pitrou commented on code in PR #41876:
URL: https://github.com/apache/arrow/pull/41876#discussion_r1633301388
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cpp/src/arrow/filesystem/s3fs.cc:
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@@ -1892,6 +1896,9 @@ class ObjectOutputStream final : public io::OutputStream {
}
// Notify completion
if (--state->parts_in_progress == 0) {
+ // GH-41862: avoid potential deadlock if the Future's callback is called
+ // with the mutex taken.
+ lock.unlock();
state->pending_parts_completed.MarkFinished(state->status);
Review Comment:
Um, you're right. @mapleFU's suggestion looks ok to me.
Note that the `pending_parts_completed` future can only be waited on in two
situations:
* the user called blocking `Close` or `Flush` and the future is waited upon
before returning from the API call;
* the user called non-blocking `CloseAsync`, which returns a cascaded future
obtained by chaining `pending_parts_completed.Then` with a continuation.
For `Close` and `CloseAsync`, it's certainly not ok to call `Write` from
another thread concurrently.
For `Flush`, it should be ok to call `Write` concurrently, but the `Flush`
does not have to wait for the completion of the concurrent `Write` call.
Moreover, more generally, it doesn't seem sound to write to an output
_stream_ (rather than random-access file) from several thread concurrently.
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