For inplace update durability, make heap_update() callers wait.

The previous commit fixed some ways of losing an inplace update.  It
remained possible to lose one when a backend working toward a
heap_update() copied a tuple into memory just before inplace update of
that tuple.  In catalogs eligible for inplace update, use LOCKTAG_TUPLE
to govern admission to the steps of copying an old tuple, modifying it,
and issuing heap_update().  This includes MERGE commands.  To avoid
changing most of the pg_class DDL, don't require LOCKTAG_TUPLE when
holding a relation lock sufficient to exclude inplace updaters.
Back-patch to v12 (all supported versions).  In v13 and v12, "UPDATE
pg_class" or "UPDATE pg_database" can still lose an inplace update.  The
v14+ UPDATE fix needs commit 86dc90056dfdbd9d1b891718d2e5614e3e432f35,
and it wasn't worth reimplementing that fix without such infrastructure.

Reviewed by Nitin Motiani and (in earlier versions) Heikki Linnakangas.

Discussion: https://postgr.es/m/20231027214946.79.nmisch@google.com

src/include/nodes/execnodes.h

@@ -556,6 +556,9 @@ typedef struct ResultRelInfo
 	 * one of its ancestors; see ExecCrossPartitionUpdateForeignKey().
 	 */
 	List	   *ri_ancestorResultRels;
+
+	/* updates do LockTuple() before oldtup read; see README.tuplock */
+	bool		ri_needLockTagTuple;
 } ResultRelInfo;
 
 /*

src/include/storage/lockdefs.h

@@ -47,6 +47,8 @@ typedef int LOCKMODE;
 
 #define MaxLockMode				8	/* highest standard lock mode */
 
+/* See README.tuplock section "Locking to write inplace-updated tables" */
+#define InplaceUpdateTupleLock ExclusiveLock
 
 /* WAL representation of an AccessExclusiveLock on a table */
 typedef struct xl_standby_lock

src/include/utils/syscache.h

@@ -139,9 +139,14 @@ extern HeapTuple SearchSysCache4(int cacheId,
 
 extern void ReleaseSysCache(HeapTuple tuple);
 
+extern HeapTuple SearchSysCacheLocked1(int cacheId,
+									   Datum key1);
+
 /* convenience routines */
 extern HeapTuple SearchSysCacheCopy(int cacheId,
 									Datum key1, Datum key2, Datum key3, Datum key4);
+extern HeapTuple SearchSysCacheLockedCopy1(int cacheId,
+										   Datum key1);
 extern bool SearchSysCacheExists(int cacheId,
 								 Datum key1, Datum key2, Datum key3, Datum key4);
 extern Oid	GetSysCacheOid(int cacheId, AttrNumber oidcol,