Work around race condition in MakeArray for ASSERTs and document assumptions.

runtime/vm/object.cc

 // Copyright (c) 2012, 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.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 19161 matching lines @@
 
   // Update the size in the header field and length of the array object.
   uword tags = array.raw_ptr()->tags_;
   ASSERT(kArrayCid == RawObject::ClassIdTag::decode(tags));
   uword old_tags;
   do {
     old_tags = tags;
     uword new_tags = RawObject::SizeTag::update(used_size, old_tags);
     tags = array.CompareAndSwapTags(old_tags, new_tags);
   } while (tags != old_tags);
+  // TODO(22501): For the heap to remain walkable by the sweeper, it must
+  // observe the creation of the filler object no later than the new length
+  // of the array. This assumption holds on ia32/x64 or if the CAS above is a
+  // full memory barrier.
+  //
+  // Also, between the CAS of the header above and the SetLength below,
+  // the array is temporarily in an inconsistent state. The header is considered
+  // the overriding source of object size by RawObject::Size, but the ASSERTs
+  // in RawObject::SizeFromClass must handle this special case.
   array.SetLength(used_len);
 
-  // Null the GrowableObjectArray, we are removing it's backing array.
+  // Null the GrowableObjectArray, we are removing its backing array.
   growable_array.SetLength(0);
   growable_array.SetData(Object::empty_array());
 
   return array.raw();
 }
 
 
 bool Array::CheckAndCanonicalizeFields(const char** error_str) const {
   Object& obj = Object::Handle();
   // Iterate over all elements, canonicalize numbers and strings, expect all
@@ skipping 1420 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart