Snapshotting and sync could cause a dead lock TranslogWriter #18481
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#18360 introduced an extra lock in order to allow writes while syncing the translog. This caused a potential deadlock (see [1]) with snapshotting code where we first acquire the instance lock, followed by a sync (which acquires the syncLock). However, the sync logic acquires the syncLock first, followed by the instance lock.
I considered solving this by not syncing the translog on snapshot - I think we can get away with just flushing it. That however will create subtleties around snapshoting and whether operations in them are persisted. I opted instead to have slightly uglier code with nest synchronized, where the scope of the change is contained to the TranslogWriter class alone. If people feel differently, I can take the other approach.
[1]