Fix data loss at inplace update after heap_update().

As previously-added tests demonstrated, heap_inplace_update() could
instead update an unrelated tuple of the same catalog.  It could lose
the update.  Losing relhasindex=t was a source of index corruption.
Inplace-updating commands like VACUUM will now wait for heap_update()
commands like GRANT TABLE and GRANT DATABASE.  That isn't ideal, but a
long-running GRANT already hurts VACUUM progress more just by keeping an
XID running.  The VACUUM will behave like a DELETE or UPDATE waiting for
the uncommitted change.

For implementation details, start at the systable_inplace_update_begin()
header comment and README.tuplock.  Back-patch to v12 (all supported
versions).  In back branches, retain a deprecated heap_inplace_update(),
for extensions.

Reported by Smolkin Grigory.  Reviewed by Nitin Motiani, (in earlier
versions) Heikki Linnakangas, and (in earlier versions) Alexander
Lakhin.

Discussion: https://postgr.es/m/CAMp+ueZQz3yDk7qg42hk6-9gxniYbp-=bG2mgqecErqR5gGGOA@mail.gmail.com

src/backend/access/heap/README.tuplock

@@ -153,3 +153,14 @@ The following infomask bits are applicable:
 
 We currently never set the HEAP_XMAX_COMMITTED when the HEAP_XMAX_IS_MULTI bit
 is set.
+
+Reading inplace-updated columns
+-------------------------------
+
+Inplace updates create an exception to the rule that tuple data won't change
+under a reader holding a pin.  A reader of a heap_fetch() result tuple may
+witness a torn read.  Current inplace-updated fields are aligned and are no
+wider than four bytes, and current readers don't need consistency across
+fields.  Hence, they get by with just fetching each field once.  XXX such a
+caller may also read a value that has not reached WAL; see
+systable_inplace_update_finish().

src/backend/access/heap/heapam.c

@@ -5880,23 +5880,245 @@ heap_abort_speculative(Relation relation, ItemPointer tid)
 }
 
 /*
- * heap_inplace_update - update a tuple "in place" (ie, overwrite it)
+ * heap_inplace_lock - protect inplace update from concurrent heap_update()
  *
- * Overwriting violates both MVCC and transactional safety, so the uses
+ * Evaluate whether the tuple's state is compatible with a no-key update.
- * of this function in Postgres are extremely limited.  Nonetheless we
+ * Current transaction rowmarks are fine, as is KEY SHARE from any
- * find some places to use it.
+ * transaction.  If compatible, return true with the buffer exclusive-locked,
+ * and the caller must release that by calling
+ * heap_inplace_update_and_unlock(), calling heap_inplace_unlock(), or raising
+ * an error.  Otherwise, return false after blocking transactions, if any,
+ * have ended.
  *
- * The tuple cannot change size, and therefore it's reasonable to assume
+ * Since this is intended for system catalogs and SERIALIZABLE doesn't cover
- * that its null bitmap (if any) doesn't change either.  So we just
+ * DDL, this doesn't guarantee any particular predicate locking.
- * overwrite the data portion of the tuple without touching the null
- * bitmap or any of the header fields.
  *
- * tuple is an in-memory tuple structure containing the data to be written
+ * One could modify this to return true for tuples with delete in progress,
- * over the target tuple.  Also, tuple->t_self identifies the target tuple.
+ * All inplace updaters take a lock that conflicts with DROP.  If explicit
+ * "DELETE FROM pg_class" is in progress, we'll wait for it like we would an
+ * update.
  *
- * Note that the tuple updated here had better not come directly from the
+ * Readers of inplace-updated fields expect changes to those fields are
- * syscache if the relation has a toast relation as this tuple could
+ * durable.  For example, vac_truncate_clog() reads datfrozenxid from
- * include toast values that have been expanded, causing a failure here.
+ * pg_database tuples via catalog snapshots.  A future snapshot must not
+ * return a lower datfrozenxid for the same database OID (lower in the
+ * FullTransactionIdPrecedes() sense).  We achieve that since no update of a
+ * tuple can start while we hold a lock on its buffer.  In cases like
+ * BEGIN;GRANT;CREATE INDEX;COMMIT we're inplace-updating a tuple visible only
+ * to this transaction.  ROLLBACK then is one case where it's okay to lose
+ * inplace updates.  (Restoring relhasindex=false on ROLLBACK is fine, since
+ * any concurrent CREATE INDEX would have blocked, then inplace-updated the
+ * committed tuple.)
+ *
+ * In principle, we could avoid waiting by overwriting every tuple in the
+ * updated tuple chain.  Reader expectations permit updating a tuple only if
+ * it's aborted, is the tail of the chain, or we already updated the tuple
+ * referenced in its t_ctid.  Hence, we would need to overwrite the tuples in
+ * order from tail to head.  That would imply either (a) mutating all tuples
+ * in one critical section or (b) accepting a chance of partial completion.
+ * Partial completion of a relfrozenxid update would have the weird
+ * consequence that the table's next VACUUM could see the table's relfrozenxid
+ * move forward between vacuum_get_cutoffs() and finishing.
+ */
+bool
+heap_inplace_lock(Relation relation,
+				  HeapTuple oldtup_ptr, Buffer buffer)
+{
+	HeapTupleData oldtup = *oldtup_ptr; /* minimize diff vs. heap_update() */
+	TM_Result	result;
+	bool		ret;
+
+	Assert(BufferIsValid(buffer));
+
+	LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
+
+	/*----------
+	 * Interpret HeapTupleSatisfiesUpdate() like heap_update() does, except:
+	 *
+	 * - wait unconditionally
+	 * - no tuple locks
+	 * - don't recheck header after wait: simpler to defer to next iteration
+	 * - don't try to continue even if the updater aborts: likewise
+	 * - no crosscheck
+	 */
+	result = HeapTupleSatisfiesUpdate(&oldtup, GetCurrentCommandId(false),
+									  buffer);
+
+	if (result == TM_Invisible)
+	{
+		/* no known way this can happen */
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg_internal("attempted to overwrite invisible tuple")));
+	}
+	else if (result == TM_SelfModified)
+	{
+		/*
+		 * CREATE INDEX might reach this if an expression is silly enough to
+		 * call e.g. SELECT ... FROM pg_class FOR SHARE.  C code of other SQL
+		 * statements might get here after a heap_update() of the same row, in
+		 * the absence of an intervening CommandCounterIncrement().
+		 */
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("tuple to be updated was already modified by an operation triggered by the current command")));
+	}
+	else if (result == TM_BeingModified)
+	{
+		TransactionId xwait;
+		uint16		infomask;
+
+		xwait = HeapTupleHeaderGetRawXmax(oldtup.t_data);
+		infomask = oldtup.t_data->t_infomask;
+
+		if (infomask & HEAP_XMAX_IS_MULTI)
+		{
+			LockTupleMode lockmode = LockTupleNoKeyExclusive;
+			MultiXactStatus mxact_status = MultiXactStatusNoKeyUpdate;
+			int			remain;
+			bool		current_is_member;
+
+			if (DoesMultiXactIdConflict((MultiXactId) xwait, infomask,
+										lockmode, &current_is_member))
+			{
+				LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+				ret = false;
+				MultiXactIdWait((MultiXactId) xwait, mxact_status, infomask,
+								relation, &oldtup.t_self, XLTW_Update,
+								&remain);
+			}
+			else
+				ret = true;
+		}
+		else if (TransactionIdIsCurrentTransactionId(xwait))
+			ret = true;
+		else if (HEAP_XMAX_IS_KEYSHR_LOCKED(infomask))
+			ret = true;
+		else
+		{
+			LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+			ret = false;
+			XactLockTableWait(xwait, relation, &oldtup.t_self,
+							  XLTW_Update);
+		}
+	}
+	else
+	{
+		ret = (result == TM_Ok);
+		if (!ret)
+		{
+			LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+		}
+	}
+
+	/*
+	 * GetCatalogSnapshot() relies on invalidation messages to know when to
+	 * take a new snapshot.  COMMIT of xwait is responsible for sending the
+	 * invalidation.  We're not acquiring heavyweight locks sufficient to
+	 * block if not yet sent, so we must take a new snapshot to ensure a later
+	 * attempt has a fair chance.  While we don't need this if xwait aborted,
+	 * don't bother optimizing that.
+	 */
+	if (!ret)
+		InvalidateCatalogSnapshot();
+	return ret;
+}
+
+/*
+ * heap_inplace_update_and_unlock - core of systable_inplace_update_finish
+ *
+ * The tuple cannot change size, and therefore its header fields and null
+ * bitmap (if any) don't change either.
+ */
+void
+heap_inplace_update_and_unlock(Relation relation,
+							   HeapTuple oldtup, HeapTuple tuple,
+							   Buffer buffer)
+{
+	HeapTupleHeader htup = oldtup->t_data;
+	uint32		oldlen;
+	uint32		newlen;
+
+	Assert(ItemPointerEquals(&oldtup->t_self, &tuple->t_self));
+	oldlen = oldtup->t_len - htup->t_hoff;
+	newlen = tuple->t_len - tuple->t_data->t_hoff;
+	if (oldlen != newlen || htup->t_hoff != tuple->t_data->t_hoff)
+		elog(ERROR, "wrong tuple length");
+
+	/* NO EREPORT(ERROR) from here till changes are logged */
+	START_CRIT_SECTION();
+
+	memcpy((char *) htup + htup->t_hoff,
+		   (char *) tuple->t_data + tuple->t_data->t_hoff,
+		   newlen);
+
+	/*----------
+	 * XXX A crash here can allow datfrozenxid() to get ahead of relfrozenxid:
+	 *
+	 * ["D" is a VACUUM (ONLY_DATABASE_STATS)]
+	 * ["R" is a VACUUM tbl]
+	 * D: vac_update_datfrozenid() -> systable_beginscan(pg_class)
+	 * D: systable_getnext() returns pg_class tuple of tbl
+	 * R: memcpy() into pg_class tuple of tbl
+	 * D: raise pg_database.datfrozenxid, XLogInsert(), finish
+	 * [crash]
+	 * [recovery restores datfrozenxid w/o relfrozenxid]
+	 */
+
+	MarkBufferDirty(buffer);
+
+	/* XLOG stuff */
+	if (RelationNeedsWAL(relation))
+	{
+		xl_heap_inplace xlrec;
+		XLogRecPtr	recptr;
+
+		xlrec.offnum = ItemPointerGetOffsetNumber(&tuple->t_self);
+
+		XLogBeginInsert();
+		XLogRegisterData((char *) &xlrec, SizeOfHeapInplace);
+
+		XLogRegisterBuffer(0, buffer, REGBUF_STANDARD);
+		XLogRegisterBufData(0, (char *) htup + htup->t_hoff, newlen);
+
+		/* inplace updates aren't decoded atm, don't log the origin */
+
+		recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_INPLACE);
+
+		PageSetLSN(BufferGetPage(buffer), recptr);
+	}
+
+	END_CRIT_SECTION();
+
+	heap_inplace_unlock(relation, oldtup, buffer);
+
+	/*
+	 * Send out shared cache inval if necessary.  Note that because we only
+	 * pass the new version of the tuple, this mustn't be used for any
+	 * operations that could change catcache lookup keys.  But we aren't
+	 * bothering with index updates either, so that's true a fortiori.
+	 *
+	 * XXX ROLLBACK discards the invalidation.  See test inplace-inval.spec.
+	 */
+	if (!IsBootstrapProcessingMode())
+		CacheInvalidateHeapTuple(relation, tuple, NULL);
+}
+
+/*
+ * heap_inplace_unlock - reverse of heap_inplace_lock
+ */
+void
+heap_inplace_unlock(Relation relation,
+					HeapTuple oldtup, Buffer buffer)
+{
+	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+}
+
+/*
+ * heap_inplace_update - deprecated
+ *
+ * This exists only to keep modules working in back branches.  Affected
+ * modules should migrate to systable_inplace_update_begin().
  */
 void
 heap_inplace_update(Relation relation, HeapTuple tuple)

src/backend/access/index/genam.c

@@ -749,3 +749,139 @@ systable_endscan_ordered(SysScanDesc sysscan)
 		UnregisterSnapshot(sysscan->snapshot);
 	pfree(sysscan);
 }
+
+/*
+ * systable_inplace_update_begin --- update a row "in place" (overwrite it)
+ *
+ * Overwriting violates both MVCC and transactional safety, so the uses of
+ * this function in Postgres are extremely limited.  Nonetheless we find some
+ * places to use it.  Standard flow:
+ *
+ * ... [any slow preparation not requiring oldtup] ...
+ * systable_inplace_update_begin([...], &tup, &inplace_state);
+ * if (!HeapTupleIsValid(tup))
+ *	elog(ERROR, [...]);
+ * ... [buffer is exclusive-locked; mutate "tup"] ...
+ * if (dirty)
+ *	systable_inplace_update_finish(inplace_state, tup);
+ * else
+ *	systable_inplace_update_cancel(inplace_state);
+ *
+ * The first several params duplicate the systable_beginscan() param list.
+ * "oldtupcopy" is an output parameter, assigned NULL if the key ceases to
+ * find a live tuple.  (In PROC_IN_VACUUM, that is a low-probability transient
+ * condition.)  If "oldtupcopy" gets non-NULL, you must pass output parameter
+ * "state" to systable_inplace_update_finish() or
+ * systable_inplace_update_cancel().
+ */
+void
+systable_inplace_update_begin(Relation relation,
+							  Oid indexId,
+							  bool indexOK,
+							  Snapshot snapshot,
+							  int nkeys, const ScanKeyData *key,
+							  HeapTuple *oldtupcopy,
+							  void **state)
+{
+	ScanKey		mutable_key = palloc(sizeof(ScanKeyData) * nkeys);
+	int			retries = 0;
+	SysScanDesc scan;
+	HeapTuple	oldtup;
+
+	/*
+	 * For now, we don't allow parallel updates.  Unlike a regular update,
+	 * this should never create a combo CID, so it might be possible to relax
+	 * this restriction, but not without more thought and testing.  It's not
+	 * clear that it would be useful, anyway.
+	 */
+	if (IsInParallelMode())
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TRANSACTION_STATE),
+				 errmsg("cannot update tuples during a parallel operation")));
+
+	/*
+	 * Accept a snapshot argument, for symmetry, but this function advances
+	 * its snapshot as needed to reach the tail of the updated tuple chain.
+	 */
+	Assert(snapshot == NULL);
+
+	Assert(IsInplaceUpdateRelation(relation) || !IsSystemRelation(relation));
+
+	/* Loop for an exclusive-locked buffer of a non-updated tuple. */
+	for (;;)
+	{
+		TupleTableSlot *slot;
+		BufferHeapTupleTableSlot *bslot;
+
+		CHECK_FOR_INTERRUPTS();
+
+		/*
+		 * Processes issuing heap_update (e.g. GRANT) at maximum speed could
+		 * drive us to this error.  A hostile table owner has stronger ways to
+		 * damage their own table, so that's minor.
+		 */
+		if (retries++ > 10000)
+			elog(ERROR, "giving up after too many tries to overwrite row");
+
+		memcpy(mutable_key, key, sizeof(ScanKeyData) * nkeys);
+		scan = systable_beginscan(relation, indexId, indexOK, snapshot,
+								  nkeys, mutable_key);
+		oldtup = systable_getnext(scan);
+		if (!HeapTupleIsValid(oldtup))
+		{
+			systable_endscan(scan);
+			*oldtupcopy = NULL;
+			return;
+		}
+
+		slot = scan->slot;
+		Assert(TTS_IS_BUFFERTUPLE(slot));
+		bslot = (BufferHeapTupleTableSlot *) slot;
+		if (heap_inplace_lock(scan->heap_rel,
+							  bslot->base.tuple, bslot->buffer))
+			break;
+		systable_endscan(scan);
+	};
+
+	*oldtupcopy = heap_copytuple(oldtup);
+	*state = scan;
+}
+
+/*
+ * systable_inplace_update_finish --- second phase of inplace update
+ *
+ * The tuple cannot change size, and therefore its header fields and null
+ * bitmap (if any) don't change either.
+ */
+void
+systable_inplace_update_finish(void *state, HeapTuple tuple)
+{
+	SysScanDesc scan = (SysScanDesc) state;
+	Relation	relation = scan->heap_rel;
+	TupleTableSlot *slot = scan->slot;
+	BufferHeapTupleTableSlot *bslot = (BufferHeapTupleTableSlot *) slot;
+	HeapTuple	oldtup = bslot->base.tuple;
+	Buffer		buffer = bslot->buffer;
+
+	heap_inplace_update_and_unlock(relation, oldtup, tuple, buffer);
+	systable_endscan(scan);
+}
+
+/*
+ * systable_inplace_update_cancel --- abandon inplace update
+ *
+ * This is an alternative to making a no-op update.
+ */
+void
+systable_inplace_update_cancel(void *state)
+{
+	SysScanDesc scan = (SysScanDesc) state;
+	Relation	relation = scan->heap_rel;
+	TupleTableSlot *slot = scan->slot;
+	BufferHeapTupleTableSlot *bslot = (BufferHeapTupleTableSlot *) slot;
+	HeapTuple	oldtup = bslot->base.tuple;
+	Buffer		buffer = bslot->buffer;
+
+	heap_inplace_unlock(relation, oldtup, buffer);
+	systable_endscan(scan);
+}

src/backend/catalog/index.c

@@ -2814,7 +2814,9 @@ index_update_stats(Relation rel,
 {
 	Oid			relid = RelationGetRelid(rel);
 	Relation	pg_class;
+	ScanKeyData key[1];
 	HeapTuple	tuple;
+	void	   *state;
 	Form_pg_class rd_rel;
 	bool		dirty;
 
@@ -2848,33 +2850,12 @@ index_update_stats(Relation rel,
 
 	pg_class = table_open(RelationRelationId, RowExclusiveLock);
 
-	/*
-	 * Make a copy of the tuple to update.  Normally we use the syscache, but
-	 * we can't rely on that during bootstrap or while reindexing pg_class
-	 * itself.
-	 */
-	if (IsBootstrapProcessingMode() ||
-		ReindexIsProcessingHeap(RelationRelationId))
-	{
-		/* don't assume syscache will work */
-		TableScanDesc pg_class_scan;
-		ScanKeyData key[1];
-
 	ScanKeyInit(&key[0],
 				Anum_pg_class_oid,
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(relid));
-
+	systable_inplace_update_begin(pg_class, ClassOidIndexId, true, NULL,
-		pg_class_scan = table_beginscan_catalog(pg_class, 1, key);
+								  1, key, &tuple, &state);
-		tuple = heap_getnext(pg_class_scan, ForwardScanDirection);
-		tuple = heap_copytuple(tuple);
-		table_endscan(pg_class_scan);
-	}
-	else
-	{
-		/* normal case, use syscache */
-		tuple = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(relid));
-	}
 
 	if (!HeapTupleIsValid(tuple))
 		elog(ERROR, "could not find tuple for relation %u", relid);
@@ -2933,11 +2914,12 @@ index_update_stats(Relation rel,
 	 */
 	if (dirty)
 	{
-		heap_inplace_update(pg_class, tuple);
+		systable_inplace_update_finish(state, tuple);
 		/* the above sends a cache inval message */
 	}
 	else
 	{
+		systable_inplace_update_cancel(state);
 		/* no need to change tuple, but force relcache inval anyway */
 		CacheInvalidateRelcacheByTuple(tuple);
 	}

src/backend/catalog/toasting.c

@@ -14,6 +14,7 @@
  */
 #include "postgres.h"
 
+#include "access/genam.h"
 #include "access/heapam.h"
 #include "access/toast_compression.h"
 #include "access/xact.h"
@@ -32,6 +33,7 @@
 #include "nodes/makefuncs.h"
 #include "storage/lock.h"
 #include "utils/builtins.h"
+#include "utils/fmgroids.h"
 #include "utils/rel.h"
 #include "utils/syscache.h"
 
@@ -337,21 +339,36 @@ create_toast_table(Relation rel, Oid toastOid, Oid toastIndexOid,
 	 */
 	class_rel = table_open(RelationRelationId, RowExclusiveLock);
 
+	if (!IsBootstrapProcessingMode())
+	{
+		/* normal case, use a transactional update */
 		reltup = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(relOid));
 		if (!HeapTupleIsValid(reltup))
 			elog(ERROR, "cache lookup failed for relation %u", relOid);
 
 		((Form_pg_class) GETSTRUCT(reltup))->reltoastrelid = toast_relid;
 
-	if (!IsBootstrapProcessingMode())
-	{
-		/* normal case, use a transactional update */
 		CatalogTupleUpdate(class_rel, &reltup->t_self, reltup);
 	}
 	else
 	{
 		/* While bootstrapping, we cannot UPDATE, so overwrite in-place */
-		heap_inplace_update(class_rel, reltup);
+
+		ScanKeyData key[1];
+		void	   *state;
+
+		ScanKeyInit(&key[0],
+					Anum_pg_class_oid,
+					BTEqualStrategyNumber, F_OIDEQ,
+					ObjectIdGetDatum(relOid));
+		systable_inplace_update_begin(class_rel, ClassOidIndexId, true,
+									  NULL, 1, key, &reltup, &state);
+		if (!HeapTupleIsValid(reltup))
+			elog(ERROR, "cache lookup failed for relation %u", relOid);
+
+		((Form_pg_class) GETSTRUCT(reltup))->reltoastrelid = toast_relid;
+
+		systable_inplace_update_finish(state, reltup);
 	}
 
 	heap_freetuple(reltup);

src/backend/commands/dbcommands.c

@@ -1587,7 +1587,7 @@ dropdb(const char *dbname, bool missing_ok, bool force)
 	Relation	pgdbrel;
 	HeapTuple	tup;
 	ScanKeyData scankey;
-	SysScanDesc scan;
+	void	   *inplace_state;
 	Form_pg_database datform;
 	int			notherbackends;
 	int			npreparedxacts;
@@ -1725,24 +1725,6 @@ dropdb(const char *dbname, bool missing_ok, bool force)
 	 */
 	pgstat_drop_database(db_id);
 
-	/*
-	 * Get the pg_database tuple to scribble on.  Note that this does not
-	 * directly rely on the syscache to avoid issues with flattened toast
-	 * values for the in-place update.
-	 */
-	ScanKeyInit(&scankey,
-				Anum_pg_database_datname,
-				BTEqualStrategyNumber, F_NAMEEQ,
-				CStringGetDatum(dbname));
-
-	scan = systable_beginscan(pgdbrel, DatabaseNameIndexId, true,
-							  NULL, 1, &scankey);
-
-	tup = systable_getnext(scan);
-	if (!HeapTupleIsValid(tup))
-		elog(ERROR, "cache lookup failed for database %u", db_id);
-	datform = (Form_pg_database) GETSTRUCT(tup);
-
 	/*
 	 * Except for the deletion of the catalog row, subsequent actions are not
 	 * transactional (consider DropDatabaseBuffers() discarding modified
@@ -1754,8 +1736,17 @@ dropdb(const char *dbname, bool missing_ok, bool force)
 	 * modification is durable before performing irreversible filesystem
 	 * operations.
 	 */
+	ScanKeyInit(&scankey,
+				Anum_pg_database_datname,
+				BTEqualStrategyNumber, F_NAMEEQ,
+				CStringGetDatum(dbname));
+	systable_inplace_update_begin(pgdbrel, DatabaseNameIndexId, true,
+								  NULL, 1, &scankey, &tup, &inplace_state);
+	if (!HeapTupleIsValid(tup))
+		elog(ERROR, "cache lookup failed for database %u", db_id);
+	datform = (Form_pg_database) GETSTRUCT(tup);
 	datform->datconnlimit = DATCONNLIMIT_INVALID_DB;
-	heap_inplace_update(pgdbrel, tup);
+	systable_inplace_update_finish(inplace_state, tup);
 	XLogFlush(XactLastRecEnd);
 
 	/*
@@ -1763,8 +1754,7 @@ dropdb(const char *dbname, bool missing_ok, bool force)
 	 * the row will be gone, but if we fail, dropdb() can be invoked again.
 	 */
 	CatalogTupleDelete(pgdbrel, &tup->t_self);
-
+	heap_freetuple(tup);
-	systable_endscan(scan);
 
 	/*
 	 * Drop db-specific replication slots.

src/backend/commands/vacuum.c

@@ -1427,7 +1427,9 @@ vac_update_relstats(Relation relation,
 {
 	Oid			relid = RelationGetRelid(relation);
 	Relation	rd;
+	ScanKeyData key[1];
 	HeapTuple	ctup;
+	void	   *inplace_state;
 	Form_pg_class pgcform;
 	bool		dirty,
 				futurexid,
@@ -1438,7 +1440,12 @@ vac_update_relstats(Relation relation,
 	rd = table_open(RelationRelationId, RowExclusiveLock);
 
 	/* Fetch a copy of the tuple to scribble on */
-	ctup = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(relid));
+	ScanKeyInit(&key[0],
+				Anum_pg_class_oid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(relid));
+	systable_inplace_update_begin(rd, ClassOidIndexId, true,
+								  NULL, 1, key, &ctup, &inplace_state);
 	if (!HeapTupleIsValid(ctup))
 		elog(ERROR, "pg_class entry for relid %u vanished during vacuuming",
 			 relid);
@@ -1546,7 +1553,9 @@ vac_update_relstats(Relation relation,
 
 	/* If anything changed, write out the tuple. */
 	if (dirty)
-		heap_inplace_update(rd, ctup);
+		systable_inplace_update_finish(inplace_state, ctup);
+	else
+		systable_inplace_update_cancel(inplace_state);
 
 	table_close(rd, RowExclusiveLock);
 
@@ -1598,6 +1607,7 @@ vac_update_datfrozenxid(void)
 	bool		bogus = false;
 	bool		dirty = false;
 	ScanKeyData key[1];
+	void	   *inplace_state;
 
 	/*
 	 * Restrict this task to one backend per database.  This avoids race
@@ -1721,20 +1731,18 @@ vac_update_datfrozenxid(void)
 	relation = table_open(DatabaseRelationId, RowExclusiveLock);
 
 	/*
-	 * Get the pg_database tuple to scribble on.  Note that this does not
+	 * Fetch a copy of the tuple to scribble on.  We could check the syscache
-	 * directly rely on the syscache to avoid issues with flattened toast
+	 * tuple first.  If that concluded !dirty, we'd avoid waiting on
-	 * values for the in-place update.
+	 * concurrent heap_update() and would avoid exclusive-locking the buffer.
+	 * For now, don't optimize that.
 	 */
 	ScanKeyInit(&key[0],
 				Anum_pg_database_oid,
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(MyDatabaseId));
 
-	scan = systable_beginscan(relation, DatabaseOidIndexId, true,
+	systable_inplace_update_begin(relation, DatabaseOidIndexId, true,
-							  NULL, 1, key);
+								  NULL, 1, key, &tuple, &inplace_state);
-	tuple = systable_getnext(scan);
-	tuple = heap_copytuple(tuple);
-	systable_endscan(scan);
 
 	if (!HeapTupleIsValid(tuple))
 		elog(ERROR, "could not find tuple for database %u", MyDatabaseId);
@@ -1768,7 +1776,9 @@ vac_update_datfrozenxid(void)
 		newMinMulti = dbform->datminmxid;
 
 	if (dirty)
-		heap_inplace_update(relation, tuple);
+		systable_inplace_update_finish(inplace_state, tuple);
+	else
+		systable_inplace_update_cancel(inplace_state);
 
 	heap_freetuple(tuple);
 	table_close(relation, RowExclusiveLock);

src/include/access/genam.h

@@ -230,5 +230,14 @@ extern SysScanDesc systable_beginscan_ordered(Relation heapRelation,
 extern HeapTuple systable_getnext_ordered(SysScanDesc sysscan,
 										  ScanDirection direction);
 extern void systable_endscan_ordered(SysScanDesc sysscan);
+extern void systable_inplace_update_begin(Relation relation,
+										  Oid indexId,
+										  bool indexOK,
+										  Snapshot snapshot,
+										  int nkeys, const ScanKeyData *key,
+										  HeapTuple *oldtupcopy,
+										  void **state);
+extern void systable_inplace_update_finish(void *state, HeapTuple tuple);
+extern void systable_inplace_update_cancel(void *state);
 
 #endif							/* GENAM_H */

src/include/access/heapam.h

@@ -256,6 +256,13 @@ extern TM_Result heap_lock_tuple(Relation relation, HeapTuple tuple,
 								 bool follow_updates,
 								 Buffer *buffer, struct TM_FailureData *tmfd);
 
+extern bool heap_inplace_lock(Relation relation,
+							  HeapTuple oldtup_ptr, Buffer buffer);
+extern void heap_inplace_update_and_unlock(Relation relation,
+										   HeapTuple oldtup, HeapTuple tuple,
+										   Buffer buffer);
+extern void heap_inplace_unlock(Relation relation,
+								HeapTuple oldtup, Buffer buffer);
 extern void heap_inplace_update(Relation relation, HeapTuple tuple);
 extern bool heap_prepare_freeze_tuple(HeapTupleHeader tuple,
 									  const struct VacuumCutoffs *cutoffs,

src/test/isolation/expected/intra-grant-inplace-db.out

@@ -9,12 +9,13 @@ step b1: BEGIN;
 step grant1: 
 	GRANT TEMP ON DATABASE isolation_regression TO regress_temp_grantee;
 
-step vac2: VACUUM (FREEZE);
+step vac2: VACUUM (FREEZE); <waiting ...>
 step snap3: 
 	INSERT INTO frozen_witness
 	SELECT datfrozenxid FROM pg_database WHERE datname = current_catalog;
 
 step c1: COMMIT;
+step vac2: <... completed>
 step cmp3: 
 	SELECT 'datfrozenxid retreated'
 	FROM pg_database
@@ -22,7 +23,6 @@ step cmp3:
 		AND age(datfrozenxid) > (SELECT min(age(x)) FROM frozen_witness);
 
 ?column?
-----------------------
+--------
-datfrozenxid retreated
+(0 rows)
-(1 row)
 

src/test/isolation/expected/intra-grant-inplace.out

@@ -14,15 +14,16 @@ relhasindex
 f          
 (1 row)
 
-step addk2: ALTER TABLE intra_grant_inplace ADD PRIMARY KEY (c);
+step addk2: ALTER TABLE intra_grant_inplace ADD PRIMARY KEY (c); <waiting ...>
 step c1: COMMIT;
+step addk2: <... completed>
 step read2: 
 	SELECT relhasindex FROM pg_class
 	WHERE oid = 'intra_grant_inplace'::regclass;
 
 relhasindex
 -----------
-f          
+t          
 (1 row)
 
 
@@ -58,8 +59,9 @@ relhasindex
 f          
 (1 row)
 
-step addk2: ALTER TABLE intra_grant_inplace ADD PRIMARY KEY (c);
+step addk2: ALTER TABLE intra_grant_inplace ADD PRIMARY KEY (c); <waiting ...>
 step r3: ROLLBACK;
+step addk2: <... completed>
 
 starting permutation: b2 sfnku2 addk2 c2
 step b2: BEGIN;
@@ -98,7 +100,7 @@ f
 step addk2: ALTER TABLE intra_grant_inplace ADD PRIMARY KEY (c);
 step c2: COMMIT;
 
-starting permutation: b3 sfu3 b1 grant1 read2 addk2 r3 c1 read2
+starting permutation: b3 sfu3 b1 grant1 read2 as3 addk2 r3 c1 read2
 step b3: BEGIN ISOLATION LEVEL READ COMMITTED;
 step sfu3: 
 	SELECT relhasindex FROM pg_class
@@ -122,17 +124,19 @@ relhasindex
 f          
 (1 row)
 
-step addk2: ALTER TABLE intra_grant_inplace ADD PRIMARY KEY (c);
+step as3: LOCK TABLE intra_grant_inplace IN ACCESS SHARE MODE;
+step addk2: ALTER TABLE intra_grant_inplace ADD PRIMARY KEY (c); <waiting ...>
 step r3: ROLLBACK;
 step grant1: <... completed>
 step c1: COMMIT;
+step addk2: <... completed>
 step read2: 
 	SELECT relhasindex FROM pg_class
 	WHERE oid = 'intra_grant_inplace'::regclass;
 
 relhasindex
 -----------
-f          
+t          
 (1 row)
 
 

src/test/isolation/specs/intra-grant-inplace-db.spec

@@ -42,5 +42,4 @@ step cmp3	{
 }
 
 
-# XXX extant bug
 permutation snap3 b1 grant1 vac2(c1) snap3 c1 cmp3

src/test/isolation/specs/intra-grant-inplace.spec

@@ -48,6 +48,7 @@ step sfu3	{
 	SELECT relhasindex FROM pg_class
 	WHERE oid = 'intra_grant_inplace'::regclass FOR UPDATE;
 }
+step as3	{ LOCK TABLE intra_grant_inplace IN ACCESS SHARE MODE; }
 step r3	{ ROLLBACK; }
 
 # Additional heap_update()
@@ -73,7 +74,7 @@ step keyshr5	{
 teardown	{ ROLLBACK; }
 
 
-# XXX extant bugs: permutation comments refer to planned post-bugfix behavior
+# XXX extant bugs: permutation comments refer to planned future LockTuple()
 
 permutation
 	b1
@@ -117,6 +118,7 @@ permutation
 	b1
 	grant1(r3)	# acquire LockTuple(), await sfu3 xmax
 	read2
+	as3			# XXX temporary until patch adds locking to addk2
 	addk2(c1)	# block in LockTuple() behind grant1
 	r3			# unblock grant1; addk2 now awaits grant1 xmax
 	c1