Disable inline expunge when in computeIfAbsent#18635
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May 9, 2026
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Pull request overview
This PR mitigates a reported ConcurrentHashMap deadlock in the weak-key cache implementation by preventing inline expunction from running while a thread is executing a computeIfAbsent mapping function, aligning with the deadlock scenario described in issue #18232.
Changes:
- Wrap the
computeIfAbsentmapping function to track “in compute” state via a per-thread counter. - Make
expungeStaleEntries()a no-op when invoked from within acomputeIfAbsentmapping function to avoid cross-map removal attempts while CHM bins are locked.
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| public static void expungeStaleEntries() { | ||
| // Skip expunging stale entries if we are inside computeIfAbsent. This check prevents a deadlock | ||
| // when there are 2 threads calling computeIfAbsent, that locks the map, and both threads try | ||
| // to remove entry from the map locked by the other thread. | ||
| // See https://github.com/open-telemetry/opentelemetry-java-instrumentation/issues/18232 | ||
| if (isInComputeIfAbsent()) { | ||
| return; | ||
| } |
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Added a test, but it isn't 100% reliable at reproducing the original issue.
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Hopefully resolves #18232
An alternative for #18629
#18629 explores removing the inline expunge which is a bit complicated because library instrumentations don't currently use the background thread.