core: Fix NPE race during hedging (1.53.x backport) #10047
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The problem was one hedge was committed before another had drained start(). This was not testable because HedgingRunnable checks whether scheduledHedgingRef is cancelled, which is racy, but there's no way to deterministically trigger either race. The same problem couldn't be triggered with retries because only one attempt will be draining at a time. Retries with cancellation also couldn't trigger it, for the surprising reason that the noop stream used in cancel() wasn't considered drained. This commit marks the noop stream as drained with cancel(), which allows memory to be garbage collected sooner and exposes the race for tests. That then showed the stream as hanging, because inFlightSubStreams wasn't being decremented. Fixes grpc#9185
YifeiZhuang
approved these changes
Apr 13, 2023
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The problem was one hedge was committed before another had drained start(). This was not testable because HedgingRunnable checks whether scheduledHedgingRef is cancelled, which is racy, but there's no way to deterministically trigger either race.
The same problem couldn't be triggered with retries because only one attempt will be draining at a time. Retries with cancellation also couldn't trigger it, for the surprising reason that the noop stream used in cancel() wasn't considered drained.
This commit marks the noop stream as drained with cancel(), which allows memory to be garbage collected sooner and exposes the race for tests. That then showed the stream as hanging, because inFlightSubStreams wasn't being decremented.
Fixes #9185
Backport of #10007