[GitHub] [arrow] rtpsw commented on pull request #34392: GH-34391: [C++] Future as-of-join-node hangs on distant times

["rtpsw (via GitHub)" <gi...@apache.org>]

rtpsw commented on PR #34392:
URL: https://github.com/apache/arrow/pull/34392#issuecomment-1501205568

   Coming back to this, I see [the same race condition](https://github.com/apache/arrow/pull/34392#issuecomment-1449560506) after merging main.
   
   One experiment I made is related to [this comment](https://github.com/apache/arrow/blob/311cc52d208034214c325983f416f2f847a2a015/cpp/src/arrow/acero/asof_join_node.cc#L186-L189):
   - When I add a lock over `mutex_` to `UnsyncFront`, I get the same results, including the race condition. This means that `UnsyncFront` is called safely per the comment, i.e., not called concurrently with `pop` or `try_pop`.
   - When  I add a lock over `mutex_` to `UnsyncSize`, I see a deadlock between one thread calling `AsofJoinNode::ProcessThread` -> `ConcurrentQueue::Pop and another calling `InputState::Push` -> `BackpressureConcurrentQueue::Push` -> `BackpressureConcurrentQueue::DoHandle` -> `ConcurrentQueue::UnsyncSize`. This isn't surprising, just shows that the comment does not apply to `UnsyncSize`.


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