Release cache pin memory

Cache pins are allocated on the CacheMemoryContext in order to survive
subtransactions, and, optionally, main transactions (in case of, e.g.,
clustering or vaccuming). However, cache pins that are released also
needs to free memory on the CacheMemoryContext in order to avoid
leaking memory in a session.

This change ensures cache pin memory is freed when a cache pin is
released. It also allocates cache pins on a separate memory context,
which is a child of the CacheMemoryContext. This separate memory
context makes it easier to track the memory used by cache pins and
also release it when necessary.

Author: erimatnor

src/cache.c

@@ -6,13 +6,28 @@
 /* List of pinned caches. A cache occurs once in this list for every pin
  * taken */
 static List *pinned_caches = NIL;
+static MemoryContext pinned_caches_mctx = NULL;
 
 typedef struct CachePin
 {
 	Cache	   *cache;
 	SubTransactionId subtxnid;
 } CachePin;
 
+
+static void
+cache_reset_pinned_caches(void)
+{
+	if (NULL != pinned_caches_mctx)
+		MemoryContextDelete(pinned_caches_mctx);
+
+	pinned_caches_mctx = AllocSetContextCreate(CacheMemoryContext,
+											   "Cache pins",
+											   ALLOCSET_DEFAULT_SIZES);
+
+	pinned_caches = NIL;
+}
+
 void
 cache_init(Cache *cache)
 {
@@ -72,7 +87,7 @@ cache_invalidate(Cache *cache)
 extern Cache *
 cache_pin(Cache *cache)
 {
-	MemoryContext old = MemoryContextSwitchTo(CacheMemoryContext);
+	MemoryContext old = MemoryContextSwitchTo(pinned_caches_mctx);
 	CachePin   *cp = palloc(sizeof(CachePin));
 
 	cp->cache = cache;
@@ -96,6 +111,7 @@ remove_pin(Cache *cache, SubTransactionId subtxnid)
 		if (cp->cache == cache && cp->subtxnid == subtxnid)
 		{
 			pinned_caches = list_delete_cell(pinned_caches, lc, prev);
+			pfree(cp);
 			return;
 		}
 
@@ -200,8 +216,8 @@ release_all_pinned_caches()
 		cp->cache->refcount--;
 		cache_destroy(cp->cache);
 	}
-	list_free(pinned_caches);
-	pinned_caches = NIL;
+
+	cache_reset_pinned_caches();
 }
 
 static void
@@ -227,6 +243,8 @@ release_subtxn_pinned_caches(SubTransactionId subtxnid, bool abort)
 			cache_release_subtxn(cp->cache, subtxnid);
 		}
 	}
+
+	list_free(pinned_caches_copy);
 }
 
 /*
@@ -251,12 +269,17 @@ cache_xact_end(XactEvent event, void *arg)
 			break;
 		default:
 			{
+				/*
+				 * Make a copy of the list of pinned caches since
+				 * cache_release() can manipulate the original list.
+				 */
+				List	   *pinned_caches_copy = list_copy(pinned_caches);
 				ListCell   *lc;
 
 				/*
 				 * Only caches left should be marked as non-released
 				 */
-				foreach(lc, pinned_caches)
+				foreach(lc, pinned_caches_copy)
 				{
 					CachePin   *cp = lfirst(lc);
 
@@ -273,6 +296,8 @@ cache_xact_end(XactEvent event, void *arg)
 					if (cp->cache->release_on_commit)
 						cache_release(cp->cache);
 				}
+
+				list_free(pinned_caches_copy);
 			}
 			break;
 	}
@@ -311,13 +336,17 @@ cache_subxact_abort(SubXactEvent event, SubTransactionId subtxn_id,
 void
 _cache_init(void)
 {
+	cache_reset_pinned_caches();
 	RegisterXactCallback(cache_xact_end, NULL);
 	RegisterSubXactCallback(cache_subxact_abort, NULL);
 }
 
 void
 _cache_fini(void)
 {
+	MemoryContextDelete(pinned_caches_mctx);
+	pinned_caches_mctx = NULL;
+	pinned_caches = NIL;
 	UnregisterXactCallback(cache_xact_end, NULL);
 	UnregisterSubXactCallback(cache_subxact_abort, NULL);
 }