Bug: Race in BatchCommitLogService in 4.1+
Status: Patch Available
Priority: Normal
Created: 16/Dec/24 17:58

Saw this crash in production on multiple hosts:
{code:java}
ERROR [MutationStage-165] 2024-12-16 04:36:35,914 JVMStabilityInspector.java:68 - Exception in thread Thread[MutationStage-165,5,SharedPool]
java.lang.RuntimeException: java.lang.Error: Maximum permit count exceeded
        at org.apache.cassandra.net.InboundSink.accept(InboundSink.java:108)
        at org.apache.cassandra.net.InboundSink.accept(InboundSink.java:45)
        at org.apache.cassandra.net.InboundMessageHandler$ProcessMessage.run(InboundMessageHandler.java:430)
        at org.apache.cassandra.concurrent.ExecutionFailure$1.run(ExecutionFailure.java:133)
        at org.apache.cassandra.concurrent.SEPWorker.run(SEPWorker.java:142)
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
        at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: java.lang.Error: Maximum permit count exceeded
        at java.base/java.util.concurrent.Semaphore$Sync.tryReleaseShared(Semaphore.java:198)
        at java.base/java.util.concurrent.locks.AbstractQueuedSynchronizer.releaseShared(AbstractQueuedSynchronizer.java:1382)
        at java.base/java.util.concurrent.Semaphore.release(Semaphore.java:619)
        at org.apache.cassandra.db.commitlog.AbstractCommitLogService.requestExtraSync(AbstractCommitLogService.java:297)
        at org.apache.cassandra.db.commitlog.BatchCommitLogService.maybeWaitForSync(BatchCommitLogService.java:40)
        at org.apache.cassandra.db.commitlog.AbstractCommitLogService.finishWriteFor(AbstractCommitLogService.java:284)
        at org.apache.cassandra.db.commitlog.CommitLog.add(CommitLog.java:330)
        at org.apache.cassandra.db.CassandraKeyspaceWriteHandler.addToCommitLog(CassandraKeyspaceWriteHandler.java:100)
        at org.apache.cassandra.db.CassandraKeyspaceWriteHandler.beginWrite(CassandraKeyspaceWriteHandler.java:54)
        at org.apache.cassandra.db.Keyspace.applyInternal(Keyspace.java:641)
        at org.apache.cassandra.db.Keyspace.applyFuture(Keyspace.java:489)
        at org.apache.cassandra.db.Mutation.applyFuture(Mutation.java:223)
        at org.apache.cassandra.db.MutationVerbHandler.doVerb(MutationVerbHandler.java:63)
        at org.apache.cassandra.net.InboundSink.lambda$new$0(InboundSink.java:78)
        at org.apache.cassandra.net.InboundSink.accept(InboundSink.java:97)
        ... 6 common frames omitted {code}
Digging through, it looks like in 4.1.0 {{AbstractCommitLogService}} was reworked to use Semaphore instead of LockSupport.  From a skim of the docs, {{LockSupport.unpark()}} is safe to run even if the permit's already available where {{Semaphore.release()}} is not.

I think that means that if more writes arrive during the right part of a flush, it double-decrements `haveWork`.  Once it's decremented and starts throwing, I think it never successfully makes it to another acquire/try

The least-invasive fix is probably to call `haveWork.drainPermits()` in `requestExtraSync()` right before `release(1)` to ensure that `havework.permits` is 0 right before we increment it.  That may technically still leave a race, but an almost-impossibly small one rather than a nontrivial one.  It might also be possible to catch that specific Exception and run acquire() or (probably better) drainPermits() and recover.  That does feel like a weird thing to do for a semaphore exception, but should be OK in this case since it's just being used to sleep the worker loop instead of actual concurrency control.