AU: Fix bug in impl of Johnson's circuit finding algorithm.

cycle_breaker.cc

 // Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "update_engine/cycle_breaker.h"
 #include <set>
 #include <utility>
 #include "update_engine/graph_utils.h"
 #include "update_engine/tarjan.h"
 #include "update_engine/utils.h"
@@ skipping 115 matching lines @@
         found = true;
     }
   }
 
   if (found) {
     Unblock(vertex);
   } else {
     for (Vertex::SubgraphEdgeMap::iterator w =
              subgraph_[vertex].subgraph_edges.begin();
          w != subgraph_[vertex].subgraph_edges.end(); ++w) {
-      subgraph_[*w].subgraph_edges.insert(vertex);
+      if (blocked_graph_[*w].out_edges.find(vertex) ==
+          blocked_graph_[*w].out_edges.end()) {
+        blocked_graph_[*w].out_edges.insert(make_pair(vertex,
+                                                      EdgeProperties()));
+      }
     }
   }
   CHECK_EQ(vertex, stack_.back());
   stack_.pop_back();
   return found;
 }
 
 }  // namespace chromeos_update_engine