Cope with inplace update making catcache stale during TOAST fetch.

nmisch · nmisch · commit b08a4b6163eb · 2024-06-27T19:21:11.000-07:00
This extends ad98fb1 to invals of inplace updates. Trouble requires an inplace update of a catalog having a TOAST table, so only pg_database was at risk. (The other catalog on which core code performs inplace updates, pg_class, has no TOAST table.) Trouble would require something like the inplace-inval.spec test. Consider GRANT ... ON DATABASE fetching a stale row from cache and discarding a datfrozenxid update that vac_truncate_clog() has already relied upon. Back-patch to v12 (all supported versions). Reviewed (in an earlier version) by Robert Haas. Discussion: https://postgr.es/m/20240114201411.d0@rfd.leadboat.com Discussion: https://postgr.es/m/20240512232923.aa.nmisch@google.com
diff --git a/src/backend/catalog/catalog.c b/src/backend/catalog/catalog.c
@@ -137,6 +137,27 @@ IsCatalogRelationOid(Oid relid)
 	return (relid < (Oid) FirstUnpinnedObjectId);
 }
 
+/*
+ * IsInplaceUpdateRelation
+ *		True iff core code performs inplace updates on the relation.
+ */
+bool
+IsInplaceUpdateRelation(Relation relation)
+{
+	return IsInplaceUpdateOid(RelationGetRelid(relation));
+}
+
+/*
+ * IsInplaceUpdateOid
+ *		Like the above, but takes an OID as argument.
+ */
+bool
+IsInplaceUpdateOid(Oid relid)
+{
+	return (relid == RelationRelationId ||
+			relid == DatabaseRelationId);
+}
+
 /*
  * IsToastRelation
  *		True iff relation is a TOAST support relation (or index).
diff --git a/src/backend/utils/cache/catcache.c b/src/backend/utils/cache/catcache.c
@@ -21,6 +21,7 @@
 #include "access/table.h"
 #include "access/valid.h"
 #include "access/xact.h"
+#include "catalog/catalog.h"
 #include "catalog/pg_collation.h"
 #include "catalog/pg_operator.h"
 #include "catalog/pg_type.h"
@@ -1843,6 +1844,23 @@ ReleaseCatCacheList(CatCList *list)
 }
 
 
+/*
+ * equalTuple
+ *		Are these tuples memcmp()-equal?
+ */
+static bool
+equalTuple(HeapTuple a, HeapTuple b)
+{
+	uint32		alen;
+	uint32		blen;
+
+	alen = a->t_len;
+	blen = b->t_len;
+	return (alen == blen &&
+			memcmp((char *) a->t_data,
+				   (char *) b->t_data, blen) == 0);
+}
+
 /*
  * CatalogCacheCreateEntry
  *		Create a new CatCTup entry, copying the given HeapTuple and other
@@ -1893,14 +1911,34 @@ CatalogCacheCreateEntry(CatCache *cache, HeapTuple ntp, SysScanDesc scandesc,
 		 */
 		if (HeapTupleHasExternal(ntp))
 		{
+			bool		need_cmp = IsInplaceUpdateOid(cache->cc_reloid);
+			HeapTuple	before = NULL;
+			bool		matches = true;
+
+			if (need_cmp)
+				before = heap_copytuple(ntp);
 			dtp = toast_flatten_tuple(ntp, cache->cc_tupdesc);
 
 			/*
 			 * The tuple could become stale while we are doing toast table
-			 * access (since AcceptInvalidationMessages can run then), so we
-			 * must recheck its visibility afterwards.
+			 * access (since AcceptInvalidationMessages can run then).
+			 * equalTuple() detects staleness from inplace updates, while
+			 * systable_recheck_tuple() detects staleness from normal updates.
+			 *
+			 * While this equalTuple() follows the usual rule of reading with
+			 * a pin and no buffer lock, it warrants suspicion since an
+			 * inplace update could appear at any moment.  It's safe because
+			 * the inplace update sends an invalidation that can't reorder
+			 * before the inplace heap change.  If the heap change reaches
+			 * this process just after equalTuple() looks, we've not missed
+			 * its inval.
 			 */
-			if (!systable_recheck_tuple(scandesc, ntp))
+			if (need_cmp)
+			{
+				matches = equalTuple(before, ntp);
+				heap_freetuple(before);
+			}
+			if (!matches || !systable_recheck_tuple(scandesc, ntp))
 			{
 				heap_freetuple(dtp);
 				return NULL;
diff --git a/src/include/catalog/catalog.h b/src/include/catalog/catalog.h
@@ -21,11 +21,13 @@
 extern bool IsSystemRelation(Relation relation);
 extern bool IsToastRelation(Relation relation);
 extern bool IsCatalogRelation(Relation relation);
+extern bool IsInplaceUpdateRelation(Relation relation);
 
 extern bool IsSystemClass(Oid relid, Form_pg_class reltuple);
 extern bool IsToastClass(Form_pg_class reltuple);
 
 extern bool IsCatalogRelationOid(Oid relid);
+extern bool IsInplaceUpdateOid(Oid relid);
 
 extern bool IsCatalogNamespace(Oid namespaceId);
 extern bool IsToastNamespace(Oid namespaceId);
