[vm, gc] Require a safepoint for heap iteration.

runtime/vm/safepoint.cc

 // Copyright (c) 2016, 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/safepoint.h"
 
 #include "vm/thread.h"
 #include "vm/thread_registry.h"
 
 namespace dart {
 
 SafepointOperationScope::SafepointOperationScope(Thread* T) : StackResource(T) {
   ASSERT(T != NULL);
   Isolate* I = T->isolate();
   ASSERT(I != NULL);
-  ASSERT(T->no_safepoint_scope_depth() == 0);
 
   SafepointHandler* handler = I->safepoint_handler();
   ASSERT(handler != NULL);
 
   // Signal all threads to get to a safepoint and wait for them to
   // get to a safepoint.
   handler->SafepointThreads(T);
 }
 
 SafepointOperationScope::~SafepointOperationScope() {
@@ skipping 17 matching lines @@
 
 SafepointHandler::~SafepointHandler() {
   ASSERT(owner_ == NULL);
   ASSERT(safepoint_operation_count_ == 0);
   delete safepoint_lock_;
   safepoint_lock_ = NULL;
   isolate_ = NULL;
 }
 
 void SafepointHandler::SafepointThreads(Thread* T) {
+  ASSERT(T->no_safepoint_scope_depth() == 0);
+  ASSERT(T->execution_state() == Thread::kThreadInVM);
+
   {
     // First grab the threads list lock for this isolate
     // and check if a safepoint is already in progress. This
     // ensures that two threads do not start a safepoint operation
     // at the same time.
     MonitorLocker sl(threads_lock());
 
     // Now check to see if a safepoint operation is already in progress
     // for this isolate, block if an operation is in progress.
     while (SafepointInProgress()) {
@@ skipping 119 matching lines @@
     while (T->IsSafepointRequested()) {
       T->SetBlockedForSafepoint(true);
       tl.Wait();
       T->SetBlockedForSafepoint(false);
     }
     T->SetAtSafepoint(false);
   }
 }
 
 }  // namespace dart