Remove AtEOXact_CatCache().

tglsfdc · tglsfdc · commit bb11ff2bc8af · 2017-08-13T16:15:14.000-04:00
The sole useful effect of this function, to check that no catcache entries have positive refcounts at transaction end, has really been obsolete since we introduced ResourceOwners in PG 8.1. We reduced the checks to assertions years ago, so that the function was a complete no-op in production builds. There have been previous discussions about removing it entirely, but consensus up to now was that it had some small value as a cross-check for bugs in the ResourceOwner logic. However, it now emerges that it's possible to trigger these assertions if you hit an assert-enabled backend with SIGTERM during a call to SearchCatCacheList, because that function temporarily increases the refcounts of entries it's intending to add to a catcache list construct. In a normal ERROR scenario, the extra refcounts are cleaned up by SearchCatCacheList's PG_CATCH block; but in a FATAL exit we do a transaction abort and exit without ever executing PG_CATCH handlers. There's a case to be made that this is a generic hazard and we should consider restructuring elog(FATAL) handling so that pending PG_CATCH handlers do get run. That's pretty scary though: it could easily create more problems than it solves. Preliminary stress testing by Andreas Seltenreich suggests that there are not many live problems of this ilk, so we rejected that idea. There are more-localized ways to fix the problem; the most principled one would be to use PG_ENSURE_ERROR_CLEANUP instead of plain PG_TRY. But adding cycles to SearchCatCacheList isn't very appealing. We could also weaken the assertions in AtEOXact_CatCache in some more or less ad-hoc way, but that just makes its raison d'etre even less compelling. In the end, the most reasonable solution seems to be to just remove AtEOXact_CatCache altogether, on the grounds that it's not worth trying to fix it. It hasn't found any bugs for us in many years. Per report from Jeevan Chalke. Back-patch to all supported branches. Discussion: https://postgr.es/m/CAM2+6=VEE30YtRQCZX7_sCFsEpoUkFBV1gZazL70fqLn8rcvBA@mail.gmail.com
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
@@ -1971,9 +1971,6 @@ CommitTransaction(void)
 	 */
 	smgrDoPendingDeletes(true);
 
-	/* Check we've released all catcache entries */
-	AtEOXact_CatCache(true);
-
 	AtCommit_Notify();
 	AtEOXact_GUC(true, 1);
 	AtEOXact_SPI(true);
@@ -2238,9 +2235,6 @@ PrepareTransaction(void)
 	 */
 	PostPrepare_Twophase();
 
-	/* Check we've released all catcache entries */
-	AtEOXact_CatCache(true);
-
 	/* PREPARE acts the same as COMMIT as far as GUC is concerned */
 	AtEOXact_GUC(true, 1);
 	AtEOXact_SPI(true);
@@ -2404,7 +2398,6 @@ AbortTransaction(void)
 							 RESOURCE_RELEASE_AFTER_LOCKS,
 							 false, true);
 		smgrDoPendingDeletes(false);
-		AtEOXact_CatCache(false);
 
 		AtEOXact_GUC(false, 1);
 		AtEOXact_SPI(false);
diff --git a/src/backend/utils/cache/catcache.c b/src/backend/utils/cache/catcache.c
@@ -539,58 +539,6 @@ CreateCacheMemoryContext(void)
 }
 
 
-/*
- *		AtEOXact_CatCache
- *
- * Clean up catcaches at end of main transaction (either commit or abort)
- *
- * As of PostgreSQL 8.1, catcache pins should get released by the
- * ResourceOwner mechanism.  This routine is just a debugging
- * cross-check that no pins remain.
- */
-void
-AtEOXact_CatCache(bool isCommit)
-{
-#ifdef USE_ASSERT_CHECKING
-	if (assert_enabled)
-	{
-		slist_iter	cache_iter;
-
-		slist_foreach(cache_iter, &CacheHdr->ch_caches)
-		{
-			CatCache   *ccp = slist_container(CatCache, cc_next, cache_iter.cur);
-			dlist_iter	iter;
-			int			i;
-
-			/* Check CatCLists */
-			dlist_foreach(iter, &ccp->cc_lists)
-			{
-				CatCList   *cl = dlist_container(CatCList, cache_elem, iter.cur);
-
-				Assert(cl->cl_magic == CL_MAGIC);
-				Assert(cl->refcount == 0);
-				Assert(!cl->dead);
-			}
-
-			/* Check individual tuples */
-			for (i = 0; i < ccp->cc_nbuckets; i++)
-			{
-				dlist_head *bucket = &ccp->cc_bucket[i];
-
-				dlist_foreach(iter, bucket)
-				{
-					CatCTup    *ct = dlist_container(CatCTup, cache_elem, iter.cur);
-
-					Assert(ct->ct_magic == CT_MAGIC);
-					Assert(ct->refcount == 0);
-					Assert(!ct->dead);
-				}
-			}
-		}
-	}
-#endif
-}
-
 /*
  *		ResetCatalogCache
  *
diff --git a/src/include/utils/catcache.h b/src/include/utils/catcache.h
@@ -162,7 +162,6 @@ typedef struct catcacheheader
 extern PGDLLIMPORT MemoryContext CacheMemoryContext;
 
 extern void CreateCacheMemoryContext(void);
-extern void AtEOXact_CatCache(bool isCommit);
 
 extern CatCache *InitCatCache(int id, Oid reloid, Oid indexoid,
 			 int nkeys, const int *key,
