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

runtime/vm/object_graph_test.cc

 // 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.
 
 #include "vm/object_graph.h"
 #include "platform/assert.h"
 #include "vm/unit_test.h"
 
 namespace dart {
 
@@ skipping 39 matching lines @@
   Array& d = Array::Handle(Array::New(0, Heap::kOld));
   a.SetAt(10, b);
   b.SetAt(0, c);
   b.SetAt(1, d);
   a.SetAt(11, d);
   intptr_t a_size = a.raw()->Size();
   intptr_t b_size = b.raw()->Size();
   intptr_t c_size = c.raw()->Size();
   intptr_t d_size = d.raw()->Size();
   {
-    // No more allocation; raw pointers ahead.
-    NoSafepointScope no_safepoint_scope;
     RawObject* b_raw = b.raw();

[danunez, 2017/08/05 17:24:11]

I think we can remove the NoSafepoint here because our mutator thread is not
going to be allocating and we shouldn't have a background thread doing any real
work in this test.

Am I correct in that?

If there is a background thread doing allocation in this test, we could see this
test come up later when background threads start allocating alongside mutators.

[rmacnak, 2017/08/07 22:37:51]

Nevermind, this is unsafe in the unlikely case that the test harness has the
background compiler running. Replaced with a SafepointOperationScope.

     // Clear handles to cut unintended retained paths.
     b = Array::null();
     c = Array::null();
     d = Array::null();
     ObjectGraph graph(thread);
     {
       HeapIterationScope iteration_scope(true);
       {
         // Compare count and size when 'b' is/isn't skipped.
         CounterVisitor with(Object::null(), Object::null());
@@ skipping 59 matching lines @@
       Array& expected_a = Array::Handle();
       expected_a ^= path.At(4);
       EXPECT(expected_c.raw() == c.raw());
       EXPECT(expected_b.raw() == a.At(10));
       EXPECT(expected_a.raw() == a.raw());
     }
   }
 }
 
 }  // namespace dart