To be clear, it's for more than just if someone filed a task without setting a project: it could be used to check the validity of various task fields like Projects or Assignee or check for a status change and kick back the task to the originator (after adding a comment as well I guess?).

If the problem to implement this is the assignee autocomplete field, would it be possible to add a new action that simply says: "Assign back to originator"?

PS: If I wanted to take a pass at this, could you tell me which PHP file implements the possible actions for Maniphest based Herald rules?

This can be built as a custom Herald action (which will eventually be documented; see T5194). Drop this in src/extensions/ and then the "Reassign to author" action should be available in "Global" Maniphest Herald rules.

<?php

class KickBackAction extends HeraldCustomAction {

  public function appliesToAdapter(HeraldAdapter $adapter) {
    return ($adapter instanceof HeraldManiphestTaskAdapter);
  }

  public function appliesToRuleType($rule_type) {
    switch ($rule_type) {
      case HeraldRuleTypeConfig::RULE_TYPE_GLOBAL:
        return true;
      case HeraldRuleTypeConfig::RULE_TYPE_PERSONAL:
      case HeraldRuleTypeConfig::RULE_TYPE_OBJECT:
      default:
        return false;
    }
  }

  public function getActionKey() {
    return "swisspol.kick";
  }

  public function getActionName() {
    return pht('Reassign to author');
  }

  public function getActionType() {
    return HeraldAdapter::VALUE_NONE;
  }

  public function applyEffect(
    HeraldAdapter $adapter,
    $object,
    HeraldEffect $effect) {

    $task = $object;

    $xactions = array();

    $xactions[] = id(new ManiphestTransaction())
      ->setTransactionType(ManiphestTransaction::TYPE_OWNER)
      ->setNewValue($original_author);

    foreach ($xactions as $xaction) {
      $adapter->queueTransaction($xaction);
    }

    return new HeraldApplyTranscript(
      $effect,
      true,
      pht('Reassigned to original author'));
  }
}

We can probably do "Task Author" / "Current Actor" eventually too, but need T4100 for the UI.

This gets a bit messy when we have something like:

• We read a commit with Fixes T123.
• We don't recognize the author.
• We close T123, acting as the Diffusion application.
• We trigger a Herald rule with this new "assign to current actor" action.

I think we have to just fail the action in this case, but it makes things a bit complicated.

Very interesting, thanks much @epriestley.

I realize I'm still missing a piece: I thought you could do that in Herald but apparently not: adding a comment to a task when a rule matches. This would be needed to not just blindly re-assign the task to the author but also add some context.

◀ Merged tasks: T6030.

the above code snippet seems not working with latest phabricator version

See T8957.

You can follow the weekly changelogs to learn about API changes:

https://secure.phabricator.com/w/changelog/2015.30/
https://secure.phabricator.com/w/changelog/2015.31/

(We don't offer support for random code I wrote in a task somewhere, so no modern version of this action is available.)

Thanks @epriestley, I'll look at that and try to modify the extension code ~