Handle subtxn for cache pinning

This commit adds logic for cache pinning to handle subtxn. It also makes
it easier to find cache pinning leaks. Finally, it fixes handling of
cross-commit operations like VACUUM and CLUSTER. Previously, such
operations incorrectly released that cache pin on the first commit
even though the object was used after that.

src/cache.c

@@ -25,6 +25,7 @@ cache_init(Cache *cache)
 	cache->htab = hash_create(cache->name, cache->numelements,
 							  &cache->hctl, cache->flags);
 	cache->refcount = 1;
+	cache->release_on_commit = true;
 }
 
 static void
@@ -155,6 +156,26 @@ cache_remove(Cache *cache, void *key)
 }
 
 
+static void
+release_all_pinned_caches()
+{
+	ListCell   *lc;
+
+	/*
+	 * release once for every occurence of a cache in the pinned caches list.
+	 * On abort, release irrespective of cache->release_on_commit.
+	 */
+	foreach(lc, pinned_caches)
+	{
+		Cache	   *cache = lfirst(lc);
+
+		cache->refcount--;
+		cache_destroy(cache);
+	}
+	list_free(pinned_caches);
+	pinned_caches = NIL;
+}
+
 /*
  * Transaction end callback that cleans up any pinned caches. This is a
  * safeguard that protects against indefinitely pinned caches (memory leaks)
@@ -163,39 +184,61 @@ cache_remove(Cache *cache, void *key)
  * cache_release() call and wrapped in a PG_TRY() block to capture and handle
  * any exceptions that occur.
  *
- * Note that this makes cache_release() optional, although timely
- * cache_release() calls are still encouraged to release memory as early as
- * possible during long-running transactions. PG_TRY() blocks are not needed,
- * however.
- */
+ * Note that this checks that cache_release() is always called by the end
+ * of a non-aborted transaction unless cache->release_on_commit is set to true.
+ * */
 static void
 cache_xact_end(XactEvent event, void *arg)
 {
 	switch (event)
 	{
-		case XACT_EVENT_COMMIT:
 		case XACT_EVENT_ABORT:
 		case XACT_EVENT_PARALLEL_ABORT:
+			release_all_pinned_caches();
+		default:
 			{
 				ListCell   *lc;
 
 				/*
-				 * release once for every occurence of a cache in the pinned
-				 * caches list
+				 * Only caches left should be marked as non-released
 				 */
 				foreach(lc, pinned_caches)
 				{
 					Cache	   *cache = lfirst(lc);
 
-					cache->refcount--;
-					cache_destroy(cache);
+					/*
+					 * This assert makes sure that that we don't have a cache
+					 * leak when running with debugging
+					 */
+					Assert(!cache->release_on_commit);
+
+					/*
+					 * This may still happen in optimized environments where
+					 * Assert is turned off. In that case, release.
+					 */
+					if (cache->release_on_commit)
+						cache_release(cache);
 				}
 			}
-		default:
 			break;
 	}
-	list_free(pinned_caches);
-	pinned_caches = NIL;
+}
+
+static void
+cache_subxact_abort(SubXactEvent event, SubTransactionId mySubid,
+					SubTransactionId parentSubid, void *arg)
+{
+	/*
+	 * Note that cache->release_on_commit is irrelevant here since can't have
+	 * cross-commit operations in subtxns
+	 */
+
+	/*
+	 * In subtxns, caches should have already been released, unless an abort
+	 * happened
+	 */
+	Assert(SUBXACT_EVENT_ABORT_SUB == event || list_length(pinned_caches) == 0);
+	release_all_pinned_caches();
 }
 
 
@@ -203,10 +246,12 @@ void
 _cache_init(void)
 {
 	RegisterXactCallback(cache_xact_end, NULL);
+	RegisterSubXactCallback(cache_subxact_abort, NULL);
 }
 
 void
 _cache_fini(void)
 {
 	UnregisterXactCallback(cache_xact_end, NULL);
+	UnregisterSubXactCallback(cache_subxact_abort, NULL);
 }

src/cache.h

@@ -31,6 +31,9 @@ typedef struct Cache
 	void	   *(*create_entry) (struct Cache *, CacheQuery *);
 	void	   *(*update_entry) (struct Cache *, CacheQuery *);
 	void		(*pre_destroy_hook) (struct Cache *);
+	bool		release_on_commit;		/* This should be false if doing
+										 * cross-commit operations like
+										 * CLUSTER or VACUUM */
 } Cache;
 
 extern void cache_init(Cache *cache);

src/process_utility.c

@@ -278,9 +278,14 @@ foreach_chunk_relid(Oid relid, process_chunk_t process_chunk, void *arg)
 	int			ret;
 
 	if (NULL == ht)
+	{
+		cache_release(hcache);
 		return -1;
+	}
 
+	hcache->release_on_commit = false;
 	ret = foreach_chunk(ht, process_chunk, arg);
+	hcache->release_on_commit = true;
 
 	cache_release(hcache);
 
@@ -477,6 +482,8 @@ process_drop_index(DropStmt *stmt)
 					 */
 					chunk_index_delete(chunk, idxrelid, false);
 			}
+
+			cache_release(hcache);
 		}
 	}
 }
@@ -1132,6 +1139,8 @@ process_cluster_start(Node *parsetree, ProcessUtilityContext context)
 		chunk_indexes = chunk_index_get_mappings(ht, index_relid);
 		MemoryContextSwitchTo(old);
 
+		hcache->release_on_commit = false;
+
 		/* Commit to get out of starting transaction */
 		PopActiveSnapshot();
 		CommitTransactionCommand();
@@ -1158,6 +1167,8 @@ process_cluster_start(Node *parsetree, ProcessUtilityContext context)
 			PopActiveSnapshot();
 			CommitTransactionCommand();
 		}
+
+		hcache->release_on_commit = true;
 		/* Start a new transaction for the cleanup work. */
 		StartTransactionCommand();
 
@@ -1166,7 +1177,6 @@ process_cluster_start(Node *parsetree, ProcessUtilityContext context)
 	}
 
 	cache_release(hcache);
-
 	return false;
 }