diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -2239,6 +2239,7 @@
     'PhabricatorWorkerTaskDetailController' => 'applications/daemon/controller/PhabricatorWorkerTaskDetailController.php',
     'PhabricatorWorkerTaskUpdateController' => 'applications/daemon/controller/PhabricatorWorkerTaskUpdateController.php',
     'PhabricatorWorkerTestCase' => 'infrastructure/daemon/workers/__tests__/PhabricatorWorkerTestCase.php',
+    'PhabricatorWorkerYieldException' => 'infrastructure/daemon/workers/exception/PhabricatorWorkerYieldException.php',
     'PhabricatorWorkingCopyDiscoveryTestCase' => 'applications/repository/engine/__tests__/PhabricatorWorkingCopyDiscoveryTestCase.php',
     'PhabricatorWorkingCopyPullTestCase' => 'applications/repository/engine/__tests__/PhabricatorWorkingCopyPullTestCase.php',
     'PhabricatorWorkingCopyTestCase' => 'applications/repository/engine/__tests__/PhabricatorWorkingCopyTestCase.php',
@@ -5126,6 +5127,7 @@
     'PhabricatorWorkerTaskDetailController' => 'PhabricatorDaemonController',
     'PhabricatorWorkerTaskUpdateController' => 'PhabricatorDaemonController',
     'PhabricatorWorkerTestCase' => 'PhabricatorTestCase',
+    'PhabricatorWorkerYieldException' => 'Exception',
     'PhabricatorWorkingCopyDiscoveryTestCase' => 'PhabricatorWorkingCopyTestCase',
     'PhabricatorWorkingCopyPullTestCase' => 'PhabricatorWorkingCopyTestCase',
     'PhabricatorWorkingCopyTestCase' => 'PhabricatorTestCase',
diff --git a/src/applications/harbormaster/query/HarbormasterBuildQuery.php b/src/applications/harbormaster/query/HarbormasterBuildQuery.php
--- a/src/applications/harbormaster/query/HarbormasterBuildQuery.php
+++ b/src/applications/harbormaster/query/HarbormasterBuildQuery.php
@@ -132,35 +132,35 @@
   private function buildWhereClause(AphrontDatabaseConnection $conn_r) {
     $where = array();
 
-    if ($this->ids) {
+    if ($this->ids !== null) {
       $where[] = qsprintf(
         $conn_r,
         'id IN (%Ld)',
         $this->ids);
     }
 
-    if ($this->phids) {
+    if ($this->phids !== null) {
       $where[] = qsprintf(
         $conn_r,
         'phid in (%Ls)',
         $this->phids);
     }
 
-    if ($this->buildStatuses) {
+    if ($this->buildStatuses !== null) {
       $where[] = qsprintf(
         $conn_r,
         'buildStatus in (%Ls)',
         $this->buildStatuses);
     }
 
-    if ($this->buildablePHIDs) {
+    if ($this->buildablePHIDs !== null) {
       $where[] = qsprintf(
         $conn_r,
         'buildablePHID IN (%Ls)',
         $this->buildablePHIDs);
     }
 
-    if ($this->buildPlanPHIDs) {
+    if ($this->buildPlanPHIDs !== null) {
       $where[] = qsprintf(
         $conn_r,
         'buildPlanPHID IN (%Ls)',
diff --git a/src/applications/harbormaster/step/HarbormasterWaitForPreviousBuildStepImplementation.php b/src/applications/harbormaster/step/HarbormasterWaitForPreviousBuildStepImplementation.php
--- a/src/applications/harbormaster/step/HarbormasterWaitForPreviousBuildStepImplementation.php
+++ b/src/applications/harbormaster/step/HarbormasterWaitForPreviousBuildStepImplementation.php
@@ -28,24 +28,17 @@
     // finished.
     $plan = $build->getBuildPlan();
 
-    $log = null;
-    $log_start = null;
-    $blockers = $this->getBlockers($object, $plan, $build);
-    while (count($blockers) > 0) {
-      if ($log === null) {
-        $log = $build->createLog($build_target, "waiting", "blockers");
-        $log_start = $log->start();
-      }
+    $log = $build->createLog($build_target, "waiting", "blockers");
+    $log_start = $log->start();
 
+    $blockers = $this->getBlockers($object, $plan, $build);
+    if ($blockers) {
       $log->append("Blocked by: ".implode(",", $blockers)."\n");
-
-      // TODO: This should fail temporarily instead after setting the target to
-      // waiting, and thereby push the build into a waiting status.
-      sleep(1);
-      $blockers = $this->getBlockers($object, $plan, $build);
     }
-    if ($log !== null) {
-      $log->finalize($log_start);
+    $log->finalize($log_start);
+
+    if ($blockers) {
+      throw new PhabricatorWorkerYieldException(15);
     }
   }
 
@@ -87,6 +80,10 @@
       ->execute();
     $buildable_phids = mpull($buildables, 'getPHID');
 
+    if (!$buildable_phids) {
+      return array();
+    }
+
     $builds = id(new HarbormasterBuildQuery())
       ->setViewer(PhabricatorUser::getOmnipotentUser())
       ->withBuildablePHIDs($buildable_phids)
@@ -101,4 +98,5 @@
 
     return $blockers;
   }
+
 }
diff --git a/src/applications/harbormaster/worker/HarbormasterTargetWorker.php b/src/applications/harbormaster/worker/HarbormasterTargetWorker.php
--- a/src/applications/harbormaster/worker/HarbormasterTargetWorker.php
+++ b/src/applications/harbormaster/worker/HarbormasterTargetWorker.php
@@ -52,6 +52,10 @@
 
       $target->setTargetStatus($next_status);
       $target->save();
+    } catch (PhabricatorWorkerYieldException $ex) {
+      // If the target wants to yield, let that escape without further
+      // processing. We'll resume after the task retries.
+      throw $ex;
     } catch (Exception $ex) {
       phlog($ex);
 
diff --git a/src/infrastructure/daemon/workers/PhabricatorTaskmasterDaemon.php b/src/infrastructure/daemon/workers/PhabricatorTaskmasterDaemon.php
--- a/src/infrastructure/daemon/workers/PhabricatorTaskmasterDaemon.php
+++ b/src/infrastructure/daemon/workers/PhabricatorTaskmasterDaemon.php
@@ -21,6 +21,8 @@
           if ($ex) {
             if ($ex instanceof PhabricatorWorkerPermanentFailureException) {
               $this->log("Task {$id} failed permanently.");
+            } else if ($ex instanceof PhabricatorWorkerYieldException) {
+              $this->log(pht('Task %s yielded.', $id));
             } else {
               $this->log("Task {$id} failed!");
               throw new PhutilProxyException(
diff --git a/src/infrastructure/daemon/workers/PhabricatorWorker.php b/src/infrastructure/daemon/workers/PhabricatorWorker.php
--- a/src/infrastructure/daemon/workers/PhabricatorWorker.php
+++ b/src/infrastructure/daemon/workers/PhabricatorWorker.php
@@ -93,7 +93,21 @@
     if (self::$runAllTasksInProcess) {
       // Do the work in-process.
       $worker = newv($task_class, array($data));
-      $worker->doWork();
+
+      while (true) {
+        try {
+          $worker->doWork();
+          break;
+        } catch (PhabricatorWorkerYieldException $ex) {
+          phlog(
+            pht(
+              'In-process task "%s" yielded for %s seconds, sleeping...',
+              $task_class,
+              $ex->getDuration()));
+
+          sleep($ex->getDuration());
+        }
+      }
 
       // Now, save a task row and immediately archive it so we can return an
       // object with a valid ID.
diff --git a/src/infrastructure/daemon/workers/exception/PhabricatorWorkerYieldException.php b/src/infrastructure/daemon/workers/exception/PhabricatorWorkerYieldException.php
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/daemon/workers/exception/PhabricatorWorkerYieldException.php
@@ -0,0 +1,22 @@
+<?php
+
+/**
+ * Allows tasks to yield to other tasks.
+ *
+ * If a worker throws this exception while processing a task, the task will be
+ * pushed toward the back of the queue and tried again later.
+ */
+final class PhabricatorWorkerYieldException extends Exception {
+
+  private $duration;
+
+  public function __construct($duration) {
+    $this->duration = $duration;
+    parent::__construct();
+  }
+
+  public function getDuration() {
+    return $this->duration;
+  }
+
+}
diff --git a/src/infrastructure/daemon/workers/storage/PhabricatorWorkerActiveTask.php b/src/infrastructure/daemon/workers/storage/PhabricatorWorkerActiveTask.php
--- a/src/infrastructure/daemon/workers/storage/PhabricatorWorkerActiveTask.php
+++ b/src/infrastructure/daemon/workers/storage/PhabricatorWorkerActiveTask.php
@@ -133,6 +133,16 @@
         PhabricatorWorkerArchiveTask::RESULT_FAILURE,
         0);
       $result->setExecutionException($ex);
+    } catch (PhabricatorWorkerYieldException $ex) {
+      $this->setExecutionException($ex);
+
+      $retry = $ex->getDuration();
+      $retry = max($retry, 5);
+
+      // NOTE: As a side effect, this saves the object.
+      $this->setLeaseDuration($retry);
+
+      $result = $this;
     } catch (Exception $ex) {
       $this->setExecutionException($ex);
       $this->setFailureCount($this->getFailureCount() + 1);