Avoid race in isolate shutdown; add assertions, error messages

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index b247e566b8934220435819e1734e2fa7738b14c7..eb79659850c0e8c3d6d3c18e01e0530acf4e7079 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1432,6 +1432,14 @@ void Isolate::Shutdown() {
     heap_->Verify(this == Dart::vm_isolate() ? kRequireMarked : kForbidMarked);
   }
 #endif  // DEBUG
+  if (heap_ != NULL) {
+    // Wait for any concurrent GC tasks to finish before shutting down.
+    PageSpace* old_space = heap_->old_space();
+    MonitorLocker ml(old_space->tasks_lock());
+    while (old_space->tasks() > 0) {
+      ml.Wait();
+    }
+  }

[siva, 2015/07/13 23:37:00]

I am wondering if we should do this wait after removing the isolate from the
list of isolates.

[koda, 2015/07/14 00:41:40]

Done.

 
   // Remove this isolate from the list *before* we start tearing it down, to
   // avoid exposing it in a state of decay.
@@ -1490,6 +1498,8 @@ void Isolate::Shutdown() {
   // TODO(5411455): For now just make sure there are no current isolates
   // as we are shutting down the isolate.
   Thread::ExitIsolate();
+  // All threads should have exited by now.
+  thread_registry()->CheckNotScheduled(this);
   Profiler::ShutdownProfilingForIsolate(this);
 }