snapshot leak and core dump with serializable transactions

The following test flashes snapshot leak warning and subsequently dumps
core. Though this looks very similar to other bug report, this is a
different issue.

postgres=# BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE ;
BEGIN
postgres=# SAVEPOINT A;
SAVEPOINT
postgres=# SELECT count(*) from pg_class;
count
-------
227
(1 row)

postgres=# RELEASE SAVEPOINT A;
WARNING: Snapshot reference leak: Snapshot 0x9e3e4d4 still referenced
RELEASE
postgres=# SELECT count(*) from pg_class;
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
!>

I looked at this briefly and ISTM that there are couple of issues here:

1. Since "SAVEPOINT A" is the first statement in the transaction, a
subtransaction is started and CurrentResourceOwner is set to the resource
owner of the subtransaction. Later when serializable snapshot is taken, its
recorded in the subtransaction resource owner. Obviously, when the
subtransaction commits, it complains about the snapshot leak because the
serializable snapshot is not yet unregistered.

So I tried to ensure that the serializable snapshot is always recorded in
the TopTransactionResourceOwner. It solved the above issue, but there is
still a core dump when the top transaction is committed. That leads to the
second issue.

2. In CommitTransaction(), I think we should call AtEOXact_Snapshot *before*
releasing the resource owners. Otherwise, ResourceOwnerReleaseInternal
complains about snapshot leak and then forcefully unregisters the snapshot.
Later when AtEOXact_Snapshot is called, it again tries to unregister the
serializable snapshot and assertion fails.

The attached patch fixes these issues.

Thanks,
Pavan

--
Pavan Deolasee
EnterpriseDB http://www.enterprisedb.com