[CF 5438] v2 - bt_index_parent_check false alarm for indexes created concurrently

This branch was automatically generated by a robot using patches from an
email thread registered at:

https://commitfest.postgresql.org/patch/5438

The branch will be overwritten each time a new patch version is posted to
the thread, and also periodically to check for bitrot caused by changes
on the master branch.

Patch(es): https://www.postgresql.org/message-id/CANtu0og_7Jvyfu2p5PJJQZ7yNY97v5cB0zan=GR_fxu8j-_7WA@mail.gmail.com
Author(s): Michail Nikolaev, Mihail Nikalayeu

Author: Commitfest Bot

contrib/amcheck/meson.build

@@ -49,6 +49,7 @@ tests += {
       't/003_cic_2pc.pl',
       't/004_verify_nbtree_unique.pl',
       't/005_pitr.pl',
+      't/006_cic_bt_index_parent_check.pl',
     ],
   },
 }

contrib/amcheck/t/006_cic_bt_index_parent_check.pl

@@ -0,0 +1,39 @@
+# Copyright (c) 2025, PostgreSQL Global Development Group
+
+# Test bt_index_parent_check with index created with CREATE INDEX CONCURRENTLY
+use strict;
+use warnings FATAL => 'all';
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+
+use Test::More;
+
+my ($node, $result);
+
+#
+# Test set-up
+#
+$node = PostgreSQL::Test::Cluster->new('CIC_bt_index_parent_check_test');
+$node->init;
+$node->start;
+$node->safe_psql('postgres', q(CREATE EXTENSION amcheck));
+$node->safe_psql('postgres', q(CREATE TABLE tbl(i int primary key)));
+# Insert two rows into index
+$node->safe_psql('postgres', q(INSERT INTO tbl SELECT i FROM generate_series(1, 2) s(i);));
+
+# start background transaction
+my $in_progress_h = $node->background_psql('postgres');
+$in_progress_h->query_safe(q(BEGIN; SELECT pg_current_xact_id();));
+
+# delete one row from table, while background transaction is in progress
+$node->safe_psql('postgres', q(DELETE FROM tbl WHERE i = 1;));
+# create index concurrently, which will skip the deleted row
+$node->safe_psql('postgres', q(CREATE INDEX CONCURRENTLY idx ON tbl(i);));
+
+# check index using bt_index_parent_check
+$result = $node->psql('postgres', q(SELECT bt_index_parent_check('idx', heapallindexed => true)));
+is($result, '0', 'bt_index_parent_check for CIC after removed row');
+
+$in_progress_h->quit;
+done_testing();

contrib/amcheck/verify_nbtree.c

@@ -382,7 +382,6 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	BTMetaPageData *metad;
 	uint32		previouslevel;
 	BtreeLevel	current;
-	Snapshot	snapshot = SnapshotAny;
 
 	if (!readonly)
 		elog(DEBUG1, "verifying consistency of tree structure for index \"%s\"",
@@ -433,38 +432,35 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		state->heaptuplespresent = 0;
 
 		/*
-		 * Register our own snapshot in !readonly case, rather than asking
+		 * Register our own snapshot for heapallindexed, rather than asking
 		 * table_index_build_scan() to do this for us later.  This needs to
 		 * happen before index fingerprinting begins, so we can later be
 		 * certain that index fingerprinting should have reached all tuples
 		 * returned by table_index_build_scan().
 		 */
-		if (!state->readonly)
-		{
-			snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
 
-			/*
-			 * GetTransactionSnapshot() always acquires a new MVCC snapshot in
-			 * READ COMMITTED mode.  A new snapshot is guaranteed to have all
-			 * the entries it requires in the index.
-			 *
-			 * We must defend against the possibility that an old xact
-			 * snapshot was returned at higher isolation levels when that
-			 * snapshot is not safe for index scans of the target index.  This
-			 * is possible when the snapshot sees tuples that are before the
-			 * index's indcheckxmin horizon.  Throwing an error here should be
-			 * very rare.  It doesn't seem worth using a secondary snapshot to
-			 * avoid this.
-			 */
-			if (IsolationUsesXactSnapshot() && rel->rd_index->indcheckxmin &&
-				!TransactionIdPrecedes(HeapTupleHeaderGetXmin(rel->rd_indextuple->t_data),
-									   snapshot->xmin))
-				ereport(ERROR,
-						(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
-						 errmsg("index \"%s\" cannot be verified using transaction snapshot",
-								RelationGetRelationName(rel))));
-		}
-	}
+		/*
+		 * GetTransactionSnapshot() always acquires a new MVCC snapshot in
+		 * READ COMMITTED mode.  A new snapshot is guaranteed to have all
+		 * the entries it requires in the index.
+		 *
+		 * We must defend against the possibility that an old xact
+		 * snapshot was returned at higher isolation levels when that
+		 * snapshot is not safe for index scans of the target index.  This
+		 * is possible when the snapshot sees tuples that are before the
+		 * index's indcheckxmin horizon.  Throwing an error here should be
+		 * very rare.  It doesn't seem worth using a secondary snapshot to
+		 * avoid this.
+		 */
+		if (IsolationUsesXactSnapshot() && rel->rd_index->indcheckxmin &&
+			!TransactionIdPrecedes(HeapTupleHeaderGetXmin(rel->rd_indextuple->t_data),
+								   state->snapshot->xmin))
+			ereport(ERROR,
+					(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
+					 errmsg("index \"%s\" cannot be verified using transaction snapshot",
+							RelationGetRelationName(rel))));
+}
 
 	/*
 	 * We need a snapshot to check the uniqueness of the index. For better
@@ -476,9 +472,7 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		state->indexinfo = BuildIndexInfo(state->rel);
 		if (state->indexinfo->ii_Unique)
 		{
-			if (snapshot != SnapshotAny)
-				state->snapshot = snapshot;
-			else
+			if (state->snapshot == InvalidSnapshot)
 				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
 		}
 	}
@@ -555,21 +549,20 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		/*
 		 * Create our own scan for table_index_build_scan(), rather than
 		 * getting it to do so for us.  This is required so that we can
-		 * actually use the MVCC snapshot registered earlier in !readonly
-		 * case.
+		 * actually use the MVCC snapshot registered earlier.
 		 *
 		 * Note that table_index_build_scan() calls heap_endscan() for us.
 		 */
 		scan = table_beginscan_strat(state->heaprel,	/* relation */
-									 snapshot,	/* snapshot */
+									 state->snapshot,	/* snapshot */
 									 0, /* number of keys */
 									 NULL,	/* scan key */
 									 true,	/* buffer access strategy OK */
 									 true); /* syncscan OK? */
 
 		/*
 		 * Scan will behave as the first scan of a CREATE INDEX CONCURRENTLY
-		 * behaves in !readonly case.
+		 * behaves.
 		 *
 		 * It's okay that we don't actually use the same lock strength for the
 		 * heap relation as any other ii_Concurrent caller would in !readonly
@@ -578,7 +571,7 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		 * that needs to be sure that there was no concurrent recycling of
 		 * TIDs.
 		 */
-		indexinfo->ii_Concurrent = !state->readonly;
+		indexinfo->ii_Concurrent = true;
 
 		/*
 		 * Don't wait for uncommitted tuple xact commit/abort when index is a
@@ -602,14 +595,11 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 								 state->heaptuplespresent, RelationGetRelationName(heaprel),
 								 100.0 * bloom_prop_bits_set(state->filter))));
 
-		if (snapshot != SnapshotAny)
-			UnregisterSnapshot(snapshot);
-
 		bloom_free(state->filter);
 	}
 
 	/* Be tidy: */
-	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+	if (state->snapshot != InvalidSnapshot)
 		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }