At present, a SyncSocket always closes its handle. This is a problem for mojo, which always closes the handle it sends over IPC, leading to a double-close. There are also double-close issues in some other cases, like in a test when an IPC call with a handle is intercepted. For example, see MockAudioRendererHost https://cs.chromium.org/chromium/src/content/browser/renderer_host/media/audio_renderer_host_unittest.cc