Fix WAL-logging of FSM and VM truncation.

When a relation is truncated, it is important that the FSM is truncated as
well. Otherwise, after recovery, the FSM can return a page that has been
truncated away, leading to errors like:

ERROR:  could not read block 28991 in file "base/16390/572026": read only 0
of 8192 bytes

We were using MarkBufferDirtyHint() to dirty the buffer holding the last
remaining page of the FSM, but during recovery, that might in fact not
dirty the page, and the FSM update might be lost.

To fix, use the stronger MarkBufferDirty() function. MarkBufferDirty()
requires us to do WAL-logging ourselves, to protect from a torn page, if
checksumming is enabled.

Also fix an oversight in visibilitymap_truncate: it also needs to WAL-log
when checksumming is enabled.

Analysis by Pavan Deolasee.

Discussion: <CABOikdNr5vKucqyZH9s1Mh0XebLs_jRhKv6eJfNnD2wxTn=_9A@mail.gmail.com>

src/backend/access/heap/visibilitymap.c

@@ -508,6 +508,9 @@ visibilitymap_truncate(Relation rel, BlockNumber nheapblocks)
 
 		LockBuffer(mapBuffer, BUFFER_LOCK_EXCLUSIVE);
 
+		/* NO EREPORT(ERROR) from here till changes are logged */
+		START_CRIT_SECTION();
+
 		/* Clear out the unwanted bytes. */
 		MemSet(&map[truncByte + 1], 0, MAPSIZE - (truncByte + 1));
 
@@ -523,7 +526,20 @@ visibilitymap_truncate(Relation rel, BlockNumber nheapblocks)
 		 */
 		map[truncByte] &= (1 << truncOffset) - 1;
 
+		/*
+		 * Truncation of a relation is WAL-logged at a higher-level, and we
+		 * will be called at WAL replay. But if checksums are enabled, we need
+		 * to still write a WAL record to protect against a torn page, if the
+		 * page is flushed to disk before the truncation WAL record. We cannot
+		 * use MarkBufferDirtyHint here, because that will not dirty the page
+		 * during recovery.
+		 */
 		MarkBufferDirty(mapBuffer);
+		if (!InRecovery && RelationNeedsWAL(rel) && XLogHintBitIsNeeded())
+			log_newpage_buffer(mapBuffer, false);
+
+		END_CRIT_SECTION();
+
 		UnlockReleaseBuffer(mapBuffer);
 	}
 	else

src/backend/storage/freespace/freespace.c

@@ -327,8 +327,26 @@ FreeSpaceMapTruncateRel(Relation rel, BlockNumber nblocks)
 		if (!BufferIsValid(buf))
 			return;				/* nothing to do; the FSM was already smaller */
 		LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
+
+		/* NO EREPORT(ERROR) from here till changes are logged */
+		START_CRIT_SECTION();
+
 		fsm_truncate_avail(BufferGetPage(buf), first_removed_slot);
-		MarkBufferDirtyHint(buf, false);
+
+		/*
+		 * Truncation of a relation is WAL-logged at a higher-level, and we
+		 * will be called at WAL replay. But if checksums are enabled, we need
+		 * to still write a WAL record to protect against a torn page, if the
+		 * page is flushed to disk before the truncation WAL record. We cannot
+		 * use MarkBufferDirtyHint here, because that will not dirty the page
+		 * during recovery.
+		 */
+		MarkBufferDirty(buf);
+		if (!InRecovery && RelationNeedsWAL(rel) && XLogHintBitIsNeeded())
+			log_newpage_buffer(buf, false);
+
+		END_CRIT_SECTION();
+
 		UnlockReleaseBuffer(buf);
 
 		new_nfsmblocks = fsm_logical_to_physical(first_removed_address) + 1;

src/test/recovery/t/008_fsm_truncation.pl

+# Test WAL replay of FSM changes.
+#
+# FSM changes don't normally need to be WAL-logged, except for truncation.
+# The FSM mustn't return a page that doesn't exist (anymore).
+use strict;
+use warnings;
+
+use PostgresNode;
+use TestLib;
+use Test::More tests => 1;
+
+my $node_master = get_new_node('master');
+$node_master->init(allows_streaming => 1);
+
+$node_master->append_conf('postgresql.conf', qq{
+fsync = on
+wal_level = replica
+wal_log_hints = on
+max_prepared_transactions = 5
+autovacuum = off
+});
+
+# Create a master node and its standby, initializing both with some data
+# at the same time.
+$node_master->start;
+
+$node_master->backup('master_backup');
+my $node_standby = get_new_node('standby');
+$node_standby->init_from_backup($node_master, 'master_backup',
+	has_streaming => 1);
+$node_standby->start;
+
+$node_master->psql('postgres', qq{
+create table testtab (a int, b char(100));
+insert into testtab select generate_series(1,1000), 'foo';
+insert into testtab select generate_series(1,1000), 'foo';
+delete from testtab where ctid > '(8,0)';
+});
+
+# Take a lock on the table to prevent following vacuum from truncating it
+$node_master->psql('postgres', qq{
+begin;
+lock table testtab in row share mode;
+prepare transaction 'p1';
+});
+
+# Vacuum, update FSM without truncation
+$node_master->psql('postgres', 'vacuum verbose testtab');
+
+# Force a checkpoint
+$node_master->psql('postgres', 'checkpoint');
+
+# Now do some more insert/deletes, another vacuum to ensure full-page writes
+# are done
+$node_master->psql('postgres', qq{
+insert into testtab select generate_series(1,1000), 'foo';
+delete from testtab where ctid > '(8,0)';
+vacuum verbose testtab;
+});
+
+# Ensure all buffers are now clean on the standby
+$node_standby->psql('postgres', 'checkpoint');
+
+# Release the lock, vacuum again which should lead to truncation
+$node_master->psql('postgres', qq{
+rollback prepared 'p1';
+vacuum verbose testtab;
+});
+
+$node_master->psql('postgres', 'checkpoint');
+my $until_lsn =
+	$node_master->safe_psql('postgres', "SELECT pg_current_xlog_location();");
+
+# Wait long enough for standby to receive and apply all WAL
+my $caughtup_query =
+	"SELECT '$until_lsn'::pg_lsn <= pg_last_xlog_replay_location()";
+$node_standby->poll_query_until('postgres', $caughtup_query)
+	or die "Timed out while waiting for standby to catch up";
+
+# Promote the standby
+$node_standby->promote;
+$node_standby->poll_query_until('postgres',
+	"SELECT NOT pg_is_in_recovery()")
+  or die "Timed out while waiting for promotion of standby";
+$node_standby->psql('postgres', 'checkpoint');
+
+# Restart to discard in-memory copy of FSM
+$node_standby->restart;
+
+# Insert should work on standby
+is($node_standby->psql('postgres',
+   qq{insert into testtab select generate_series(1,1000), 'foo';}),
+   0, 'INSERT succeeds with truncated relation FSM');