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nmischnmisch
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amcheck: Distinguish interrupted page deletion from corruption.
This prevents false-positive reports about "the first child of leftmost target page is not leftmost of its level", "block %u is not leftmost" and "left link/right link pair". They appeared if amcheck ran before VACUUM cleaned things, after a cluster exited recovery between the first-stage and second-stage WAL records of a deletion. Back-patch to v11 (all supported versions). Reviewed by Peter Geoghegan. Discussion: https://postgr.es/m/20231005025232.c7.nmisch@google.com
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4 files changed

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‎contrib/amcheck/Makefile‎

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -12,6 +12,7 @@ PGFILEDESC = "amcheck - function for verifying relation integrity"
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REGRESS = check check_btree check_heap
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15+
EXTRA_INSTALL = contrib/pg_walinspect
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TAP_TESTS = 1
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1718
ifdef USE_PGXS

‎contrib/amcheck/meson.build‎

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -44,6 +44,7 @@ tests += {
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't/002_cic.pl',
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't/003_cic_2pc.pl',
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't/004_verify_nbtree_unique.pl',
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't/005_pitr.pl',
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],
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},
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}

‎contrib/amcheck/t/005_pitr.pl‎

Lines changed: 82 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,82 @@
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# Copyright (c) 2021-2023, PostgreSQL Global Development Group
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# Test integrity of intermediate states by PITR to those states
4+
use strict;
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use warnings;
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use PostgreSQL::Test::Cluster;
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use PostgreSQL::Test::Utils;
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use Test::More;
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# origin node: generate WAL records of interest.
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my $origin = PostgreSQL::Test::Cluster->new('origin');
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$origin->init(has_archiving => 1, allows_streaming => 1);
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$origin->append_conf('postgresql.conf', 'autovacuum = off');
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$origin->start;
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$origin->backup('my_backup');
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# Create a table with each of 6 PK values spanning 1/4 of a block. Delete the
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# first four, so one index leaf is eligible for deletion. Make a replication
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# slot just so pg_walinspect will always have access to later WAL.
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my $setup = <<EOSQL;
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BEGIN;
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CREATE EXTENSION amcheck;
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CREATE EXTENSION pg_walinspect;
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CREATE TABLE not_leftmost (c text STORAGE PLAIN);
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INSERT INTO not_leftmost
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SELECT repeat(n::text, database_block_size / 4)
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FROM generate_series(1,6) t(n), pg_control_init();
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ALTER TABLE not_leftmost ADD CONSTRAINT not_leftmost_pk PRIMARY KEY (c);
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DELETE FROM not_leftmost WHERE c ~ '^[1-4]';
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SELECT pg_create_physical_replication_slot('for_walinspect', true, false);
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COMMIT;
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EOSQL
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$origin->safe_psql('postgres', $setup);
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my $before_vacuum_lsn =
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$origin->safe_psql('postgres', "SELECT pg_current_wal_lsn()");
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# VACUUM to delete the aforementioned leaf page. Force an XLogFlush() by
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# dropping a permanent table. That way, the XLogReader infrastructure can
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# always see VACUUM's records, even under synchronous_commit=off. Finally,
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# find the LSN of that VACUUM's last UNLINK_PAGE record.
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my $vacuum = <<EOSQL;
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SET synchronous_commit = off;
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VACUUM (VERBOSE, INDEX_CLEANUP ON) not_leftmost;
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CREATE TABLE XLogFlush ();
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DROP TABLE XLogFlush;
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SELECT max(start_lsn)
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FROM pg_get_wal_records_info('$before_vacuum_lsn', 'FFFFFFFF/FFFFFFFF')
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WHERE resource_manager = 'Btree' AND record_type = 'UNLINK_PAGE';
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EOSQL
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my $unlink_lsn = $origin->safe_psql('postgres', $vacuum);
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$origin->stop;
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die "did not find UNLINK_PAGE record" unless $unlink_lsn;
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# replica node: amcheck at notable points in the WAL stream
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my $replica = PostgreSQL::Test::Cluster->new('replica');
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$replica->init_from_backup($origin, 'my_backup', has_restoring => 1);
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$replica->append_conf('postgresql.conf',
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"recovery_target_lsn = '$unlink_lsn'");
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$replica->append_conf('postgresql.conf', 'recovery_target_inclusive = off');
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$replica->append_conf('postgresql.conf', 'recovery_target_action = promote');
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$replica->start;
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$replica->poll_query_until('postgres', "SELECT pg_is_in_recovery() = 'f';")
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or die "Timed out while waiting for PITR promotion";
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# recovery done; run amcheck
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my $debug = "SET client_min_messages = 'debug1'";
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my ($rc, $stderr);
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$rc = $replica->psql(
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'postgres',
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"$debug; SELECT bt_index_parent_check('not_leftmost_pk', true)",
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stderr => \$stderr);
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print STDERR $stderr, "\n";
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is($rc, 0, "bt_index_parent_check passes");
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like(
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$stderr,
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qr/interrupted page deletion detected/,
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"bt_index_parent_check: interrupted page deletion detected");
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$rc = $replica->psql(
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'postgres',
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"$debug; SELECT bt_index_check('not_leftmost_pk', true)",
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stderr => \$stderr);
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print STDERR $stderr, "\n";
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is($rc, 0, "bt_index_check passes");
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done_testing();

‎contrib/amcheck/verify_nbtree.c‎

Lines changed: 79 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -157,6 +157,9 @@ static void bt_check_every_level(Relation rel, Relation heaprel,
157157
bool rootdescend, bool checkunique);
158158
static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
159159
BtreeLevel level);
160+
static bool bt_leftmost_ignoring_half_dead(BtreeCheckState *state,
161+
BlockNumber start,
162+
BTPageOpaque start_opaque);
160163
static void bt_recheck_sibling_links(BtreeCheckState *state,
161164
BlockNumber btpo_prev_from_target,
162165
BlockNumber leftcurrent);
@@ -826,7 +829,7 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
826829
*/
827830
if (state->readonly)
828831
{
829-
if (!P_LEFTMOST(opaque))
832+
if (!bt_leftmost_ignoring_half_dead(state, current, opaque))
830833
ereport(ERROR,
831834
(errcode(ERRCODE_INDEX_CORRUPTED),
832835
errmsg("block %u is not leftmost in index \"%s\"",
@@ -880,8 +883,16 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
880883
*/
881884
}
882885

883-
/* Sibling links should be in mutual agreement */
884-
if (opaque->btpo_prev != leftcurrent)
886+
/*
887+
* Sibling links should be in mutual agreement. There arises
888+
* leftcurrent == P_NONE && btpo_prev != P_NONE when the left sibling
889+
* of the parent's low-key downlink is half-dead. (A half-dead page
890+
* has no downlink from its parent.) Under heavyweight locking, the
891+
* last bt_leftmost_ignoring_half_dead() validated this btpo_prev.
892+
* Without heavyweight locking, validation of the P_NONE case remains
893+
* unimplemented.
894+
*/
895+
if (opaque->btpo_prev != leftcurrent && leftcurrent != P_NONE)
885896
bt_recheck_sibling_links(state, opaque->btpo_prev, leftcurrent);
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887898
/* Check level */
@@ -1117,6 +1128,66 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
11171128
}
11181129
}
11191130

1131+
/*
1132+
* Like P_LEFTMOST(start_opaque), but accept an arbitrarily-long chain of
1133+
* half-dead, sibling-linked pages to the left. If a half-dead page appears
1134+
* under state->readonly, the database exited recovery between the first-stage
1135+
* and second-stage WAL records of a deletion.
1136+
*/
1137+
static bool
1138+
bt_leftmost_ignoring_half_dead(BtreeCheckState *state,
1139+
BlockNumber start,
1140+
BTPageOpaque start_opaque)
1141+
{
1142+
BlockNumber reached = start_opaque->btpo_prev,
1143+
reached_from = start;
1144+
bool all_half_dead = true;
1145+
1146+
/*
1147+
* To handle the !readonly case, we'd need to accept BTP_DELETED pages and
1148+
* potentially observe nbtree/README "Page deletion and backwards scans".
1149+
*/
1150+
Assert(state->readonly);
1151+
1152+
while (reached != P_NONE && all_half_dead)
1153+
{
1154+
Page page = palloc_btree_page(state, reached);
1155+
BTPageOpaque reached_opaque = BTPageGetOpaque(page);
1156+
1157+
CHECK_FOR_INTERRUPTS();
1158+
1159+
/*
1160+
* Try to detect btpo_prev circular links. _bt_unlink_halfdead_page()
1161+
* writes that side-links will continue to point to the siblings.
1162+
* Check btpo_next for that property.
1163+
*/
1164+
all_half_dead = P_ISHALFDEAD(reached_opaque) &&
1165+
reached != start &&
1166+
reached != reached_from &&
1167+
reached_opaque->btpo_next == reached_from;
1168+
if (all_half_dead)
1169+
{
1170+
XLogRecPtr pagelsn = PageGetLSN(page);
1171+
1172+
/* pagelsn should point to an XLOG_BTREE_MARK_PAGE_HALFDEAD */
1173+
ereport(DEBUG1,
1174+
(errcode(ERRCODE_NO_DATA),
1175+
errmsg_internal("harmless interrupted page deletion detected in index \"%s\"",
1176+
RelationGetRelationName(state->rel)),
1177+
errdetail_internal("Block=%u right block=%u page lsn=%X/%X.",
1178+
reached, reached_from,
1179+
LSN_FORMAT_ARGS(pagelsn))));
1180+
1181+
reached_from = reached;
1182+
reached = reached_opaque->btpo_prev;
1183+
}
1184+
1185+
pfree(page);
1186+
}
1187+
1188+
return all_half_dead;
1189+
}
1190+
11201191
/*
11211192
* Raise an error when target page's left link does not point back to the
11221193
* previous target page, called leftcurrent here. The leftcurrent page's
@@ -1157,6 +1228,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
11571228
BlockNumber btpo_prev_from_target,
11581229
BlockNumber leftcurrent)
11591230
{
1231+
/* passing metapage to BTPageGetOpaque() would give irrelevant findings */
1232+
Assert(leftcurrent != P_NONE);
1233+
11601234
if (!state->readonly)
11611235
{
11621236
Buffer lbuf;
@@ -2235,7 +2309,8 @@ bt_child_highkey_check(BtreeCheckState *state,
22352309
opaque = BTPageGetOpaque(page);
22362310

22372311
/* The first page we visit at the level should be leftmost */
2238-
if (first && !BlockNumberIsValid(state->prevrightlink) && !P_LEFTMOST(opaque))
2312+
if (first && !BlockNumberIsValid(state->prevrightlink) &&
2313+
!bt_leftmost_ignoring_half_dead(state, blkno, opaque))
22392314
ereport(ERROR,
22402315
(errcode(ERRCODE_INDEX_CORRUPTED),
22412316
errmsg("the first child of leftmost target page is not leftmost of its level in index \"%s\"",

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