void RenderThreadImpl::RegisterPendingRenderFrameConnect(
    int routing_id,
    mojo::ScopedMessagePipeHandle handle) {
  std::pair<std::map<int, mojo::MessagePipeHandle>::iterator, bool> result =
      pending_render_frame_connects_.insert(
          std::make_pair(routing_id, handle.release()));
  CHECK(result.second) << "Inserting a duplicate item.";
}

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.