Avoid memory leak if pgstat_vacuum_stat is interrupted partway through.

The temporary hash tables made by pgstat_collect_oids should be allocated
in a short-term memory context, which is not the default behavior of
hash_create.  Noted while looking through hash_create calls in connection
with Robert Haas' recent complaint.

This is a pre-existing bug, but it doesn't seem important enough to
back-patch.  The hash table is not so large that it would matter unless this
happened many times within a session, which seems quite unlikely.

src/backend/postmaster/pgstat.c

@@ -13,7 +13,7 @@
  *
  *	Copyright (c) 2001-2009, PostgreSQL Global Development Group
  *
- *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.193 2009/11/28 23:38:07 tgl Exp $
+ *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.194 2009/12/27 19:40:07 tgl Exp $
  * ----------
  */
 #include "postgres.h"
@@ -1033,7 +1033,8 @@ pgstat_vacuum_stat(void)
  *
  *	Collect the OIDs of all objects listed in the specified system catalog
  *	into a temporary hash table.  Caller should hash_destroy the result
- *	when done with it.
+ *	when done with it.  (However, we make the table in CurrentMemoryContext
+ *	so that it will be freed properly in event of an error.)
  * ----------
  */
 static HTAB *
@@ -1049,10 +1050,11 @@ pgstat_collect_oids(Oid catalogid)
 	hash_ctl.keysize = sizeof(Oid);
 	hash_ctl.entrysize = sizeof(Oid);
 	hash_ctl.hash = oid_hash;
+	hash_ctl.hcxt = CurrentMemoryContext;
 	htab = hash_create("Temporary table of OIDs",
 					   PGSTAT_TAB_HASH_SIZE,
 					   &hash_ctl,
-					   HASH_ELEM | HASH_FUNCTION);
+					   HASH_ELEM | HASH_FUNCTION | HASH_CONTEXT);
 
 	rel = heap_open(catalogid, AccessShareLock);
 	scan = heap_beginscan(rel, SnapshotNow, 0, NULL);