For now, hard limit task graph at 100 nodes

Summary:
Ref T4788. One install has some particularly impressive task graphs which are thousands of nodes large.

The current graph is pretty broken in these cases. For now, just render a "too big to show" message. In the future, I'd plan to finesse this (e.g., show parents/children, show links to parents/children, etc).

Test Plan:
  - Viewed a normal task.
  - Set limit to 3, viewed a task with graph size 6, saw an error message.
  - Viewed a revision stack graph (unaffected).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4788

Differential Revision: https://secure.phabricator.com/D16295

src/applications/maniphest/controller/ManiphestTaskDetailController.php

@@ -87,12 +87,25 @@ final class ManiphestTaskDetailController extends ManiphestController {
       ->addPropertySection(pht('Description'), $description)
       ->addPropertySection(pht('Details'), $details);
 
+    $graph_limit = 100;
     $task_graph = id(new ManiphestTaskGraph())
       ->setViewer($viewer)
       ->setSeedPHID($task->getPHID())
+      ->setLimit($graph_limit)
       ->loadGraph();
     if (!$task_graph->isEmpty()) {
-      $graph_table = $task_graph->newGraphTable();
+      if ($task_graph->isOverLimit()) {
+        $message = pht(
+          'Task graph too large to display (this task is connected to '.
+          'more than %s other tasks).',
+          $graph_limit);
+        $message = phutil_tag('em', array(), $message);
+        $graph_table = id(new PHUIPropertyListView())
+          ->addTextContent($message);
+      } else {
+        $graph_table = $task_graph->newGraphTable();
+      }
+
       $view->addPropertySection(pht('Task Graph'), $graph_table);
     }
 

src/infrastructure/graph/PhabricatorObjectGraph.php

@@ -9,6 +9,7 @@ abstract class PhabricatorObjectGraph
   private $seedPHID;
   private $objects;
   private $loadEntireGraph = false;
+  private $limit;
 
   public function setViewer(PhabricatorUser $viewer) {
     $this->viewer = $viewer;
@@ -23,6 +24,15 @@ abstract class PhabricatorObjectGraph
     return $this->viewer;
   }
 
+  public function setLimit($limit) {
+    $this->limit = $limit;
+    return $this;
+  }
+
+  public function getLimit() {
+    return $this->limit;
+  }
+
   abstract protected function getEdgeTypes();
   abstract protected function getParentEdgeType();
   abstract protected function newQuery();
@@ -44,6 +54,16 @@ abstract class PhabricatorObjectGraph
     return (count($this->getNodes()) <= 2);
   }
 
+  final public function isOverLimit() {
+    $limit = $this->getLimit();
+
+    if (!$limit) {
+      return false;
+    }
+
+    return (count($this->edgeReach) > $limit);
+  }
+
   final public function getEdges($type) {
     $edges = idx($this->edges, $type, array());
 
@@ -72,6 +92,10 @@ abstract class PhabricatorObjectGraph
   }
 
   final protected function loadEdges(array $nodes) {
+    if ($this->isOverLimit()) {
+      return array_fill_keys($nodes, array());
+    }
+
     $edge_types = $this->getEdgeTypes();
 
     $query = id(new PhabricatorEdgeQuery())