v3: fix dependency checkpoint race #1171
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This affects
Task.triggerAndWait()andTask.batchTriggerAndWait(). A race condition could result where the dependency completes just before the checkpoint does.If the timing is just right, the created checkpoint causes the resume message from dependency completion to be ACKed. This means the parent task stays stuck in the frozen state. Another issues is that even if the resume comes through, there's won't be a running task to send it to, as checkpoint completion also results in parent task termination.
The fix involves checking for dependency completion after creating the checkpoint event, and only ACKing if the dependency hasn't completed yet. Also, if the dependency has completed, we will keep the parent task alive so it can resume immediately.