Fix race condition with HasMutatorThread that was being used without a lock.

runtime/vm/isolate.cc

 // Copyright (c) 2013, 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.
 
 #include "vm/isolate.h"
 
 #include "include/dart_api.h"
 #include "include/dart_native_api.h"
 #include "platform/assert.h"
 #include "platform/text_buffer.h"
@@ skipping 971 matching lines @@
     result->LowLevelShutdown();
     Thread::ExitIsolate();
     delete result;
     return NULL;
   }
 
   return result;
 }
 
 
+Thread* Isolate::mutator_thread() const {
+  ASSERT(thread_registry() != NULL);
+  return thread_registry()->mutator_thread();
+}
+
+
 void Isolate::SetupInstructionsSnapshotPage(
     const uint8_t* instructions_snapshot_buffer) {
   InstructionsSnapshot snapshot(instructions_snapshot_buffer);
 #if defined(DEBUG)
   if (FLAG_trace_isolates) {
     OS::Print("Precompiled instructions are at [0x%" Px ", 0x%" Px ")\n",
               reinterpret_cast<uword>(snapshot.instructions_start()),
               reinterpret_cast<uword>(snapshot.instructions_start()) +
               snapshot.instructions_size());
   }
@@ skipping 1576 matching lines @@
   Thread* thread = NULL;
   OSThread* os_thread = OSThread::Current();
   if (os_thread != NULL) {
     // We are about to associate the thread with an isolate and it would
     // not be possible to correctly track no_safepoint_scope_depth for the
     // thread in the constructor/destructor of MonitorLocker,
     // so we create a MonitorLocker object which does not do any
     // no_safepoint_scope_depth increments/decrements.
     MonitorLocker ml(threads_lock(), false);
 
+    // Check to make sure we don't already have a mutator thread.
+    if (is_mutator && mutator_thread_ != NULL) {
+      return NULL;
+    }
+
     // If a safepoint operation is in progress wait for it
     // to finish before scheduling this thread in.
     while (!bypass_safepoint && safepoint_handler()->SafepointInProgress()) {
       ml.Wait();
     }
 
     // Now get a free Thread structure.
     thread = thread_registry()->GetFreeThreadLocked(this, is_mutator);
     ASSERT(thread != NULL);
 
@@ skipping 293 matching lines @@
 void IsolateSpawnState::DecrementSpawnCount() {
   ASSERT(spawn_count_monitor_ != NULL);
   ASSERT(spawn_count_ != NULL);
   MonitorLocker ml(spawn_count_monitor_);
   ASSERT(*spawn_count_ > 0);
   *spawn_count_ = *spawn_count_ - 1;
   ml.Notify();
 }
 
 }  // namespace dart