Transactions deadlock

com.couchbase.transactions.threadsafety.ThreadSafetyKVGoldenPathTest.replaceRegularNumDocuments{boolean}[2]  (queryMode = false, 1.2.4) timed out due to a deadlock.  AFAIK this is the first time it's been seen.

Log is here: https://gist.github.com/programmatix/88f8ca7c130ae6e1bce3794073f12338

AFAICT, operation 32716 gets the lock.  It should now be adding itself to the KV wait group, under a synchronized (this) block.  But this line doesn't get logged, indicating that either we're blocked waiting for that synchronized block, or somehow this operation just died without unlocking.

On a static investigation the cause is unclear.  There's nothing blocking inside any of the synchronized blocks.  I can't see anything that could cause the operation to die.