Don't subscribe bots implicitly when they act on objects, or when they are mentioned

Summary:
See PHI1098. When users comment on objects, they are automatically subscribed. And when `@alice` mentions `@bailey` on a task, that usually subscribes `@bailey`.

These rules make less sense if the user is a bot. There's generally no reason for a bot to automatically subscribe to objects it acts on (it's not going to read email and follow up later), and it can always subscribe itself pretty easily if it wants (since everything is `*.edit` now and supports subscribe transactions).

Also, don't subscribe bots when they're mentioned for similar reasons. If users really want to subscribe bots, they can do so explicitly.

These rules seem slightly like "bad implicit magic" since it's not immediately obvious why `@abc` subscribes that user but `@xyz` may not, but some of these rules are fairly complicated already (e.g., `@xyz` doesn't subscribe them if they unsubscribed or are implicitly subscribed) and this feels like it gets the right/desired result almost-always.

Test Plan:
On a fresh task:

  - Mentioned a bot in a comment with `@bot`.
    - Before patch: bot got CC'd.
    - After patch: no CC.
  - Called `maniphest.edit` via the API to add a comment as a bot.
    - Before patch: bot got CC'd.
    - After patch: no CC.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20284

src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php

@@ -1816,31 +1816,52 @@ abstract class PhabricatorApplicationTransactionEditor
       $users = mpull($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)
-        ) {
+        $user = idx($users, $phid);
+
+        // Don't subscribe invalid users.
+        if (!$user) {
           unset($phids[$key]);
-        } else {
-          if ($object->isAutomaticallySubscribed($phid)) {
+          continue;
+        }
+
+        // Don't subscribe bots that get mentioned. If users truly intend
+        // to subscribe them, they can add them explicitly, but it's generally
+        // not useful to subscribe bots to objects.
+        if ($user->getIsSystemAgent()) {
+          unset($phids[$key]);
+          continue;
+        }
+
+        // Do not subscribe mentioned users who do not have permission to see
+        // the object.
+        if ($object instanceof PhabricatorPolicyInterface) {
+          $can_view = PhabricatorPolicyFilter::hasCapability(
+            $user,
+            $object,
+            PhabricatorPolicyCapability::CAN_VIEW);
+          if (!$can_view) {
             unset($phids[$key]);
+            continue;
           }
         }
+
+        // Don't subscribe users who are already automatically subscribed.
+        if ($object->isAutomaticallySubscribed($phid)) {
+          unset($phids[$key]);
+          continue;
+        }
       }
+
       $phids = array_values($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));
+    $xaction = $object->getApplicationTransactionTemplate()
+      ->setTransactionType(PhabricatorTransactions::TYPE_SUBSCRIBERS)
+      ->setNewValue(array('+' => $phids));
 
     return $xaction;
   }
@@ -2876,6 +2897,24 @@ abstract class PhabricatorApplicationTransactionEditor
       }
     }
 
+    $actor = $this->getActor();
+
+    $user = id(new PhabricatorPeopleQuery())
+      ->setViewer($actor)
+      ->withPHIDs(array($actor_phid))
+      ->executeOne();
+    if (!$user) {
+      return $xactions;
+    }
+
+    // When a bot acts (usually via the API), don't automatically subscribe
+    // them as a side effect. They can always subscribe explicitly if they
+    // want, and bot subscriptions normally just clutter things up since bots
+    // usually do not read email.
+    if ($user->getIsSystemAgent()) {
+      return $xactions;
+    }
+
     $xaction = newv(get_class(head($xactions)), array());
     $xaction->setTransactionType(PhabricatorTransactions::TYPE_SUBSCRIBERS);
     $xaction->setNewValue(array('+' => array($actor_phid)));