sass/dart-sass#2312 made embedded compiler shutdown upon ProtocolError significantly faster, which revealed a race condition in SyncMessagePort that caused flaky test failures: https://github.com/sass/dart-sass/actions/runs/10552748019/job/29232018679

During shutdown the worker would post a {"type":"exit","code":76} event. For most of the cases, the main thread is fast enough that it's already passed the check of Atomics.compareExchange(this.buffer, 0, BufferState.MessageSent, BufferState.AwaitingMessage) === BufferState.Closed, and already waiting at Atomics.wait(this.buffer, 0, BufferState.AwaitingMessage). In such case, the exit event is consumed correctly.

However, when the main thread is running slow, the port may have been closed before the check of Atomics.compareExchange(this.buffer, 0, BufferState.MessageSent, BufferState.AwaitingMessage) === BufferState.Closed, and by the time it runs the check, the error the SyncMessagePort's channel is closed is thrown, with the last exit event message not able to be consumed.