Publisher#flatMapConcatIterable may skip emitting items #3108
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Scottmitch
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Motivation: Publisher#flatMapConcatIterable may not emit some items due to race conditions and visibility issues. The iterator state is written to outside the scope of holding the lock. After a drain loop completes we may request 1 more iterator. However it is possible the thread emitting holds the lock while another thread invokes onNext(t). The emitting thread may not see the iterator, and instead see `EmptyIterator.instance()` and cause it to request 1 more item, but then the not-visible iterator contents won't be emitted. Modifications: - Make FlatMapIterableSubscriber iterator state volatile and atomically update it. There is only ever 1 valid iterator because only 1 outstanding demand is issued only after the current iterator `!hasNext()`. The iterator state is re-read on each drain loop, and the terminal condition must atomically set to EmptyIterator.
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lgtm as well. Nice find. |
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How did we find this? Was it reported by a user? |
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@mgodave yes, and that user provided reproducer |
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Motivation:
Publisher#flatMapConcatIterable may not emit some items due to race conditions and visibility issues. The iterator state is written to outside the scope of holding the lock. After a drain loop completes we may request 1 more iterator. However it is possible the thread emitting holds the lock while another thread invokes onNext(t). The emitting thread may not see the iterator, and instead see
EmptyIterator.instance()and cause it to request 1 more item, but then the not-visible iterator contents won't be emitted.Modifications:
!hasNext(). The iterator state is re-read on each drain loop, and the terminal condition must atomically set to EmptyIterator.