Allow edge query filtering by destination PHIDs

Summary: See title. Adds features needed for D3136.

Test Plan:
Observe sanity (or run D3136 in a sandbox
and observe that voting works).

Reviewers: epriestley

Reviewed By: epriestley

CC: gmarcotte, aran, Korvin

Differential Revision: https://secure.phabricator.com/D3205

src/infrastructure/edges/query/PhabricatorEdgeQuery.php

@@ -38,6 +38,7 @@
 final class PhabricatorEdgeQuery extends PhabricatorQuery {
 
   private $sourcePHIDs;
+  private $destPHIDs;
   private $edgeTypes;
   private $resultSet;
 
@@ -62,6 +63,19 @@ final class PhabricatorEdgeQuery extends PhabricatorQuery {
   }
 
 
+  /**
+   * Find edges terminating at one or more destination PHIDs.
+   *
+   * @param list List of destination PHIDs.
+   * @return this
+   *
+   */
+  public function withDestinationPHIDs(array $dest_phids) {
+    $this->destPHIDs = $dest_phids;
+    return $this;
+  }
+
+
   /**
    * Find edges of specific types.
    *
@@ -110,6 +124,31 @@ final class PhabricatorEdgeQuery extends PhabricatorQuery {
     return array_keys($edges[$src_phid][$edge_type]);
   }
 
+  /**
+   * Convenience method for loading a single edge's metadata for
+   * a given source, destination, and edge type. Returns null
+   * if the edge does not exist or does not have metadata. Builds
+   * and immediately executes a full query.
+   *
+   * @param phid  Source PHID.
+   * @param const Edge type.
+   * @param phid  Destination PHID.
+   * @return wild Edge annotation (or null).
+   */
+  public static function loadSingleEdgeData($src_phid, $edge_type, $dest_phid) {
+    $edges = id(new PhabricatorEdgeQuery())
+      ->withSourcePHIDs(array($src_phid))
+      ->withEdgeTypes(array($edge_type))
+      ->withDestinationPHIDs(array($dest_phid))
+      ->needEdgeData(true)
+      ->execute();
+
+    if (isset($edges[$src_phid][$edge_type][$dest_phid]['data'])) {
+      return $edges[$src_phid][$edge_type][$dest_phid]['data'];
+    }
+    return null;
+  }
+
 
   /**
    * Load specified edges.
@@ -262,6 +301,14 @@ final class PhabricatorEdgeQuery extends PhabricatorQuery {
         $this->edgeTypes);
     }
 
+    if ($this->destPHIDs) {
+      // potentially complain if $this->edgeType was not set
+      $where[] = qsprintf(
+        $conn_r,
+        'edge.dst IN (%Ls)',
+        $this->destPHIDs);
+    }
+
     return $this->formatWhereClause($where);
   }