Synchronization needed on Schema.cfIdMap?

[Dan LaRocque]

Hi,

I think cfIdMap in config/Schema.java may be subject to unsynchronized 
access by distinct threads.

Say one thread adds a CF, maybe triggering resizes on cfIdMap's internal 
tables.  What guarantees that other threads calling 
Schema.instance.getId concurrent with CF addition see an 
internally-consistent cfIdMap?  HashBiMap is not threadsafe, and 
Schema's methods that touch cfIdMap have no explicit synchronization or 
locking, except for clear().  I think this scenario could lead to 
spurious and rare "Unknown table/cf" exceptions on reads/writes during 
unrelated schema migrations in 1.2 (reworded to "Unknown keyspace/cf" in 
2.0), which is how I got here in the first place.

I could be misreading the access pattern, maybe by missing external 
synchronization somewhere.  I brought this to the list instead of JIRA 
because I'm uncertain about the problem.  I'm hoping for a sanity check.

If this is actually a bug and not a misunderstanding, then a fix should 
be pretty straightforward.  Even though Maps.synchronizedBiMap could be 
deemed unacceptable for read throughput reasons, it should be possible 
to get decent reads by changing cfIdMap into a volatile reference to an 
unmodifiable bimap and guarding all modifications with a single write lock.

thanks,
Dan