Fix asserts in StackFrameIterator which were effectively disabled

runtime/vm/thread.cc

 // Copyright (c) 2015, 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/thread.h"
 
 #include "vm/compiler_stats.h"
 #include "vm/dart_api_state.h"
 #include "vm/growable_array.h"
 #include "vm/isolate.h"
@@ skipping 691 matching lines @@
   visitor->VisitPointer(reinterpret_cast<RawObject**>(&sticky_error_));
   visitor->VisitPointer(reinterpret_cast<RawObject**>(&async_stack_trace_));
 
   // Visit the api local scope as it has all the api local handles.
   ApiLocalScope* scope = api_top_scope_;
   while (scope != NULL) {
     scope->local_handles()->VisitObjectPointers(visitor);
     scope = scope->previous();
   }
 
+  // The MarkTask, which calls this method, can run on a different thread.  We
+  // therefore assume the mutator is at a safepoint and we can iterate it's
+  // stack.
+  // TODO(vm-team): It would be beneficial to be able to ask the mutator thread
+  // whether it is in fact blocked at the moment (at a "safepoint") so we can
+  // safely iterate it's stack.
+  //
+  // Unfortunately we cannot use `this->IsAtSafepoint()` here because that will
+  // return `false` even though the mutator thread is waiting for mark tasks
+  // (which iterate it's stack) to finish.
+  const StackFrameIterator::CrossThreadPolicy cross_thread_policy =
+      StackFrameIterator::kAllowCrossThreadIteration;
+
+  const StackFrameIterator::ValidationPolicy validation_policy =
+      validate_frames ? StackFrameIterator::kValidateFrames
+                      : StackFrameIterator::kDontValidateFrames;
+
   // Iterate over all the stack frames and visit objects on the stack.
-  StackFrameIterator frames_iterator(top_exit_frame_info(), validate_frames);
+  StackFrameIterator frames_iterator(top_exit_frame_info(), validation_policy,
+                                     this, cross_thread_policy);
   StackFrame* frame = frames_iterator.NextFrame();
   while (frame != NULL) {
     frame->VisitObjectPointers(visitor);
     frame = frames_iterator.NextFrame();
   }
 }
 
 
 bool Thread::CanLoadFromThread(const Object& object) {
 #define CHECK_OBJECT(type_name, member_name, expr, default_init_value)         \
@@ skipping 181 matching lines @@
 
 DisableThreadInterruptsScope::~DisableThreadInterruptsScope() {
   if (thread() != NULL) {
     OSThread* os_thread = thread()->os_thread();
     ASSERT(os_thread != NULL);
     os_thread->EnableThreadInterrupts();
   }
 }
 
 }  // namespace dart