Prevent Files from requiring infinite policy checks

Summary:
Fixes T6726. Currently, a file may be attached to itself (or to other files, ultimately forming a loop). In this case, we currently run around the loop forever trying to load all the files.

Instead, decline to load objects if we're inside a query which is already loading them. This produces the right policy result //and// completes in finite time.

Test Plan:
  - Looped two files by writing `{F123}` and `{F124}` on the other files, respectively.
  - Loaded `F123`.
  - Saw long hang; used `debug.time-limit` to see huge stack trace instead.
  - Wrote patch.
  - `F123` now loads correctly.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6726

Differential Revision: https://secure.phabricator.com/D12756

src/applications/phid/query/PhabricatorObjectQuery.php

@@ -104,6 +104,16 @@ final class PhabricatorObjectQuery
   }
 
   private function loadObjectsByPHID(array $types, array $phids) {
+    // Don't try to load PHIDs which are already "in flight"; this prevents us
+    // from recursing indefinitely if policy checks or edges form a loop. We
+    // will decline to load the corresponding objects.
+    $in_flight = $this->getPHIDsInFlight();
+    foreach ($phids as $key => $phid) {
+      if (isset($in_flight[$phid])) {
+        unset($phids[$key]);
+      }
+    }
+
     $results = array();
 
     $workspace = $this->getObjectsFromWorkspace($phids);
@@ -119,6 +129,8 @@ final class PhabricatorObjectQuery
       return $results;
     }
 
+    $this->putPHIDsInFlight($phids);
+
     $groups = array();
     foreach ($phids as $phid) {
       $type = phid_get_type($phid);