core: event: prevent libevent timer socket deadlock - #445
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The libevent timer callback writes notifications from the event-loop thread. If its socket buffer fills, a blocking send waits for the same loop to drain the read side and deadlocks the loop. Make the write endpoint nonblocking and treat a would-block result as an already pending timer notification. Read errors from the write socket instead of the timer callback descriptor. Signed-off-by: zshuang0316 <zshuang0316@163.com>
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Summary
The libevent timeout callback writes timer notifications from the event-loop
thread. If unread notifications fill the socket buffer, the blocking
send()waits for that same event loop to drain the read endpoint and theloop deadlocks.
Make the timer write endpoint nonblocking and treat
EAGAIN/EWOULDBLOCKas an already-pending notification. Also retrieve send errorsfrom the actual write endpoint instead of the libevent timer callback fd,
which is
-1.The issue was observed in Fluent Bit on Windows. A Windows Performance
Recorder trace showed the engine thread blocked through:
cb_timeout() -> send() -> WSPSend() -> SockWaitForSingleObject()The same issue can affect any platform explicitly using Monkey's libevent
backend.
Related downstream PR: fluent/fluent-bit#12241
Testing
Built Monkey with the libevent backend on Windows and ran:
mk-test-event_timeout.exeResult:
SUCCESS: All unit tests have passed.