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

contrib/amcheck/Makefile

@@ -12,6 +12,7 @@ PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
 
+EXTRA_INSTALL = contrib/pg_walinspect
 TAP_TESTS = 1
 
 ifdef USE_PGXS

contrib/amcheck/t/005_pitr.pl

@@ -0,0 +1,83 @@
+# Copyright (c) 2021-2023, PostgreSQL Global Development Group
+
+# Test integrity of intermediate states by PITR to those states
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# origin node: generate WAL records of interest.
+my $origin = PostgreSQL::Test::Cluster->new('origin');
+$origin->init(has_archiving => 1, allows_streaming => 1);
+$origin->append_conf('postgresql.conf', 'autovacuum = off');
+$origin->start;
+$origin->backup('my_backup');
+# Create a table with each of 6 PK values spanning 1/4 of a block.  Delete the
+# first four, so one index leaf is eligible for deletion.  Make a replication
+# slot just so pg_walinspect will always have access to later WAL.
+my $setup = <<EOSQL;
+BEGIN;
+CREATE EXTENSION amcheck;
+CREATE EXTENSION pg_walinspect;
+CREATE TABLE not_leftmost (c text);
+ALTER TABLE not_leftmost ALTER c SET STORAGE PLAIN;
+INSERT INTO not_leftmost
+  SELECT repeat(n::text, database_block_size / 4)
+  FROM generate_series(1,6) t(n), pg_control_init();
+ALTER TABLE not_leftmost ADD CONSTRAINT not_leftmost_pk PRIMARY KEY (c);
+DELETE FROM not_leftmost WHERE c ~ '^[1-4]';
+SELECT pg_create_physical_replication_slot('for_walinspect', true, false);
+COMMIT;
+EOSQL
+$origin->safe_psql('postgres', $setup);
+my $before_vacuum_lsn =
+  $origin->safe_psql('postgres', "SELECT pg_current_wal_lsn()");
+# VACUUM to delete the aforementioned leaf page.  Force an XLogFlush() by
+# dropping a permanent table.  That way, the XLogReader infrastructure can
+# always see VACUUM's records, even under synchronous_commit=off.  Finally,
+# find the LSN of that VACUUM's last UNLINK_PAGE record.
+my $vacuum = <<EOSQL;
+SET synchronous_commit = off;
+VACUUM (VERBOSE, INDEX_CLEANUP ON) not_leftmost;
+CREATE TABLE XLogFlush ();
+DROP TABLE XLogFlush;
+SELECT max(start_lsn)
+  FROM pg_get_wal_records_info('$before_vacuum_lsn', pg_current_wal_flush_lsn())
+  WHERE resource_manager = 'Btree' AND record_type = 'UNLINK_PAGE';
+EOSQL
+my $unlink_lsn = $origin->safe_psql('postgres', $vacuum);
+$origin->stop;
+die "did not find UNLINK_PAGE record" unless $unlink_lsn;
+
+# replica node: amcheck at notable points in the WAL stream
+my $replica = PostgreSQL::Test::Cluster->new('replica');
+$replica->init_from_backup($origin, 'my_backup', has_restoring => 1);
+$replica->append_conf('postgresql.conf',
+	"recovery_target_lsn = '$unlink_lsn'");
+$replica->append_conf('postgresql.conf', 'recovery_target_inclusive = off');
+$replica->append_conf('postgresql.conf', 'recovery_target_action = promote');
+$replica->start;
+$replica->poll_query_until('postgres', "SELECT pg_is_in_recovery() = 'f';")
+  or die "Timed out while waiting for PITR promotion";
+# recovery done; run amcheck
+my $debug = "SET client_min_messages = 'debug1'";
+my ($rc, $stderr);
+$rc = $replica->psql(
+	'postgres',
+	"$debug; SELECT bt_index_parent_check('not_leftmost_pk', true)",
+	stderr => \$stderr);
+print STDERR $stderr, "\n";
+is($rc, 0, "bt_index_parent_check passes");
+like(
+	$stderr,
+	qr/interrupted page deletion detected/,
+	"bt_index_parent_check: interrupted page deletion detected");
+$rc = $replica->psql(
+	'postgres',
+	"$debug; SELECT bt_index_check('not_leftmost_pk', true)",
+	stderr => \$stderr);
+print STDERR $stderr, "\n";
+is($rc, 0, "bt_index_check passes");
+
+done_testing();

contrib/amcheck/verify_nbtree.c

@@ -147,6 +147,9 @@ static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool rootdescend);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
+static bool bt_leftmost_ignoring_half_dead(BtreeCheckState *state,
+										   BlockNumber start,
+										   BTPageOpaque start_opaque);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
@@ -775,7 +778,7 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
 			 */
 			if (state->readonly)
 			{
-				if (!P_LEFTMOST(opaque))
+				if (!bt_leftmost_ignoring_half_dead(state, current, opaque))
 					ereport(ERROR,
 							(errcode(ERRCODE_INDEX_CORRUPTED),
 							 errmsg("block %u is not leftmost in index \"%s\"",
@@ -829,8 +832,16 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
 			 */
 		}
 
-		/* Sibling links should be in mutual agreement */
+		/*
-		if (opaque->btpo_prev != leftcurrent)
+		 * Sibling links should be in mutual agreement.  There arises
+		 * leftcurrent == P_NONE && btpo_prev != P_NONE when the left sibling
+		 * of the parent's low-key downlink is half-dead.  (A half-dead page
+		 * has no downlink from its parent.)  Under heavyweight locking, the
+		 * last bt_leftmost_ignoring_half_dead() validated this btpo_prev.
+		 * Without heavyweight locking, validation of the P_NONE case remains
+		 * unimplemented.
+		 */
+		if (opaque->btpo_prev != leftcurrent && leftcurrent != P_NONE)
 			bt_recheck_sibling_links(state, opaque->btpo_prev, leftcurrent);
 
 		/* Check level */
@@ -911,6 +922,66 @@ nextpage:
 	return nextleveldown;
 }
 
+/*
+ * Like P_LEFTMOST(start_opaque), but accept an arbitrarily-long chain of
+ * half-dead, sibling-linked pages to the left.  If a half-dead page appears
+ * under state->readonly, the database exited recovery between the first-stage
+ * and second-stage WAL records of a deletion.
+ */
+static bool
+bt_leftmost_ignoring_half_dead(BtreeCheckState *state,
+							   BlockNumber start,
+							   BTPageOpaque start_opaque)
+{
+	BlockNumber reached = start_opaque->btpo_prev,
+				reached_from = start;
+	bool		all_half_dead = true;
+
+	/*
+	 * To handle the !readonly case, we'd need to accept BTP_DELETED pages and
+	 * potentially observe nbtree/README "Page deletion and backwards scans".
+	 */
+	Assert(state->readonly);
+
+	while (reached != P_NONE && all_half_dead)
+	{
+		Page		page = palloc_btree_page(state, reached);
+		BTPageOpaque reached_opaque = BTPageGetOpaque(page);
+
+		CHECK_FOR_INTERRUPTS();
+
+		/*
+		 * Try to detect btpo_prev circular links.  _bt_unlink_halfdead_page()
+		 * writes that side-links will continue to point to the siblings.
+		 * Check btpo_next for that property.
+		 */
+		all_half_dead = P_ISHALFDEAD(reached_opaque) &&
+			reached != start &&
+			reached != reached_from &&
+			reached_opaque->btpo_next == reached_from;
+		if (all_half_dead)
+		{
+			XLogRecPtr	pagelsn = PageGetLSN(page);
+
+			/* pagelsn should point to an XLOG_BTREE_MARK_PAGE_HALFDEAD */
+			ereport(DEBUG1,
+					(errcode(ERRCODE_NO_DATA),
+					 errmsg_internal("harmless interrupted page deletion detected in index \"%s\"",
+									 RelationGetRelationName(state->rel)),
+					 errdetail_internal("Block=%u right block=%u page lsn=%X/%X.",
+										reached, reached_from,
+										LSN_FORMAT_ARGS(pagelsn))));
+
+			reached_from = reached;
+			reached = reached_opaque->btpo_prev;
+		}
+
+		pfree(page);
+	}
+
+	return all_half_dead;
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -951,6 +1022,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
 						 BlockNumber btpo_prev_from_target,
 						 BlockNumber leftcurrent)
 {
+	/* passing metapage to BTPageGetOpaque() would give irrelevant findings */
+	Assert(leftcurrent != P_NONE);
+
 	if (!state->readonly)
 	{
 		Buffer		lbuf;
@@ -1934,7 +2008,8 @@ bt_child_highkey_check(BtreeCheckState *state,
 		opaque = BTPageGetOpaque(page);
 
 		/* The first page we visit at the level should be leftmost */
-		if (first && !BlockNumberIsValid(state->prevrightlink) && !P_LEFTMOST(opaque))
+		if (first && !BlockNumberIsValid(state->prevrightlink) &&
+			!bt_leftmost_ignoring_half_dead(state, blkno, opaque))
 			ereport(ERROR,
 					(errcode(ERRCODE_INDEX_CORRUPTED),
 					 errmsg("the first child of leftmost target page is not leftmost of its level in index \"%s\"",