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

runtime/vm/raw_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/raw_object.h"
 
 #include "vm/class_table.h"
 #include "vm/dart.h"
 #include "vm/freelist.h"
 #include "vm/isolate.h"
@@ skipping 198 matching lines @@
       }
       default:
         UNREACHABLE();
         break;
     }
   }
   ASSERT(instance_size != 0);
 #if defined(DEBUG)
   uword tags = ptr()->tags_;
   intptr_t tags_size = SizeTag::decode(tags);
+  if ((class_id == kArrayCid) && (instance_size > tags_size && tags_size > 0)) {
+    // TODO(22501): Array::MakeArray could be in the process of shrinking
+    // the array (see comment therein), having already updated the tags but not
+    // yet set the new length. Wait a millisecond and try again.
+    int retries_remaining = 1000;  // ... but not forever.
+    do {
+      OS::Sleep(1);
+      const RawArray* raw_array = reinterpret_cast<const RawArray*>(this);
+      intptr_t array_length = Smi::Value(raw_array->ptr()->length_);
+      instance_size = Array::InstanceSize(array_length);
+    } while ((instance_size > tags_size) && (--retries_remaining > 0));
+  }
   if ((instance_size != tags_size) && (tags_size != 0)) {
     FATAL3("Size mismatch: %" Pd " from class vs %" Pd " from tags %" Px "\n",
            instance_size, tags_size, tags);
   }
 #endif  // DEBUG
   return instance_size;
 }
 
 
 #if defined(DEBUG)
@@ skipping 695 matching lines @@
 intptr_t RawUserTag::VisitUserTagPointers(
     RawUserTag* raw_obj, ObjectPointerVisitor* visitor) {
   // Make sure that we got here with the tagged pointer as this.
   ASSERT(raw_obj->IsHeapObject());
   visitor->VisitPointers(raw_obj->from(), raw_obj->to());
   return UserTag::InstanceSize();
 }
 
 
 }  // namespace dart