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Wez Furlong
cd6b01dfe9
The queue logic assumes that if we get Some(duration) back from a throttle check that it must be a non-zero duration. We found a configuration that used a custom lua delivery handler together with a max_message_rate throttle. When running with local (non-redis) throttles it is possible that the in-memory throttle implementation can return a 0ns delay. This results in the queue subsystem choosing to requeue the message using the throttle duration as the delay, which results in the message being due immediately, which causes recursion into the insert-ready flow, which performs the same throttle check with the same 0ns result, and repeat until a stack overflow occurs on the spool in thread. The circumstances to trigger this are a bit niche and racy: if you turn up debug logging you can perturb the timing so that the stack overflow is not 100% repeatable. It is unlikely to cause a stack overflow on the inbound processing side because the threads doing that processing are multiplexing many other events that can also perturb the timing. This commit addresses this edge case in a very simple way: if the duration is 0, then we return `None` for the duration. This is actually how we handle this for one of the redis throttle implementations already. This commit makes the other two cases consistent with that.