array());
     unset($new['+']);
     $new_rem = idx($new, '-', array());
     unset($new['-']);
     $new_set = idx($new, '=', null);
     unset($new['=']);
 
     if ($new) {
       throw new Exception(
         pht(
           "Invalid '%s' value for Edge transaction. Value should contain only ".
           "keys '%s' (add edges), '%s' (remove edges) and '%s' (set edges).",
           'new',
           '+',
           '-',
           '='));
     }
 
     $old = $xaction->getOldValue();
 
     $lists = array($new_set, $new_add, $new_rem);
     foreach ($lists as $list) {
       $this->checkEdgeList($list, $xaction->getMetadataValue('edge:type'));
     }
 
     $result = array();
     foreach ($old as $dst_phid => $edge) {
       if ($new_set !== null && empty($new_set[$dst_phid])) {
         continue;
       }
       $result[$dst_phid] = $this->normalizeEdgeTransactionValue(
         $xaction,
         $edge,
         $dst_phid);
     }
 
     if ($new_set !== null) {
       foreach ($new_set as $dst_phid => $edge) {
         $result[$dst_phid] = $this->normalizeEdgeTransactionValue(
           $xaction,
           $edge,
           $dst_phid);
       }
     }
 
     foreach ($new_add as $dst_phid => $edge) {
       $result[$dst_phid] = $this->normalizeEdgeTransactionValue(
         $xaction,
         $edge,
         $dst_phid);
     }
 
     foreach ($new_rem as $dst_phid => $edge) {
       unset($result[$dst_phid]);
     }
 
     return $result;
   }
 
   private function checkEdgeList($list, $edge_type) {
     if (!$list) {
       return;
     }
     foreach ($list as $key => $item) {
       if (phid_get_type($key) === PhabricatorPHIDConstants::PHID_TYPE_UNKNOWN) {
         throw new Exception(
           pht(
             'Edge transactions must have destination PHIDs as in edge '.
             'lists (found key "%s" on transaction of type "%s").',
             $key,
             $edge_type));
       }
       if (!is_array($item) && $item !== $key) {
         throw new Exception(
           pht(
             'Edge transactions must have PHIDs or edge specs as values '.
             '(found value "%s" on transaction of type "%s").',
             $item,
             $edge_type));
       }
     }
   }
 
   private function normalizeEdgeTransactionValue(
     PhabricatorApplicationTransaction $xaction,
     $edge,
     $dst_phid) {
 
     if (!is_array($edge)) {
       if ($edge != $dst_phid) {
         throw new Exception(
           pht(
             'Transaction edge data must either be the edge PHID or an edge '.
             'specification dictionary.'));
       }
       $edge = array();
     } else {
       foreach ($edge as $key => $value) {
         switch ($key) {
           case 'src':
           case 'dst':
           case 'type':
           case 'data':
           case 'dateCreated':
           case 'dateModified':
           case 'seq':
           case 'dataID':
             break;
           default:
             throw new Exception(
               pht(
                 'Transaction edge specification contains unexpected key "%s".',
                 $key));
         }
       }
     }
 
     $edge['dst'] = $dst_phid;
 
     $edge_type = $xaction->getMetadataValue('edge:type');
     if (empty($edge['type'])) {
       $edge['type'] = $edge_type;
     } else {
       if ($edge['type'] != $edge_type) {
         $this_type = $edge['type'];
         throw new Exception(
           pht(
             "Edge transaction includes edge of type '%s', but ".
             "transaction is of type '%s'. Each edge transaction ".
             "must alter edges of only one type.",
             $this_type,
             $edge_type));
       }
     }
 
     if (!isset($edge['data'])) {
       $edge['data'] = array();
     }
 
     return $edge;
   }
 
   protected function sortTransactions(array $xactions) {
     $head = array();
     $tail = array();
 
     // Move bare comments to the end, so the actions precede them.
     foreach ($xactions as $xaction) {
       $type = $xaction->getTransactionType();
       if ($type == PhabricatorTransactions::TYPE_COMMENT) {
         $tail[] = $xaction;
       } else {
         $head[] = $xaction;
       }
     }
 
     return array_values(array_merge($head, $tail));
   }
 
 
   protected function filterTransactions(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $type_comment = PhabricatorTransactions::TYPE_COMMENT;
 
     $no_effect = array();
     $has_comment = false;
     $any_effect = false;
     foreach ($xactions as $key => $xaction) {
       if ($this->transactionHasEffect($object, $xaction)) {
         if ($xaction->getTransactionType() != $type_comment) {
           $any_effect = true;
         }
       } else if ($xaction->getIgnoreOnNoEffect()) {
         unset($xactions[$key]);
       } else {
         $no_effect[$key] = $xaction;
       }
       if ($xaction->hasComment()) {
         $has_comment = true;
       }
     }
 
     if (!$no_effect) {
       return $xactions;
     }
 
     if (!$this->getContinueOnNoEffect() && !$this->getIsPreview()) {
       throw new PhabricatorApplicationTransactionNoEffectException(
         $no_effect,
         $any_effect,
         $has_comment);
     }
 
     if (!$any_effect && !$has_comment) {
       // If we only have empty comment transactions, just drop them all.
       return array();
     }
 
     foreach ($no_effect as $key => $xaction) {
       if ($xaction->hasComment()) {
         $xaction->setTransactionType($type_comment);
         $xaction->setOldValue(null);
         $xaction->setNewValue(null);
       } else {
         unset($xactions[$key]);
       }
     }
 
     return $xactions;
   }
 
 
   /**
    * Hook for validating transactions. This callback will be invoked for each
    * available transaction type, even if an edit does not apply any transactions
    * of that type. This allows you to raise exceptions when required fields are
    * missing, by detecting that the object has no field value and there is no
    * transaction which sets one.
    *
    * @param PhabricatorLiskDAO Object being edited.
    * @param string Transaction type to validate.
    * @param list<PhabricatorApplicationTransaction> Transactions of given type,
    *   which may be empty if the edit does not apply any transactions of the
    *   given type.
    * @return list<PhabricatorApplicationTransactionValidationError> List of
    *   validation errors.
    */
   protected function validateTransaction(
     PhabricatorLiskDAO $object,
     $type,
     array $xactions) {
 
     $errors = array();
 
     $xtype = $this->getModularTransactionType($type);
     if ($xtype) {
       $errors[] = $xtype->validateTransactions($object, $xactions);
     }
 
     switch ($type) {
       case PhabricatorTransactions::TYPE_VIEW_POLICY:
         $errors[] = $this->validatePolicyTransaction(
           $object,
           $xactions,
           $type,
           PhabricatorPolicyCapability::CAN_VIEW);
         break;
       case PhabricatorTransactions::TYPE_EDIT_POLICY:
         $errors[] = $this->validatePolicyTransaction(
           $object,
           $xactions,
           $type,
           PhabricatorPolicyCapability::CAN_EDIT);
         break;
       case PhabricatorTransactions::TYPE_SPACE:
         $errors[] = $this->validateSpaceTransactions(
           $object,
           $xactions,
           $type);
         break;
       case PhabricatorTransactions::TYPE_CUSTOMFIELD:
         $groups = array();
         foreach ($xactions as $xaction) {
           $groups[$xaction->getMetadataValue('customfield:key')][] = $xaction;
         }
 
         $field_list = PhabricatorCustomField::getObjectFields(
           $object,
           PhabricatorCustomField::ROLE_EDIT);
         $field_list->setViewer($this->getActor());
 
         $role_xactions = PhabricatorCustomField::ROLE_APPLICATIONTRANSACTIONS;
         foreach ($field_list->getFields() as $field) {
           if (!$field->shouldEnableForRole($role_xactions)) {
             continue;
           }
           $errors[] = $field->validateApplicationTransactions(
             $this,
             $type,
             idx($groups, $field->getFieldKey(), array()));
         }
         break;
     }
 
     return array_mergev($errors);
   }
 
   private function validatePolicyTransaction(
     PhabricatorLiskDAO $object,
     array $xactions,
     $transaction_type,
     $capability) {
 
     $actor = $this->requireActor();
     $errors = array();
     // Note $this->xactions is necessary; $xactions is $this->xactions of
     // $transaction_type
     $policy_object = $this->adjustObjectForPolicyChecks(
       $object,
       $this->xactions);
 
     // Make sure the user isn't editing away their ability to $capability this
     // object.
     foreach ($xactions as $xaction) {
       try {
         PhabricatorPolicyFilter::requireCapabilityWithForcedPolicy(
           $actor,
           $policy_object,
           $capability,
           $xaction->getNewValue());
       } catch (PhabricatorPolicyException $ex) {
         $errors[] = new PhabricatorApplicationTransactionValidationError(
           $transaction_type,
           pht('Invalid'),
           pht(
             'You can not select this %s policy, because you would no longer '.
             'be able to %s the object.',
             $capability,
             $capability),
           $xaction);
       }
     }
 
     if ($this->getIsNewObject()) {
       if (!$xactions) {
         $has_capability = PhabricatorPolicyFilter::hasCapability(
           $actor,
           $policy_object,
           $capability);
         if (!$has_capability) {
           $errors[] = new PhabricatorApplicationTransactionValidationError(
             $transaction_type,
             pht('Invalid'),
             pht(
               'The selected %s policy excludes you. Choose a %s policy '.
               'which allows you to %s the object.',
               $capability,
               $capability,
               $capability));
         }
       }
     }
 
     return $errors;
   }
 
 
   private function validateSpaceTransactions(
     PhabricatorLiskDAO $object,
     array $xactions,
     $transaction_type) {
     $errors = array();
 
     $actor = $this->getActor();
 
     $has_spaces = PhabricatorSpacesNamespaceQuery::getViewerSpacesExist($actor);
     $actor_spaces = PhabricatorSpacesNamespaceQuery::getViewerSpaces($actor);
     $active_spaces = PhabricatorSpacesNamespaceQuery::getViewerActiveSpaces(
       $actor);
     foreach ($xactions as $xaction) {
       $space_phid = $xaction->getNewValue();
 
       if ($space_phid === null) {
         if (!$has_spaces) {
           // The install doesn't have any spaces, so this is fine.
           continue;
         }
 
         // The install has some spaces, so every object needs to be put
         // in a valid space.
         $errors[] = new PhabricatorApplicationTransactionValidationError(
           $transaction_type,
           pht('Invalid'),
           pht('You must choose a space for this object.'),
           $xaction);
         continue;
       }
 
       // If the PHID isn't `null`, it needs to be a valid space that the
       // viewer can see.
       if (empty($actor_spaces[$space_phid])) {
         $errors[] = new PhabricatorApplicationTransactionValidationError(
           $transaction_type,
           pht('Invalid'),
           pht(
             'You can not shift this object in the selected space, because '.
             'the space does not exist or you do not have access to it.'),
           $xaction);
       } else if (empty($active_spaces[$space_phid])) {
 
         // It's OK to edit objects in an archived space, so just move on if
         // we aren't adjusting the value.
         $old_space_phid = $this->getTransactionOldValue($object, $xaction);
         if ($space_phid == $old_space_phid) {
           continue;
         }
 
         $errors[] = new PhabricatorApplicationTransactionValidationError(
           $transaction_type,
           pht('Archived'),
           pht(
             'You can not shift this object into the selected space, because '.
             'the space is archived. Objects can not be created inside (or '.
             'moved into) archived spaces.'),
           $xaction);
       }
     }
 
     return $errors;
   }
 
 
   protected function adjustObjectForPolicyChecks(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $copy = clone $object;
 
     foreach ($xactions as $xaction) {
       switch ($xaction->getTransactionType()) {
         case PhabricatorTransactions::TYPE_SUBSCRIBERS:
           $clone_xaction = clone $xaction;
           $clone_xaction->setOldValue(array_values($this->subscribers));
           $clone_xaction->setNewValue(
             $this->getPHIDTransactionNewValue(
               $clone_xaction));
 
           PhabricatorPolicyRule::passTransactionHintToRule(
             $copy,
             new PhabricatorSubscriptionsSubscribersPolicyRule(),
             array_fuse($clone_xaction->getNewValue()));
 
           break;
         case PhabricatorTransactions::TYPE_SPACE:
           $space_phid = $this->getTransactionNewValue($object, $xaction);
           $copy->setSpacePHID($space_phid);
           break;
       }
     }
 
     return $copy;
   }
 
   protected function validateAllTransactions(
     PhabricatorLiskDAO $object,
     array $xactions) {
     return array();
   }
 
   /**
    * Check for a missing text field.
    *
    * A text field is missing if the object has no value and there are no
    * transactions which set a value, or if the transactions remove the value.
    * This method is intended to make implementing @{method:validateTransaction}
    * more convenient:
    *
    *   $missing = $this->validateIsEmptyTextField(
    *     $object->getName(),
    *     $xactions);
    *
    * This will return `true` if the net effect of the object and transactions
    * is an empty field.
    *
    * @param wild Current field value.
    * @param list<PhabricatorApplicationTransaction> Transactions editing the
    *          field.
    * @return bool True if the field will be an empty text field after edits.
    */
   protected function validateIsEmptyTextField($field_value, array $xactions) {
     if (strlen($field_value) && empty($xactions)) {
       return false;
     }
 
     if ($xactions && strlen(last($xactions)->getNewValue())) {
       return false;
     }
 
     return true;
   }
 
   /**
    * Check that text field input isn't longer than a specified length.
    *
    * A text field input is invalid if the length of the input is longer than a
    * specified length. This length can be determined by the space allotted in
    * the database, or given arbitrarily.
    * This method is intended to make implementing @{method:validateTransaction}
    * more convenient:
    *
    *   $overdrawn = $this->validateIsTextFieldTooLong(
    *     $object->getName(),
    *     $xactions,
    *     $field_length);
    *
    * This will return `true` if the net effect of the object and transactions
    * is a field that is too long.
    *
    * @param wild Current field value.
    * @param list<PhabricatorApplicationTransaction> Transactions editing the
    *          field.
    * @param integer for maximum field length.
    * @return bool True if the field will be too long after edits.
    */
   protected function validateIsTextFieldTooLong(
     $field_value,
     array $xactions,
     $length) {
 
     if ($xactions) {
       $new_value_length = phutil_utf8_strlen(last($xactions)->getNewValue());
       if ($new_value_length <= $length) {
         return false;
       } else {
         return true;
       }
     }
 
     $old_value_length = phutil_utf8_strlen($field_value);
     if ($old_value_length <= $length) {
       return false;
     }
 
     return true;
   }
 
 
 /* -(  Implicit CCs  )------------------------------------------------------- */
 
 
   /**
    * When a user interacts with an object, we might want to add them to CC.
    */
   final public function applyImplicitCC(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     if (!($object instanceof PhabricatorSubscribableInterface)) {
       // If the object isn't subscribable, we can't CC them.
       return $xactions;
     }
 
     $actor_phid = $this->getActingAsPHID();
 
     $type_user = PhabricatorPeopleUserPHIDType::TYPECONST;
     if (phid_get_type($actor_phid) != $type_user) {
       // Transactions by application actors like Herald, Harbormaster and
       // Diffusion should not CC the applications.
       return $xactions;
     }
 
     if ($object->isAutomaticallySubscribed($actor_phid)) {
       // If they're auto-subscribed, don't CC them.
       return $xactions;
     }
 
     $should_cc = false;
     foreach ($xactions as $xaction) {
       if ($this->shouldImplyCC($object, $xaction)) {
         $should_cc = true;
         break;
       }
     }
 
     if (!$should_cc) {
       // Only some types of actions imply a CC (like adding a comment).
       return $xactions;
     }
 
     if ($object->getPHID()) {
       if (isset($this->subscribers[$actor_phid])) {
         // If the user is already subscribed, don't implicitly CC them.
         return $xactions;
       }
 
       $unsub = PhabricatorEdgeQuery::loadDestinationPHIDs(
         $object->getPHID(),
         PhabricatorObjectHasUnsubscriberEdgeType::EDGECONST);
       $unsub = array_fuse($unsub);
       if (isset($unsub[$actor_phid])) {
         // If the user has previously unsubscribed from this object explicitly,
         // don't implicitly CC them.
         return $xactions;
       }
     }
 
     $xaction = newv(get_class(head($xactions)), array());
     $xaction->setTransactionType(PhabricatorTransactions::TYPE_SUBSCRIBERS);
     $xaction->setNewValue(array('+' => array($actor_phid)));
 
     array_unshift($xactions, $xaction);
 
     return $xactions;
   }
 
   protected function shouldImplyCC(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     return $xaction->isCommentTransaction();
   }
 
 
 /* -(  Sending Mail  )------------------------------------------------------- */
 
 
   /**
    * @task mail
    */
   protected function shouldSendMail(
     PhabricatorLiskDAO $object,
     array $xactions) {
     return false;
   }
 
 
   /**
    * @task mail
    */
   private function buildMail(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $email_to = $this->mailToPHIDs;
     $email_cc = $this->mailCCPHIDs;
     $email_cc = array_merge($email_cc, $this->heraldEmailPHIDs);
 
     $targets = $this->buildReplyHandler($object)
       ->getMailTargets($email_to, $email_cc);
 
     // Set this explicitly before we start swapping out the effective actor.
     $this->setActingAsPHID($this->getActingAsPHID());
 
     $messages = array();
     foreach ($targets as $target) {
       $original_actor = $this->getActor();
 
       $viewer = $target->getViewer();
       $this->setActor($viewer);
       $locale = PhabricatorEnv::beginScopedLocale($viewer->getTranslation());
 
       $caught = null;
       $mail = null;
       try {
         // Reload handles for the new viewer.
         $this->loadHandles($xactions);
 
         $mail = $this->buildMailForTarget($object, $xactions, $target);
       } catch (Exception $ex) {
         $caught = $ex;
       }
 
       $this->setActor($original_actor);
       unset($locale);
 
       if ($caught) {
         throw $ex;
       }
 
       if ($mail) {
         $messages[] = $mail;
       }
     }
 
     $this->runHeraldMailRules($messages);
 
     return $messages;
   }
 
   private function buildMailForTarget(
     PhabricatorLiskDAO $object,
     array $xactions,
     PhabricatorMailTarget $target) {
 
     // Check if any of the transactions are visible for this viewer. If we
     // don't have any visible transactions, don't send the mail.
 
     $any_visible = false;
     foreach ($xactions as $xaction) {
       if (!$xaction->shouldHideForMail($xactions)) {
         $any_visible = true;
         break;
       }
     }
 
     if (!$any_visible) {
       return null;
     }
 
     $mail = $this->buildMailTemplate($object);
     $body = $this->buildMailBody($object, $xactions);
 
     $mail_tags = $this->getMailTags($object, $xactions);
     $action = $this->getMailAction($object, $xactions);
 
     if (PhabricatorEnv::getEnvConfig('metamta.email-preferences')) {
       $this->addEmailPreferenceSectionToMailBody(
         $body,
         $object,
         $xactions);
     }
 
     $mail
       ->setSensitiveContent(false)
       ->setFrom($this->getActingAsPHID())
       ->setSubjectPrefix($this->getMailSubjectPrefix())
       ->setVarySubjectPrefix('['.$action.']')
       ->setThreadID($this->getMailThreadID($object), $this->getIsNewObject())
       ->setRelatedPHID($object->getPHID())
       ->setExcludeMailRecipientPHIDs($this->getExcludeMailRecipientPHIDs())
       ->setForceHeraldMailRecipientPHIDs($this->heraldForcedEmailPHIDs)
       ->setMailTags($mail_tags)
       ->setIsBulk(true)
       ->setBody($body->render())
       ->setHTMLBody($body->renderHTML());
 
     foreach ($body->getAttachments() as $attachment) {
       $mail->addAttachment($attachment);
     }
 
     if ($this->heraldHeader) {
       $mail->addHeader('X-Herald-Rules', $this->heraldHeader);
     }
 
     if ($object instanceof PhabricatorProjectInterface) {
       $this->addMailProjectMetadata($object, $mail);
     }
 
     if ($this->getParentMessageID()) {
       $mail->setParentMessageID($this->getParentMessageID());
     }
 
     return $target->willSendMail($mail);
   }
 
   private function addMailProjectMetadata(
     PhabricatorLiskDAO $object,
     PhabricatorMetaMTAMail $template) {
 
     $project_phids = PhabricatorEdgeQuery::loadDestinationPHIDs(
       $object->getPHID(),
       PhabricatorProjectObjectHasProjectEdgeType::EDGECONST);
 
     if (!$project_phids) {
       return;
     }
 
     // TODO: This viewer isn't quite right. It would be slightly better to use
     // the mail recipient, but that's not very easy given the way rendering
     // works today.
 
     $handles = id(new PhabricatorHandleQuery())
       ->setViewer($this->requireActor())
       ->withPHIDs($project_phids)
       ->execute();
 
     $project_tags = array();
     foreach ($handles as $handle) {
       if (!$handle->isComplete()) {
         continue;
       }
       $project_tags[] = '<'.$handle->getObjectName().'>';
     }
 
     if (!$project_tags) {
       return;
     }
 
     $project_tags = implode(', ', $project_tags);
     $template->addHeader('X-Phabricator-Projects', $project_tags);
   }
 
 
   protected function getMailThreadID(PhabricatorLiskDAO $object) {
     return $object->getPHID();
   }
 
 
   /**
    * @task mail
    */
   protected function getStrongestAction(
     PhabricatorLiskDAO $object,
     array $xactions) {
     return last(msort($xactions, 'getActionStrength'));
   }
 
 
   /**
    * @task mail
    */
   protected function buildReplyHandler(PhabricatorLiskDAO $object) {
     throw new Exception(pht('Capability not supported.'));
   }
 
   /**
    * @task mail
    */
   protected function getMailSubjectPrefix() {
     throw new Exception(pht('Capability not supported.'));
   }
 
 
   /**
    * @task mail
    */
   protected function getMailTags(
     PhabricatorLiskDAO $object,
     array $xactions) {
     $tags = array();
 
     foreach ($xactions as $xaction) {
       $tags[] = $xaction->getMailTags();
     }
 
     return array_mergev($tags);
   }
 
   /**
    * @task mail
    */
   public function getMailTagsMap() {
     // TODO: We should move shared mail tags, like "comment", here.
     return array();
   }
 
 
   /**
    * @task mail
    */
   protected function getMailAction(
     PhabricatorLiskDAO $object,
     array $xactions) {
     return $this->getStrongestAction($object, $xactions)->getActionName();
   }
 
 
   /**
    * @task mail
    */
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
     throw new Exception(pht('Capability not supported.'));
   }
 
 
   /**
    * @task mail
    */
   protected function getMailTo(PhabricatorLiskDAO $object) {
     throw new Exception(pht('Capability not supported.'));
   }
 
 
   /**
    * @task mail
    */
   protected function getMailCC(PhabricatorLiskDAO $object) {
     $phids = array();
     $has_support = false;
 
     if ($object instanceof PhabricatorSubscribableInterface) {
       $phid = $object->getPHID();
       $phids[] = PhabricatorSubscribersQuery::loadSubscribersForPHID($phid);
       $has_support = true;
     }
 
     if ($object instanceof PhabricatorProjectInterface) {
       $project_phids = PhabricatorEdgeQuery::loadDestinationPHIDs(
         $object->getPHID(),
         PhabricatorProjectObjectHasProjectEdgeType::EDGECONST);
 
       if ($project_phids) {
         $projects = id(new PhabricatorProjectQuery())
           ->setViewer(PhabricatorUser::getOmnipotentUser())
           ->withPHIDs($project_phids)
           ->needWatchers(true)
           ->execute();
 
         $watcher_phids = array();
         foreach ($projects as $project) {
           foreach ($project->getAllAncestorWatcherPHIDs() as $phid) {
             $watcher_phids[$phid] = $phid;
           }
         }
 
         if ($watcher_phids) {
           // We need to do a visibility check for all the watchers, as
           // watching a project is not a guarantee that you can see objects
           // associated with it.
           $users = id(new PhabricatorPeopleQuery())
             ->setViewer($this->requireActor())
             ->withPHIDs($watcher_phids)
             ->execute();
 
           $watchers = array();
           foreach ($users as $user) {
             $can_see = PhabricatorPolicyFilter::hasCapability(
               $user,
               $object,
               PhabricatorPolicyCapability::CAN_VIEW);
             if ($can_see) {
               $watchers[] = $user->getPHID();
             }
           }
           $phids[] = $watchers;
         }
       }
 
       $has_support = true;
     }
 
     if (!$has_support) {
       throw new Exception(pht('Capability not supported.'));
     }
 
     return array_mergev($phids);
   }
 
 
   /**
    * @task mail
    */
   protected function buildMailBody(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $body = new PhabricatorMetaMTAMailBody();
     $body->setViewer($this->requireActor());
 
     $this->addHeadersAndCommentsToMailBody($body, $xactions);
     $this->addCustomFieldsToMailBody($body, $object, $xactions);
     return $body;
   }
 
 
   /**
    * @task mail
    */
   protected function addEmailPreferenceSectionToMailBody(
     PhabricatorMetaMTAMailBody $body,
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $href = PhabricatorEnv::getProductionURI(
       '/settings/panel/emailpreferences/');
     $body->addLinkSection(pht('EMAIL PREFERENCES'), $href);
   }
 
 
   /**
    * @task mail
    */
   protected function addHeadersAndCommentsToMailBody(
     PhabricatorMetaMTAMailBody $body,
     array $xactions,
     $object_label = null,
     $object_href = null) {
 
     $headers = array();
     $headers_html = array();
     $comments = array();
     $details = array();
 
     foreach ($xactions as $xaction) {
       if ($xaction->shouldHideForMail($xactions)) {
         continue;
       }
 
       $header = $xaction->getTitleForMail();
       if ($header !== null) {
         $headers[] = $header;
       }
 
       $header_html = $xaction->getTitleForHTMLMail();
       if ($header_html !== null) {
         $headers_html[] = $header_html;
       }
 
       $comment = $xaction->getBodyForMail();
       if ($comment !== null) {
         $comments[] = $comment;
       }
 
       if ($xaction->hasChangeDetailsForMail()) {
         $details[] = $xaction;
       }
     }
 
     $headers_text = implode("\n", $headers);
     $body->addRawPlaintextSection($headers_text);
 
     $headers_html = phutil_implode_html(phutil_tag('br'), $headers_html);
 
     $header_button = null;
     if ($object_label !== null) {
       $button_style = array(
         'text-decoration: none;',
         'padding: 4px 8px;',
         'margin: 0 8px 8px;',
         'float: right;',
         'color: #464C5C;',
         'font-weight: bold;',
         'border-radius: 3px;',
         'background-color: #F7F7F9;',
         'background-image: linear-gradient(to bottom,#fff,#f1f0f1);',
         'display: inline-block;',
         'border: 1px solid rgba(71,87,120,.2);',
       );
 
       $header_button = phutil_tag(
         'a',
         array(
           'style' => implode(' ', $button_style),
           'href' => $object_href,
         ),
         $object_label);
     }
 
     $xactions_style = array(
     );
 
     $header_action = phutil_tag(
       'td',
       array(),
       $header_button);
 
     $header_action = phutil_tag(
       'td',
       array(
         'style' => implode(' ', $xactions_style),
       ),
       array(
         $headers_html,
         // Add an extra newline to prevent the "View Object" button from
         // running into the transaction text in Mail.app text snippet
         // previews.
         "\n",
       ));
 
     $headers_html = phutil_tag(
       'table',
       array(),
       phutil_tag('tr', array(), array($header_action, $header_button)));
 
     $body->addRawHTMLSection($headers_html);
 
     foreach ($comments as $comment) {
       $body->addRemarkupSection(null, $comment);
     }
 
     foreach ($details as $xaction) {
       $details = $xaction->renderChangeDetailsForMail($body->getViewer());
       if ($details !== null) {
         $label = $this->getMailDiffSectionHeader($xaction);
         $body->addHTMLSection($label, $details);
       }
     }
 
   }
 
   private function getMailDiffSectionHeader($xaction) {
     $type = $xaction->getTransactionType();
 
     $xtype = $this->getModularTransactionType($type);
     if ($xtype) {
       return $xtype->getMailDiffSectionHeader();
     }
 
     return pht('EDIT DETAILS');
   }
 
   /**
    * @task mail
    */
   protected function addCustomFieldsToMailBody(
     PhabricatorMetaMTAMailBody $body,
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     if ($object instanceof PhabricatorCustomFieldInterface) {
       $field_list = PhabricatorCustomField::getObjectFields(
         $object,
         PhabricatorCustomField::ROLE_TRANSACTIONMAIL);
       $field_list->setViewer($this->getActor());
       $field_list->readFieldsFromStorage($object);
 
       foreach ($field_list->getFields() as $field) {
         $field->updateTransactionMailBody(
           $body,
           $this,
           $xactions);
       }
     }
   }
 
 
   /**
    * @task mail
    */
   private function runHeraldMailRules(array $messages) {
     foreach ($messages as $message) {
       $engine = new HeraldEngine();
       $adapter = id(new PhabricatorMailOutboundMailHeraldAdapter())
         ->setObject($message);
 
       $rules = $engine->loadRulesForAdapter($adapter);
       $effects = $engine->applyRules($rules, $adapter);
       $engine->applyEffects($effects, $adapter, $rules);
     }
   }
 
 
 /* -(  Publishing Feed Stories  )-------------------------------------------- */
 
 
   /**
    * @task feed
    */
   protected function shouldPublishFeedStory(
     PhabricatorLiskDAO $object,
     array $xactions) {
     return false;
   }
 
 
   /**
    * @task feed
    */
   protected function getFeedStoryType() {
     return 'PhabricatorApplicationTransactionFeedStory';
   }
 
 
   /**
    * @task feed
    */
   protected function getFeedRelatedPHIDs(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $phids = array(
       $object->getPHID(),
       $this->getActingAsPHID(),
     );
 
     if ($object instanceof PhabricatorProjectInterface) {
       $project_phids = PhabricatorEdgeQuery::loadDestinationPHIDs(
         $object->getPHID(),
         PhabricatorProjectObjectHasProjectEdgeType::EDGECONST);
       foreach ($project_phids as $project_phid) {
         $phids[] = $project_phid;
       }
     }
 
     return $phids;
   }
 
 
   /**
    * @task feed
    */
   protected function getFeedNotifyPHIDs(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     return array_unique(array_merge(
       $this->getMailTo($object),
       $this->getMailCC($object)));
   }
 
 
   /**
    * @task feed
    */
   protected function getFeedStoryData(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $xactions = msort($xactions, 'getActionStrength');
     $xactions = array_reverse($xactions);
 
     return array(
       'objectPHID'        => $object->getPHID(),
       'transactionPHIDs'  => mpull($xactions, 'getPHID'),
     );
   }
 
 
   /**
    * @task feed
    */
   protected function publishFeedStory(
     PhabricatorLiskDAO $object,
     array $xactions,
     array $mailed_phids) {
 
     $xactions = mfilter($xactions, 'shouldHideForFeed', true);
 
     if (!$xactions) {
       return;
     }
 
     $related_phids = $this->feedRelatedPHIDs;
     $subscribed_phids = $this->feedNotifyPHIDs;
 
     $story_type = $this->getFeedStoryType();
     $story_data = $this->getFeedStoryData($object, $xactions);
 
     id(new PhabricatorFeedStoryPublisher())
       ->setStoryType($story_type)
       ->setStoryData($story_data)
       ->setStoryTime(time())
       ->setStoryAuthorPHID($this->getActingAsPHID())
       ->setRelatedPHIDs($related_phids)
       ->setPrimaryObjectPHID($object->getPHID())
       ->setSubscribedPHIDs($subscribed_phids)
       ->setMailRecipientPHIDs($mailed_phids)
       ->setMailTags($this->getMailTags($object, $xactions))
       ->publish();
   }
 
 
 /* -(  Search Index  )------------------------------------------------------- */
 
 
   /**
    * @task search
    */
   protected function supportsSearch() {
     return false;
   }
 
 
 /* -(  Herald Integration )-------------------------------------------------- */
 
 
   protected function shouldApplyHeraldRules(
     PhabricatorLiskDAO $object,
     array $xactions) {
     return false;
   }
 
   protected function buildHeraldAdapter(
     PhabricatorLiskDAO $object,
     array $xactions) {
     throw new Exception(pht('No herald adapter specified.'));
   }
 
   private function setHeraldAdapter(HeraldAdapter $adapter) {
     $this->heraldAdapter = $adapter;
     return $this;
   }
 
   protected function getHeraldAdapter() {
     return $this->heraldAdapter;
   }
 
   private function setHeraldTranscript(HeraldTranscript $transcript) {
     $this->heraldTranscript = $transcript;
     return $this;
   }
 
   protected function getHeraldTranscript() {
     return $this->heraldTranscript;
   }
 
   private function applyHeraldRules(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $adapter = $this->buildHeraldAdapter($object, $xactions)
       ->setContentSource($this->getContentSource())
       ->setIsNewObject($this->getIsNewObject())
       ->setAppliedTransactions($xactions);
 
     if ($this->getApplicationEmail()) {
       $adapter->setApplicationEmail($this->getApplicationEmail());
     }
 
     $xscript = HeraldEngine::loadAndApplyRules($adapter);
 
     $this->setHeraldAdapter($adapter);
     $this->setHeraldTranscript($xscript);
 
     if ($adapter instanceof HarbormasterBuildableAdapterInterface) {
       HarbormasterBuildable::applyBuildPlans(
         $adapter->getHarbormasterBuildablePHID(),
         $adapter->getHarbormasterContainerPHID(),
         $adapter->getQueuedHarbormasterBuildRequests());
     }
 
     return array_merge(
       $this->didApplyHeraldRules($object, $adapter, $xscript),
       $adapter->getQueuedTransactions());
   }
 
   protected function didApplyHeraldRules(
     PhabricatorLiskDAO $object,
     HeraldAdapter $adapter,
     HeraldTranscript $transcript) {
     return array();
   }
 
 
 /* -(  Custom Fields  )------------------------------------------------------ */
 
 
   /**
    * @task customfield
    */
   private function getCustomFieldForTransaction(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     $field_key = $xaction->getMetadataValue('customfield:key');
     if (!$field_key) {
       throw new Exception(
         pht(
         "Custom field transaction has no '%s'!",
         'customfield:key'));
     }
 
     $field = PhabricatorCustomField::getObjectField(
       $object,
       PhabricatorCustomField::ROLE_APPLICATIONTRANSACTIONS,
       $field_key);
 
     if (!$field) {
       throw new Exception(
         pht(
           "Custom field transaction has invalid '%s'; field '%s' ".
           "is disabled or does not exist.",
           'customfield:key',
           $field_key));
     }
 
     if (!$field->shouldAppearInApplicationTransactions()) {
       throw new Exception(
         pht(
           "Custom field transaction '%s' does not implement ".
           "integration for %s.",
           $field_key,
           'ApplicationTransactions'));
     }
 
     $field->setViewer($this->getActor());
 
     return $field;
   }
 
 
 /* -(  Files  )-------------------------------------------------------------- */
 
 
   /**
    * Extract the PHIDs of any files which these transactions attach.
    *
    * @task files
    */
   private function extractFilePHIDs(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $changes = $this->getRemarkupChanges($xactions);
     $blocks = mpull($changes, 'getNewValue');
 
     $phids = array();
     if ($blocks) {
       $phids[] = PhabricatorMarkupEngine::extractFilePHIDsFromEmbeddedFiles(
         $this->getActor(),
         $blocks);
     }
 
     foreach ($xactions as $xaction) {
       $type = $xaction->getTransactionType();
 
       $xtype = $this->getModularTransactionType($type);
       if ($xtype) {
         $phids[] = $xtype->extractFilePHIDs($object, $xaction->getNewValue());
       } else {
         $phids[] = $this->extractFilePHIDsFromCustomTransaction(
           $object,
           $xaction);
       }
     }
 
     $phids = array_unique(array_filter(array_mergev($phids)));
     if (!$phids) {
       return array();
     }
 
     // Only let a user attach files they can actually see, since this would
     // otherwise let you access any file by attaching it to an object you have
     // view permission on.
 
     $files = id(new PhabricatorFileQuery())
       ->setViewer($this->getActor())
       ->withPHIDs($phids)
       ->execute();
 
     return mpull($files, 'getPHID');
   }
 
   /**
    * @task files
    */
   protected function extractFilePHIDsFromCustomTransaction(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
     return array();
   }
 
 
   /**
    * @task files
    */
   private function attachFiles(
     PhabricatorLiskDAO $object,
     array $file_phids) {
 
     if (!$file_phids) {
       return;
     }
 
     $editor = new PhabricatorEdgeEditor();
 
     $src = $object->getPHID();
     $type = PhabricatorObjectHasFileEdgeType::EDGECONST;
     foreach ($file_phids as $dst) {
       $editor->addEdge($src, $type, $dst);
     }
 
     $editor->save();
   }
 
   private function applyInverseEdgeTransactions(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction,
     $inverse_type) {
 
     $old = $xaction->getOldValue();
     $new = $xaction->getNewValue();
 
     $add = array_keys(array_diff_key($new, $old));
     $rem = array_keys(array_diff_key($old, $new));
 
     $add = array_fuse($add);
     $rem = array_fuse($rem);
     $all = $add + $rem;
 
     $nodes = id(new PhabricatorObjectQuery())
       ->setViewer($this->requireActor())
       ->withPHIDs($all)
       ->execute();
 
     foreach ($nodes as $node) {
       if (!($node instanceof PhabricatorApplicationTransactionInterface)) {
         continue;
       }
 
       if ($node instanceof PhabricatorUser) {
         // TODO: At least for now, don't record inverse edge transactions
         // for users (for example, "alincoln joined project X"): Feed fills
         // this role instead.
         continue;
       }
 
       $editor = $node->getApplicationTransactionEditor();
       $template = $node->getApplicationTransactionTemplate();
       $target = $node->getApplicationTransactionObject();
 
       if (isset($add[$node->getPHID()])) {
         $edge_edit_type = '+';
       } else {
         $edge_edit_type = '-';
       }
 
       $template
         ->setTransactionType($xaction->getTransactionType())
         ->setMetadataValue('edge:type', $inverse_type)
         ->setNewValue(
           array(
             $edge_edit_type => array($object->getPHID() => $object->getPHID()),
           ));
 
       $editor
         ->setContinueOnNoEffect(true)
         ->setContinueOnMissingFields(true)
         ->setParentMessageID($this->getParentMessageID())
         ->setIsInverseEdgeEditor(true)
         ->setActor($this->requireActor())
         ->setActingAsPHID($this->getActingAsPHID())
         ->setContentSource($this->getContentSource());
 
       $editor->applyTransactions($target, array($template));
     }
   }
 
 
 /* -(  Workers  )------------------------------------------------------------ */
 
 
   /**
    * Load any object state which is required to publish transactions.
    *
    * This hook is invoked in the main process before we compute data related
    * to publishing transactions (like email "To" and "CC" lists), and again in
    * the worker before publishing occurs.
    *
    * @return object Publishable object.
    * @task workers
    */
   protected function willPublish(PhabricatorLiskDAO $object, array $xactions) {
     return $object;
   }
 
 
   /**
    * Convert the editor state to a serializable dictionary which can be passed
    * to a worker.
    *
    * This data will be loaded with @{method:loadWorkerState} in the worker.
    *
    * @return dict<string, wild> Serializable editor state.
    * @task workers
    */
   final private function getWorkerState() {
     $state = array();
     foreach ($this->getAutomaticStateProperties() as $property) {
       $state[$property] = $this->$property;
     }
 
     $custom_state = $this->getCustomWorkerState();
     $custom_encoding = $this->getCustomWorkerStateEncoding();
 
     $state += array(
       'excludeMailRecipientPHIDs' => $this->getExcludeMailRecipientPHIDs(),
       'custom' => $this->encodeStateForStorage($custom_state, $custom_encoding),
       'custom.encoding' => $custom_encoding,
     );
 
     return $state;
   }
 
 
   /**
    * Hook; return custom properties which need to be passed to workers.
    *
    * @return dict<string, wild> Custom properties.
    * @task workers
    */
   protected function getCustomWorkerState() {
     return array();
   }
 
 
   /**
    * Hook; return storage encoding for custom properties which need to be
    * passed to workers.
    *
    * This primarily allows binary data to be passed to workers and survive
    * JSON encoding.
    *
    * @return dict<string, string> Property encodings.
    * @task workers
    */
   protected function getCustomWorkerStateEncoding() {
     return array();
   }
 
 
   /**
    * Load editor state using a dictionary emitted by @{method:getWorkerState}.
    *
    * This method is used to load state when running worker operations.
    *
    * @param dict<string, wild> Editor state, from @{method:getWorkerState}.
    * @return this
    * @task workers
    */
   final public function loadWorkerState(array $state) {
     foreach ($this->getAutomaticStateProperties() as $property) {
       $this->$property = idx($state, $property);
     }
 
     $exclude = idx($state, 'excludeMailRecipientPHIDs', array());
     $this->setExcludeMailRecipientPHIDs($exclude);
 
     $custom_state = idx($state, 'custom', array());
     $custom_encodings = idx($state, 'custom.encoding', array());
     $custom = $this->decodeStateFromStorage($custom_state, $custom_encodings);
 
     $this->loadCustomWorkerState($custom);
 
     return $this;
   }
 
 
   /**
    * Hook; set custom properties on the editor from data emitted by
    * @{method:getCustomWorkerState}.
    *
    * @param dict<string, wild> Custom state,
    *   from @{method:getCustomWorkerState}.
    * @return this
    * @task workers
    */
   protected function loadCustomWorkerState(array $state) {
     return $this;
   }
 
 
   /**
    * Get a list of object properties which should be automatically sent to
    * workers in the state data.
    *
    * These properties will be automatically stored and loaded by the editor in
    * the worker.
    *
    * @return list<string> List of properties.
    * @task workers
    */
   private function getAutomaticStateProperties() {
     return array(
       'parentMessageID',
       'disableEmail',
       'isNewObject',
       'heraldEmailPHIDs',
       'heraldForcedEmailPHIDs',
       'heraldHeader',
       'mailToPHIDs',
       'mailCCPHIDs',
       'feedNotifyPHIDs',
       'feedRelatedPHIDs',
     );
   }
 
   /**
    * Apply encodings prior to storage.
    *
    * See @{method:getCustomWorkerStateEncoding}.
    *
    * @param map<string, wild> Map of values to encode.
    * @param map<string, string> Map of encodings to apply.
    * @return map<string, wild> Map of encoded values.
    * @task workers
    */
   final private function encodeStateForStorage(
     array $state,
     array $encodings) {
 
     foreach ($state as $key => $value) {
       $encoding = idx($encodings, $key);
       switch ($encoding) {
         case self::STORAGE_ENCODING_BINARY:
           // The mechanics of this encoding (serialize + base64) are a little
           // awkward, but it allows us encode arrays and still be JSON-safe
           // with binary data.
 
           $value = @serialize($value);
           if ($value === false) {
             throw new Exception(
               pht(
                 'Failed to serialize() value for key "%s".',
                 $key));
           }
 
           $value = base64_encode($value);
           if ($value === false) {
             throw new Exception(
               pht(
                 'Failed to base64 encode value for key "%s".',
                 $key));
           }
           break;
       }
       $state[$key] = $value;
     }
 
     return $state;
   }
 
 
   /**
    * Undo storage encoding applied when storing state.
    *
    * See @{method:getCustomWorkerStateEncoding}.
    *
    * @param map<string, wild> Map of encoded values.
    * @param map<string, string> Map of encodings.
    * @return map<string, wild> Map of decoded values.
    * @task workers
    */
   final private function decodeStateFromStorage(
     array $state,
     array $encodings) {
 
     foreach ($state as $key => $value) {
       $encoding = idx($encodings, $key);
       switch ($encoding) {
         case self::STORAGE_ENCODING_BINARY:
           $value = base64_decode($value);
           if ($value === false) {
             throw new Exception(
               pht(
                 'Failed to base64_decode() value for key "%s".',
                 $key));
           }
 
           $value = unserialize($value);
           break;
       }
       $state[$key] = $value;
     }
 
     return $state;
   }
 
 
   /**
    * Remove conflicts from a list of projects.
    *
    * Objects aren't allowed to be tagged with multiple milestones in the same
    * group, nor projects such that one tag is the ancestor of any other tag.
    * If the list of PHIDs include mutually exclusive projects, remove the
    * conflicting projects.
    *
    * @param list<phid> List of project PHIDs.
    * @return list<phid> List with conflicts removed.
    */
   private function applyProjectConflictRules(array $phids) {
     if (!$phids) {
       return array();
     }
 
     // Overall, the last project in the list wins in cases of conflict (so when
     // you add something, the thing you just added sticks and removes older
     // values).
 
     // Beyond that, there are two basic cases:
 
     // Milestones: An object can't be in "A > Sprint 3" and "A > Sprint 4".
     // If multiple projects are milestones of the same parent, we only keep the
     // last one.
 
     // Ancestor: You can't be in "A" and "A > B". If "A > B" comes later
     // in the list, we remove "A" and keep "A > B". If "A" comes later, we
     // remove "A > B" and keep "A".
 
     // Note that it's OK to be in "A > B" and "A > C". There's only a conflict
     // if one project is an ancestor of another. It's OK to have something
     // tagged with multiple projects which share a common ancestor, so long as
     // they are not mutual ancestors.
 
     $viewer = PhabricatorUser::getOmnipotentUser();
 
     $projects = id(new PhabricatorProjectQuery())
       ->setViewer($viewer)
       ->withPHIDs(array_keys($phids))
       ->execute();
     $projects = mpull($projects, null, 'getPHID');
 
     // We're going to build a map from each project with milestones to the last
     // milestone in the list. This last milestone is the milestone we'll keep.
     $milestone_map = array();
 
     // We're going to build a set of the projects which have no descendants
     // later in the list. This allows us to apply both ancestor rules.
     $ancestor_map = array();
 
     foreach ($phids as $phid => $ignored) {
       $project = idx($projects, $phid);
       if (!$project) {
         continue;
       }
 
       // This is the last milestone we've seen, so set it as the selection for
       // the project's parent. This might be setting a new value or overwriting
       // an earlier value.
       if ($project->isMilestone()) {
         $parent_phid = $project->getParentProjectPHID();
         $milestone_map[$parent_phid] = $phid;
       }
 
       // Since this is the last item in the list we've examined so far, add it
       // to the set of projects with no later descendants.
       $ancestor_map[$phid] = $phid;
 
       // Remove any ancestors from the set, since this is a later descendant.
       foreach ($project->getAncestorProjects() as $ancestor) {
         $ancestor_phid = $ancestor->getPHID();
         unset($ancestor_map[$ancestor_phid]);
       }
     }
 
     // Now that we've built the maps, we can throw away all the projects which
     // have conflicts.
     foreach ($phids as $phid => $ignored) {
       $project = idx($projects, $phid);
 
       if (!$project) {
         // If a PHID is invalid, we just leave it as-is. We could clean it up,
         // but leaving it untouched is less likely to cause collateral damage.
         continue;
       }
 
       // If this was a milestone, check if it was the last milestone from its
       // group in the list. If not, remove it from the list.
       if ($project->isMilestone()) {
         $parent_phid = $project->getParentProjectPHID();
         if ($milestone_map[$parent_phid] !== $phid) {
           unset($phids[$phid]);
           continue;
         }
       }
 
       // If a later project in the list is a subproject of this one, it will
       // have removed ancestors from the map. If this project does not point
       // at itself in the ancestor map, it should be discarded in favor of a
       // subproject that comes later.
       if (idx($ancestor_map, $phid) !== $phid) {
         unset($phids[$phid]);
         continue;
       }
 
       // If a later project in the list is an ancestor of this one, it will
       // have added itself to the map. If any ancestor of this project points
       // at itself in the map, this project should be dicarded in favor of
       // that later ancestor.
       foreach ($project->getAncestorProjects() as $ancestor) {
         $ancestor_phid = $ancestor->getPHID();
         if (isset($ancestor_map[$ancestor_phid])) {
           unset($phids[$phid]);
           continue 2;
         }
       }
     }
 
     return $phids;
   }
 
   /**
    * When the view policy for an object is changed, scramble the secret keys
    * for attached files to invalidate existing URIs.
    */
   private function scrambleFileSecrets($object) {
     // If this is a newly created object, we don't need to scramble anything
     // since it couldn't have been previously published.
     if ($this->getIsNewObject()) {
       return;
     }
 
     // If the object is a file itself, scramble it.
     if ($object instanceof PhabricatorFile) {
       if ($this->shouldScramblePolicy($object->getViewPolicy())) {
         $object->scrambleSecret();
         $object->save();
       }
     }
 
     $phid = $object->getPHID();
 
     $attached_phids = PhabricatorEdgeQuery::loadDestinationPHIDs(
       $phid,
       PhabricatorObjectHasFileEdgeType::EDGECONST);
     if (!$attached_phids) {
       return;
     }
 
     $omnipotent_viewer = PhabricatorUser::getOmnipotentUser();
 
     $files = id(new PhabricatorFileQuery())
       ->setViewer($omnipotent_viewer)
       ->withPHIDs($attached_phids)
       ->execute();
     foreach ($files as $file) {
       $view_policy = $file->getViewPolicy();
       if ($this->shouldScramblePolicy($view_policy)) {
         $file->scrambleSecret();
         $file->save();
       }
     }
   }
 
 
   /**
    * Check if a policy is strong enough to justify scrambling. Objects which
    * are set to very open policies don't need to scramble their files, and
    * files with very open policies don't need to be scrambled when associated
    * objects change.
    */
   private function shouldScramblePolicy($policy) {
     switch ($policy) {
       case PhabricatorPolicies::POLICY_PUBLIC:
       case PhabricatorPolicies::POLICY_USER:
         return false;
     }
 
     return true;
   }
 
   private function updateWorkboardColumns($object, $const, $old, $new) {
     // If an object is removed from a project, remove it from any proxy
     // columns for that project. This allows a task which is moved up from a
     // milestone to the parent to move back into the "Backlog" column on the
     // parent workboard.
 
     if ($const != PhabricatorProjectObjectHasProjectEdgeType::EDGECONST) {
       return;
     }
 
     // TODO: This should likely be some future WorkboardInterface.
     $appears_on_workboards = ($object instanceof ManiphestTask);
     if (!$appears_on_workboards) {
       return;
     }
 
     $removed_phids = array_keys(array_diff_key($old, $new));
     if (!$removed_phids) {
       return;
     }
 
     // Find any proxy columns for the removed projects.
     $proxy_columns = id(new PhabricatorProjectColumnQuery())
       ->setViewer(PhabricatorUser::getOmnipotentUser())
       ->withProxyPHIDs($removed_phids)
       ->execute();
     if (!$proxy_columns) {
       return array();
     }
 
     $proxy_phids = mpull($proxy_columns, 'getPHID');
 
     $position_table = new PhabricatorProjectColumnPosition();
     $conn_w = $position_table->establishConnection('w');
 
     queryfx(
       $conn_w,
       'DELETE FROM %T WHERE objectPHID = %s AND columnPHID IN (%Ls)',
       $position_table->getTableName(),
       $object->getPHID(),
       $proxy_phids);
   }
 
   private function getModularTransactionTypes() {
     if ($this->modularTypes === null) {
       $template = $this->object->getApplicationTransactionTemplate();
       if ($template instanceof PhabricatorModularTransaction) {
         $xtypes = $template->newModularTransactionTypes();
         foreach ($xtypes as $key => $xtype) {
           $xtype = clone $xtype;
           $xtype->setEditor($this);
           $xtypes[$key] = $xtype;
         }
       } else {
         $xtypes = array();
       }
 
       $this->modularTypes = $xtypes;
     }
 
     return $this->modularTypes;
   }
 
   private function getModularTransactionType($type) {
     $types = $this->getModularTransactionTypes();
     return idx($types, $type);
   }
 
   private function willApplyTransactions($object, array $xactions) {
     foreach ($xactions as $xaction) {
       $type = $xaction->getTransactionType();
 
       $xtype = $this->getModularTransactionType($type);
       if (!$xtype) {
         continue;
       }
 
       $xtype->willApplyTransactions($object, $xactions);
     }
   }
 
   public function getCreateObjectTitle($author, $object) {
     return pht('%s created this object.', $author);
   }
 
   public function getCreateObjectTitleForFeed($author, $object) {
     return pht('%s created an object: %s.', $author, $object);
   }
 
 }
diff --git a/src/applications/transactions/storage/PhabricatorModularTransactionType.php b/src/applications/transactions/storage/PhabricatorModularTransactionType.php
index f57d91b08f..55ad678539 100644
--- a/src/applications/transactions/storage/PhabricatorModularTransactionType.php
+++ b/src/applications/transactions/storage/PhabricatorModularTransactionType.php
@@ -1,312 +1,319 @@
 <?php
 
 abstract class PhabricatorModularTransactionType
   extends Phobject {
 
   private $storage;
   private $viewer;
   private $editor;
 
   final public function getTransactionTypeConstant() {
     return $this->getPhobjectClassConstant('TRANSACTIONTYPE');
   }
 
   public function generateOldValue($object) {
     throw new PhutilMethodNotImplementedException();
   }
 
   public function generateNewValue($object, $value) {
     return $value;
   }
 
   public function validateTransactions($object, array $xactions) {
     return array();
   }
 
   public function willApplyTransactions($object, array $xactions) {
     return;
   }
 
   public function applyInternalEffects($object, $value) {
     return;
   }
 
   public function applyExternalEffects($object, $value) {
     return;
   }
 
   public function getTransactionHasEffect($object, $old, $new) {
     return ($old !== $new);
   }
 
   public function extractFilePHIDs($object, $value) {
     return array();
   }
 
   public function shouldHide() {
     return false;
   }
 
   public function getIcon() {
     return null;
   }
 
   public function getTitle() {
     return null;
   }
 
   public function getTitleForFeed() {
     return null;
   }
 
   public function getActionName() {
     return null;
   }
 
   public function getActionStrength() {
     return null;
   }
 
   public function getColor() {
     return null;
   }
 
   public function hasChangeDetailView() {
     return false;
   }
 
   public function newChangeDetailView() {
     return null;
   }
 
   public function getMailDiffSectionHeader() {
     return pht('EDIT DETAILS');
   }
 
   public function newRemarkupChanges() {
     return array();
   }
 
+  public function mergeTransactions(
+    $object,
+    PhabricatorApplicationTransaction $u,
+    PhabricatorApplicationTransaction $v) {
+    return null;
+  }
+
   final public function setStorage(
     PhabricatorApplicationTransaction $xaction) {
     $this->storage = $xaction;
     return $this;
   }
 
   private function getStorage() {
     return $this->storage;
   }
 
   final public function setViewer(PhabricatorUser $viewer) {
     $this->viewer = $viewer;
     return $this;
   }
 
   final protected function getViewer() {
     return $this->viewer;
   }
 
   final public function getActor() {
     return $this->getEditor()->getActor();
   }
 
   final public function getActingAsPHID() {
     return $this->getEditor()->getActingAsPHID();
   }
 
   final public function setEditor(
     PhabricatorApplicationTransactionEditor $editor) {
     $this->editor = $editor;
     return $this;
   }
 
   final protected function getEditor() {
     if (!$this->editor) {
       throw new PhutilInvalidStateException('setEditor');
     }
     return $this->editor;
   }
 
   final protected function getAuthorPHID() {
     return $this->getStorage()->getAuthorPHID();
   }
 
   final protected function getObjectPHID() {
     return $this->getStorage()->getObjectPHID();
   }
 
   final protected function getObject() {
     return $this->getStorage()->getObject();
   }
 
   final protected function getOldValue() {
     return $this->getStorage()->getOldValue();
   }
 
   final protected function getNewValue() {
     return $this->getStorage()->getNewValue();
   }
 
   final protected function renderAuthor() {
     $author_phid = $this->getAuthorPHID();
     return $this->getStorage()->renderHandleLink($author_phid);
   }
 
   final protected function renderObject() {
     $object_phid = $this->getObjectPHID();
     return $this->getStorage()->renderHandleLink($object_phid);
   }
 
   final protected function renderHandle($phid) {
     $viewer = $this->getViewer();
     $display = $viewer->renderHandle($phid);
 
     if ($this->isTextMode()) {
       $display->setAsText(true);
     }
 
     return $display;
   }
 
   final protected function renderOldHandle() {
     return $this->renderHandle($this->getOldValue());
   }
 
   final protected function renderNewHandle() {
     return $this->renderHandle($this->getNewValue());
   }
 
   final protected function renderHandleList(array $phids) {
     $viewer = $this->getViewer();
     $display = $viewer->renderHandleList($phids)
       ->setAsInline(true);
 
     if ($this->isTextMode()) {
       $display->setAsText(true);
     }
 
     return $display;
   }
 
   final protected function renderValue($value) {
     if ($this->isTextMode()) {
       return sprintf('"%s"', $value);
     }
 
     return phutil_tag(
       'span',
       array(
         'class' => 'phui-timeline-value',
       ),
       $value);
   }
 
   final protected function renderOldValue() {
     return $this->renderValue($this->getOldValue());
   }
 
   final protected function renderNewValue() {
     return $this->renderValue($this->getNewValue());
   }
 
   final protected function renderDate($epoch) {
     $viewer = $this->getViewer();
 
     // We accept either epoch timestamps or dictionaries describing a
     // PhutilCalendarDateTime.
     if (is_array($epoch)) {
       $datetime = PhutilCalendarAbsoluteDateTime::newFromDictionary($epoch)
         ->setViewerTimezone($viewer->getTimezoneIdentifier());
 
       $all_day = $datetime->getIsAllDay();
 
       $epoch = $datetime->getEpoch();
     } else {
       $all_day = false;
     }
 
     if ($all_day) {
       $display = phabricator_date($epoch, $viewer);
     } else {
       $display = phabricator_datetime($epoch, $viewer);
 
       // When rendering to text, we explicitly render the offset from UTC to
       // provide context to the date: the mail may be generating with the
       // server's settings, or the user may later refer back to it after
       // changing timezones.
 
       if ($this->isRenderingTargetExternal()) {
         $offset = $viewer->getTimeZoneOffsetInHours();
         if ($offset >= 0) {
           $display = pht('%s (UTC+%d)', $display, $offset);
         } else {
           $display = pht('%s (UTC-%d)', $display, abs($offset));
         }
       }
     }
 
     return $this->renderValue($display);
   }
 
   final protected function renderOldDate() {
     return $this->renderDate($this->getOldValue());
   }
 
   final protected function renderNewDate() {
     return $this->renderDate($this->getNewValue());
   }
 
   final protected function newError($title, $message, $xaction = null) {
     return new PhabricatorApplicationTransactionValidationError(
       $this->getTransactionTypeConstant(),
       $title,
       $message,
       $xaction);
   }
 
   final protected function newRequiredError($message, $xaction = null) {
     return $this->newError(pht('Required'), $message, $xaction)
       ->setIsMissingFieldError(true);
   }
 
   final protected function newInvalidError($message, $xaction = null) {
     return $this->newError(pht('Invalid'), $message, $xaction);
   }
 
   final protected function isNewObject() {
     return $this->getEditor()->getIsNewObject();
   }
 
   final protected function isEmptyTextTransaction($value, array $xactions) {
     foreach ($xactions as $xaction) {
       $value = $xaction->getNewValue();
     }
 
     return !strlen($value);
   }
 
   /**
    * When rendering to external targets (Email/Asana/etc), we need to include
    * more information that users can't obtain later.
    */
   final protected function isRenderingTargetExternal() {
     // Right now, this is our best proxy for this:
     return $this->isTextMode();
     // "TARGET_TEXT" means "EMail" and "TARGET_HTML" means "Web".
   }
 
   final protected function isTextMode() {
     $target = $this->getStorage()->getRenderingTarget();
     return ($target == PhabricatorApplicationTransaction::TARGET_TEXT);
   }
 
   final protected function newRemarkupChange() {
     return id(new PhabricatorTransactionRemarkupChange())
       ->setTransaction($this->getStorage());
   }
 
   final protected function isCreateTransaction() {
     return $this->getStorage()->getIsCreateTransaction();
   }
 
 }