Fix back-pressure in Netty frame handler #1681
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A race condition can cause to see the channel as non-writable but end up with a new count down latch, not the one that was present when the writability flag was true. So the code waits on this new latch which will be never counted down. This can happen when the writability flips from false to true very fast. This commit makes sure to get the latch first, then check the writability a second time. In case we get an "old" latch, it is counted down automatically, so the enqueuing code will not be blocked.
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A race condition can cause to see the channel as non-writable but end up with a new count down latch, not the one that was present when the writability flag was true. So the code waits on this new latch which will be never counted down. This can happen when the writability flips from false to true very fast.
This commit makes sure to get the latch first, then check the writability a second time. In case we get an "old" latch, it is counted down automatically, so the enqueuing code will not be blocked.