patch: fix SSI finished list corruption

There's a corner case in the SSI cleanup code that isn't handled
correctly. It can arise when running workloads that are comprised
mostly (but not 100%) of READ ONLY transactions, and can corrupt the
finished SERIALIZABLEXACT list, potentially causing a segfault. The
attached patch fixes it.

Specifically, when the only remaining active transactions are READ
ONLY, we do a "partial cleanup" of committed transactions because
certain types of conflicts aren't possible anymore. For committed r/w
transactions, we release the SIREAD locks but keep the
SERIALIZABLEXACT. However, for committed r/o transactions, we can go
further and release the SERIALIZABLEXACT too. The problem was with the
latter case: we were returning the SERIALIZABLEXACT to the free list
without removing it from the finished list.

The only real change in the patch is the SHMQueueDelete line, but I
also reworked some of the surrounding code to make it obvious that r/o
and r/w transactions are handled differently -- the existing code felt
a bit too clever.

Dan

-- 
Dan R. K. Ports              MIT CSAIL                http://drkp.net/