Make query engines "overheat" instead of stalling when filtering too many results

Summary: Ref T11773. This is an initial first step toward a more complete solution, but should make the worst case much less bad: prior to this change, the worst case was "30 second exeuction timeout". After this patch, the worst case is "no results + explanatory message", which is strictly better.

Test Plan:
Made all feed stories fail policy checks, loaded home page.

  - Before adding overheating: 9,600 queries / 20 seconds
  - After adding overheating: 376 queries / 800ms

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11773

Differential Revision: https://secure.phabricator.com/D16735

src/applications/search/controller/PhabricatorApplicationSearchController.php

@@ -238,6 +238,8 @@ final class PhabricatorApplicationSearchController
           $nux_view = null;
         }
 
+        $is_overheated = $query->getIsOverheated();
+
         if ($nux_view) {
           $box->appendChild($nux_view);
         } else {
@@ -265,6 +267,10 @@ final class PhabricatorApplicationSearchController
             $box->appendChild($list->getContent());
           }
 
+          if ($is_overheated) {
+            $box->appendChild($this->newOverheatedView($objects));
+          }
+
           $result_header = $list->getHeader();
           if ($result_header) {
             $box->setHeader($result_header);
@@ -525,4 +531,27 @@ final class PhabricatorApplicationSearchController
       ->setDropdownMenu($action_list);
   }
 
+  private function newOverheatedView(array $results) {
+    if ($results) {
+      $message = pht(
+        'Most objects matching your query are not visible to you, so '.
+        'filtering results is taking a long time. Only some results are '.
+        'shown. Refine your query to find results more quickly.');
+    } else {
+      $message = pht(
+        'Most objects matching your query are not visible to you, so '.
+        'filtering results is taking a long time. Refine your query to '.
+        'find results more quickly.');
+    }
+
+    return id(new PHUIInfoView())
+      ->setSeverity(PHUIInfoView::SEVERITY_WARNING)
+      ->setFlush(true)
+      ->setTitle(pht('Query Overheated'))
+      ->setErrors(
+        array(
+          $message,
+        ));
+  }
+
 }