Fix deferred load errors / bug #26482

runtime/lib/lib_prefix.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "dart:async";
 import "dart:isolate";
 
 // This type corresponds to the VM-internal class LibraryPrefix.
 class _LibraryPrefix {
   bool _load() native "LibraryPrefix_load";
@@ skipping 14 matching lines @@
     pair.add(this);
     pair.add(completer);
     _outstandingLoadRequests.add(pair);
     Timer.run(() {
       var hasCompleted = this._load();
       // Loading can complete immediately, for example when the same
       // library has been loaded eagerly or through another deferred
       // prefix. If that is the case, we must invalidate the dependent
       // code and complete the future now since there will be no callback
       // from the VM.
-      if (hasCompleted) {
+      if (hasCompleted && !completer.isCompleted) {
         _invalidateDependentCode();
         completer.complete(true);
         _outstandingLoadRequests.remove(pair);
       }
     });
     return completer.future;
   }
 }
 
 // A list of two element lists. The first element is the _LibraryPrefix. The
 // second element is the Completer for the load request.
 var _outstandingLoadRequests = new List<List>();
 
 // Called from the VM when an outstanding load request has finished.
 _completeDeferredLoads() {
   // Determine which outstanding load requests have completed and complete
   // their completer (with an error or true). For outstanding load requests
   // which have not completed, remember them for next time in
   // stillOutstandingLoadRequests.
   var stillOutstandingLoadRequests = new List<List>();
-  for (int i = 0; i < _outstandingLoadRequests.length; i++) {
-    var prefix = _outstandingLoadRequests[i][0];
-    if (prefix._load()) {
-      var completer = _outstandingLoadRequests[i][1];
-      var error = prefix._loadError();
-      if (error != null) {
-        completer.completeError(error);
-      } else {
-        prefix._invalidateDependentCode();
-        completer.complete(true);
-      }
+  var completedLoadRequests = new List<List>();
+
+  // Make a copy of the outstandingRequests because the call to _load below
+  // may recursively trigger another call to |_completeDeferredLoads|, which
+  // can cause |_outstandingLoadRequests| to be modified.
+  var outstandingRequests = _outstandingLoadRequests.toList();
+  for (int i = 0; i < outstandingRequests.length; i++) {
+    var prefix = outstandingRequests[i][0];
+    var completer = outstandingRequests[i][1];
+    var error = prefix._loadError();
+    if (completer.isCompleted) {
+      // Already completed. Skip.
+      continue;
+    }
+    if (error != null) {
+      completer.completeError(error);
+    } else if (prefix._load()) {
+      prefix._invalidateDependentCode();
+      completer.complete(true);
     } else {
-      stillOutstandingLoadRequests.add(_outstandingLoadRequests[i]);
+      stillOutstandingLoadRequests.add(outstandingRequests[i]);
     }
   }
   _outstandingLoadRequests = stillOutstandingLoadRequests;
 }