Adding upstream version 14.5.upstream/14.5upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

18 files changed, 6288 insertions, 0 deletions

diff --git a/contrib/amcheck/.gitignore b/contrib/amcheck/.gitignore
new file mode 100644
index 0000000..5dcb3ff
--- /dev/null
+++ b/contrib/amcheck/.gitignore
@@ -0,0 +1,4 @@
+# Generated subdirectories
+/log/
+/results/
+/tmp_check/
diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
new file mode 100644
index 0000000..b82f221
--- /dev/null
+++ b/contrib/amcheck/Makefile
@@ -0,0 +1,26 @@
+# contrib/amcheck/Makefile
+
+MODULE_big	= amcheck
+OBJS = \
+	$(WIN32RES) \
+	verify_heapam.o \
+	verify_nbtree.o
+
+EXTENSION = amcheck
+DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+PGFILEDESC = "amcheck - function for verifying relation integrity"
+
+REGRESS = check check_btree check_heap
+
+TAP_TESTS = 1
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = contrib/amcheck
+top_builddir = ../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif
diff --git a/contrib/amcheck/amcheck--1.0--1.1.sql b/contrib/amcheck/amcheck--1.0--1.1.sql
new file mode 100644
index 0000000..088416e
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.0--1.1.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.0--1.1.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.1'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.1,
+-- create new, overloaded versions of the 1.0 functions
+
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- Don't want these to be available to public
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck--1.0.sql b/contrib/amcheck/amcheck--1.0.sql
new file mode 100644
index 0000000..a6612d1
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.0.sql
@@ -0,0 +1,24 @@
+/* contrib/amcheck/amcheck--1.0.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION amcheck" to load this file. \quit
+
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- Don't want these to be available to public
+REVOKE ALL ON FUNCTION bt_index_check(regclass) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck--1.1--1.2.sql b/contrib/amcheck/amcheck--1.1--1.2.sql
new file mode 100644
index 0000000..883530d
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.1--1.2.sql
@@ -0,0 +1,19 @@
+/* contrib/amcheck/amcheck--1.1--1.2.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.2'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.2,
+-- create new, overloaded version of the 1.1 function signature
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- Don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck--1.2--1.3.sql b/contrib/amcheck/amcheck--1.2--1.3.sql
new file mode 100644
index 0000000..7237ab7
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.2--1.3.sql
@@ -0,0 +1,30 @@
+/* contrib/amcheck/amcheck--1.2--1.3.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.3'" to load this file. \quit
+
+--
+-- verify_heapam()
+--
+CREATE FUNCTION verify_heapam(relation regclass,
+							  on_error_stop boolean default false,
+							  check_toast boolean default false,
+							  skip text default 'none',
+							  startblock bigint default null,
+							  endblock bigint default null,
+							  blkno OUT bigint,
+							  offnum OUT integer,
+							  attnum OUT integer,
+							  msg OUT text)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'verify_heapam'
+LANGUAGE C;
+
+-- Don't want this to be available to public
+REVOKE ALL ON FUNCTION verify_heapam(regclass,
+									 boolean,
+									 boolean,
+									 text,
+									 bigint,
+									 bigint)
+FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
new file mode 100644
index 0000000..ab50931
--- /dev/null
+++ b/contrib/amcheck/amcheck.control
@@ -0,0 +1,5 @@
+# amcheck extension
+comment = 'functions for verifying relation integrity'
+default_version = '1.3'
+module_pathname = '$libdir/amcheck'
+relocatable = true
diff --git a/contrib/amcheck/expected/check.out b/contrib/amcheck/expected/check.out
new file mode 100644
index 0000000..5b3e6d5
--- /dev/null
+++ b/contrib/amcheck/expected/check.out
@@ -0,0 +1 @@
+CREATE EXTENSION amcheck;
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
new file mode 100644
index 0000000..38791bb
--- /dev/null
+++ b/contrib/amcheck/expected/check_btree.out
@@ -0,0 +1,210 @@
+CREATE TABLE bttest_a(id int8);
+CREATE TABLE bttest_b(id int8);
+CREATE TABLE bttest_multi(id int8, data int8);
+CREATE TABLE delete_test_table (a bigint, b bigint, c bigint, d bigint);
+-- Stabalize tests
+ALTER TABLE bttest_a SET (autovacuum_enabled = false);
+ALTER TABLE bttest_b SET (autovacuum_enabled = false);
+ALTER TABLE bttest_multi SET (autovacuum_enabled = false);
+ALTER TABLE delete_test_table SET (autovacuum_enabled = false);
+INSERT INTO bttest_a SELECT * FROM generate_series(1, 100000);
+INSERT INTO bttest_b SELECT * FROM generate_series(100000, 1, -1);
+INSERT INTO bttest_multi SELECT i, i%2  FROM generate_series(1, 100000) as i;
+CREATE INDEX bttest_a_idx ON bttest_a USING btree (id) WITH (deduplicate_items = ON);
+CREATE INDEX bttest_b_idx ON bttest_b USING btree (id);
+CREATE UNIQUE INDEX bttest_multi_idx ON bttest_multi
+USING btree (id) INCLUDE (data);
+CREATE ROLE regress_bttest_role;
+-- verify permissions are checked (error due to function not callable)
+SET ROLE regress_bttest_role;
+SELECT bt_index_check('bttest_a_idx'::regclass);
+ERROR:  permission denied for function bt_index_check
+SELECT bt_index_parent_check('bttest_a_idx'::regclass);
+ERROR:  permission denied for function bt_index_parent_check
+RESET ROLE;
+-- we, intentionally, don't check relation permissions - it's useful
+-- to run this cluster-wide with a restricted account, and as tested
+-- above explicit permission has to be granted for that.
+GRANT EXECUTE ON FUNCTION bt_index_check(regclass) TO regress_bttest_role;
+GRANT EXECUTE ON FUNCTION bt_index_parent_check(regclass) TO regress_bttest_role;
+GRANT EXECUTE ON FUNCTION bt_index_check(regclass, boolean) TO regress_bttest_role;
+GRANT EXECUTE ON FUNCTION bt_index_parent_check(regclass, boolean) TO regress_bttest_role;
+SET ROLE regress_bttest_role;
+SELECT bt_index_check('bttest_a_idx');
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx');
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+RESET ROLE;
+-- verify plain tables are rejected (error)
+SELECT bt_index_check('bttest_a');
+ERROR:  "bttest_a" is not an index
+SELECT bt_index_parent_check('bttest_a');
+ERROR:  "bttest_a" is not an index
+-- verify non-existing indexes are rejected (error)
+SELECT bt_index_check(17);
+ERROR:  could not open relation with OID 17
+SELECT bt_index_parent_check(17);
+ERROR:  could not open relation with OID 17
+-- verify wrong index types are rejected (error)
+BEGIN;
+CREATE INDEX bttest_a_brin_idx ON bttest_a USING brin(id);
+SELECT bt_index_parent_check('bttest_a_brin_idx');
+ERROR:  only B-Tree indexes are supported as targets for verification
+DETAIL:  Relation "bttest_a_brin_idx" is not a B-Tree index.
+ROLLBACK;
+-- normal check outside of xact
+SELECT bt_index_check('bttest_a_idx');
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+-- more expansive tests
+SELECT bt_index_check('bttest_a_idx', true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+BEGIN;
+SELECT bt_index_check('bttest_a_idx');
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx');
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- make sure we don't have any leftover locks
+SELECT * FROM pg_locks
+WHERE relation = ANY(ARRAY['bttest_a', 'bttest_a_idx', 'bttest_b', 'bttest_b_idx']::regclass[])
+    AND pid = pg_backend_pid();
+ locktype | database | relation | page | tuple | virtualxid | transactionid | classid | objid | objsubid | virtualtransaction | pid | mode | granted | fastpath | waitstart 
+----------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+-----+------+---------+----------+-----------
+(0 rows)
+
+COMMIT;
+-- Deduplication
+TRUNCATE bttest_a;
+INSERT INTO bttest_a SELECT 42 FROM generate_series(1, 2000);
+SELECT bt_index_check('bttest_a_idx', true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+-- normal check outside of xact for index with included columns
+SELECT bt_index_check('bttest_multi_idx');
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+-- more expansive tests for index with included columns
+SELECT bt_index_parent_check('bttest_multi_idx', true, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- repeat expansive tests for index built using insertions
+TRUNCATE bttest_multi;
+INSERT INTO bttest_multi SELECT i, i%2  FROM generate_series(1, 100000) as i;
+SELECT bt_index_parent_check('bttest_multi_idx', true, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+--
+-- Test for multilevel page deletion/downlink present checks, and rootdescend
+-- checks
+--
+INSERT INTO delete_test_table SELECT i, 1, 2, 3 FROM generate_series(1,80000) i;
+ALTER TABLE delete_test_table ADD PRIMARY KEY (a,b,c,d);
+-- Delete most entries, and vacuum, deleting internal pages and creating "fast
+-- root"
+DELETE FROM delete_test_table WHERE a < 79990;
+VACUUM delete_test_table;
+SELECT bt_index_parent_check('delete_test_table_pkey', true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+--
+-- BUG #15597: must not assume consistent input toasting state when forming
+-- tuple.  Bloom filter must fingerprint normalized index tuple representation.
+--
+CREATE TABLE toast_bug(buggy text);
+ALTER TABLE toast_bug ALTER COLUMN buggy SET STORAGE extended;
+CREATE INDEX toasty ON toast_bug(buggy);
+-- pg_attribute entry for toasty.buggy (the index) will have plain storage:
+UPDATE pg_attribute SET attstorage = 'p'
+WHERE attrelid = 'toasty'::regclass AND attname = 'buggy';
+-- Whereas pg_attribute entry for toast_bug.buggy (the table) still has extended storage:
+SELECT attstorage FROM pg_attribute
+WHERE attrelid = 'toast_bug'::regclass AND attname = 'buggy';
+ attstorage 
+------------
+ x
+(1 row)
+
+-- Insert compressible heap tuple (comfortably exceeds TOAST_TUPLE_THRESHOLD):
+INSERT INTO toast_bug SELECT repeat('a', 2200);
+-- Should not get false positive report of corruption:
+SELECT bt_index_check('toasty', true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+--
+-- Check that index expressions and predicates are run as the table's owner
+--
+TRUNCATE bttest_a;
+INSERT INTO bttest_a SELECT * FROM generate_series(1, 1000);
+ALTER TABLE bttest_a OWNER TO regress_bttest_role;
+-- A dummy index function checking current_user
+CREATE FUNCTION ifun(int8) RETURNS int8 AS $$
+BEGIN
+	ASSERT current_user = 'regress_bttest_role',
+		format('ifun(%s) called by %s', $1, current_user);
+	RETURN $1;
+END;
+$$ LANGUAGE plpgsql IMMUTABLE;
+CREATE INDEX bttest_a_expr_idx ON bttest_a ((ifun(id) + ifun(0)))
+	WHERE ifun(id + 10) > ifun(10);
+SELECT bt_index_check('bttest_a_expr_idx', true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+-- cleanup
+DROP TABLE bttest_a;
+DROP TABLE bttest_b;
+DROP TABLE bttest_multi;
+DROP TABLE delete_test_table;
+DROP TABLE toast_bug;
+DROP FUNCTION ifun(int8);
+DROP OWNED BY regress_bttest_role; -- permissions
+DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/expected/check_heap.out b/contrib/amcheck/expected/check_heap.out
new file mode 100644
index 0000000..1fb3823
--- /dev/null
+++ b/contrib/amcheck/expected/check_heap.out
@@ -0,0 +1,194 @@
+CREATE TABLE heaptest (a integer, b text);
+REVOKE ALL ON heaptest FROM PUBLIC;
+-- Check that invalid skip option is rejected
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'rope');
+ERROR:  invalid skip option
+HINT:  Valid skip options are "all-visible", "all-frozen", and "none".
+-- Check specifying invalid block ranges when verifying an empty table
+SELECT * FROM verify_heapam(relation := 'heaptest', startblock := 0, endblock := 0);
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', startblock := 5, endblock := 8);
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+-- Check that valid options are not rejected nor corruption reported
+-- for an empty table, and that skip enum-like parameter is case-insensitive
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'none');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'all-frozen');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'all-visible');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'None');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'All-Frozen');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'All-Visible');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'NONE');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'ALL-FROZEN');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'ALL-VISIBLE');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+-- Add some data so subsequent tests are not entirely trivial
+INSERT INTO heaptest (a, b)
+	(SELECT gs, repeat('x', gs)
+		FROM generate_series(1,50) gs);
+-- Check that valid options are not rejected nor corruption reported
+-- for a non-empty table
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'none');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'all-frozen');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'all-visible');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', startblock := 0, endblock := 0);
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+CREATE ROLE regress_heaptest_role;
+-- verify permissions are checked (error due to function not callable)
+SET ROLE regress_heaptest_role;
+SELECT * FROM verify_heapam(relation := 'heaptest');
+ERROR:  permission denied for function verify_heapam
+RESET ROLE;
+GRANT EXECUTE ON FUNCTION verify_heapam(regclass, boolean, boolean, text, bigint, bigint) TO regress_heaptest_role;
+-- verify permissions are now sufficient
+SET ROLE regress_heaptest_role;
+SELECT * FROM verify_heapam(relation := 'heaptest');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+RESET ROLE;
+-- Check specifying invalid block ranges when verifying a non-empty table.
+SELECT * FROM verify_heapam(relation := 'heaptest', startblock := 0, endblock := 10000);
+ERROR:  ending block number must be between 0 and 0
+SELECT * FROM verify_heapam(relation := 'heaptest', startblock := 10000, endblock := 11000);
+ERROR:  starting block number must be between 0 and 0
+-- Vacuum freeze to change the xids encountered in subsequent tests
+VACUUM (FREEZE, DISABLE_PAGE_SKIPPING) heaptest;
+-- Check that valid options are not rejected nor corruption reported
+-- for a non-empty frozen table
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'none');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'all-frozen');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'all-visible');
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+SELECT * FROM verify_heapam(relation := 'heaptest', startblock := 0, endblock := 0);
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+-- Check that partitioned tables (the parent ones) which don't have visibility
+-- maps are rejected
+CREATE TABLE test_partitioned (a int, b text default repeat('x', 5000))
+			 PARTITION BY list (a);
+SELECT * FROM verify_heapam('test_partitioned',
+							startblock := NULL,
+							endblock := NULL);
+ERROR:  "test_partitioned" is not a table, materialized view, or TOAST table
+-- Check that valid options are not rejected nor corruption reported
+-- for an empty partition table (the child one)
+CREATE TABLE test_partition partition OF test_partitioned FOR VALUES IN (1);
+SELECT * FROM verify_heapam('test_partition',
+							startblock := NULL,
+							endblock := NULL);
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+-- Check that valid options are not rejected nor corruption reported
+-- for a non-empty partition table (the child one)
+INSERT INTO test_partitioned (a) (SELECT 1 FROM generate_series(1,1000) gs);
+SELECT * FROM verify_heapam('test_partition',
+							startblock := NULL,
+							endblock := NULL);
+ blkno | offnum | attnum | msg 
+-------+--------+--------+-----
+(0 rows)
+
+-- Check that indexes are rejected
+CREATE INDEX test_index ON test_partition (a);
+SELECT * FROM verify_heapam('test_index',
+							startblock := NULL,
+							endblock := NULL);
+ERROR:  "test_index" is not a table, materialized view, or TOAST table
+-- Check that views are rejected
+CREATE VIEW test_view AS SELECT 1;
+SELECT * FROM verify_heapam('test_view',
+							startblock := NULL,
+							endblock := NULL);
+ERROR:  "test_view" is not a table, materialized view, or TOAST table
+-- Check that sequences are rejected
+CREATE SEQUENCE test_sequence;
+SELECT * FROM verify_heapam('test_sequence',
+							startblock := NULL,
+							endblock := NULL);
+ERROR:  "test_sequence" is not a table, materialized view, or TOAST table
+-- Check that foreign tables are rejected
+CREATE FOREIGN DATA WRAPPER dummy;
+CREATE SERVER dummy_server FOREIGN DATA WRAPPER dummy;
+CREATE FOREIGN TABLE test_foreign_table () SERVER dummy_server;
+SELECT * FROM verify_heapam('test_foreign_table',
+							startblock := NULL,
+							endblock := NULL);
+ERROR:  "test_foreign_table" is not a table, materialized view, or TOAST table
+-- cleanup
+DROP TABLE heaptest;
+DROP TABLE test_partition;
+DROP TABLE test_partitioned;
+DROP OWNED BY regress_heaptest_role; -- permissions
+DROP ROLE regress_heaptest_role;
diff --git a/contrib/amcheck/sql/check.sql b/contrib/amcheck/sql/check.sql
new file mode 100644
index 0000000..5b3e6d5
--- /dev/null
+++ b/contrib/amcheck/sql/check.sql
@@ -0,0 +1 @@
+CREATE EXTENSION amcheck;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
new file mode 100644
index 0000000..033c04b
--- /dev/null
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -0,0 +1,146 @@
+CREATE TABLE bttest_a(id int8);
+CREATE TABLE bttest_b(id int8);
+CREATE TABLE bttest_multi(id int8, data int8);
+CREATE TABLE delete_test_table (a bigint, b bigint, c bigint, d bigint);
+
+-- Stabalize tests
+ALTER TABLE bttest_a SET (autovacuum_enabled = false);
+ALTER TABLE bttest_b SET (autovacuum_enabled = false);
+ALTER TABLE bttest_multi SET (autovacuum_enabled = false);
+ALTER TABLE delete_test_table SET (autovacuum_enabled = false);
+
+INSERT INTO bttest_a SELECT * FROM generate_series(1, 100000);
+INSERT INTO bttest_b SELECT * FROM generate_series(100000, 1, -1);
+INSERT INTO bttest_multi SELECT i, i%2  FROM generate_series(1, 100000) as i;
+
+CREATE INDEX bttest_a_idx ON bttest_a USING btree (id) WITH (deduplicate_items = ON);
+CREATE INDEX bttest_b_idx ON bttest_b USING btree (id);
+CREATE UNIQUE INDEX bttest_multi_idx ON bttest_multi
+USING btree (id) INCLUDE (data);
+
+CREATE ROLE regress_bttest_role;
+
+-- verify permissions are checked (error due to function not callable)
+SET ROLE regress_bttest_role;
+SELECT bt_index_check('bttest_a_idx'::regclass);
+SELECT bt_index_parent_check('bttest_a_idx'::regclass);
+RESET ROLE;
+
+-- we, intentionally, don't check relation permissions - it's useful
+-- to run this cluster-wide with a restricted account, and as tested
+-- above explicit permission has to be granted for that.
+GRANT EXECUTE ON FUNCTION bt_index_check(regclass) TO regress_bttest_role;
+GRANT EXECUTE ON FUNCTION bt_index_parent_check(regclass) TO regress_bttest_role;
+GRANT EXECUTE ON FUNCTION bt_index_check(regclass, boolean) TO regress_bttest_role;
+GRANT EXECUTE ON FUNCTION bt_index_parent_check(regclass, boolean) TO regress_bttest_role;
+SET ROLE regress_bttest_role;
+SELECT bt_index_check('bttest_a_idx');
+SELECT bt_index_parent_check('bttest_a_idx');
+RESET ROLE;
+
+-- verify plain tables are rejected (error)
+SELECT bt_index_check('bttest_a');
+SELECT bt_index_parent_check('bttest_a');
+
+-- verify non-existing indexes are rejected (error)
+SELECT bt_index_check(17);
+SELECT bt_index_parent_check(17);
+
+-- verify wrong index types are rejected (error)
+BEGIN;
+CREATE INDEX bttest_a_brin_idx ON bttest_a USING brin(id);
+SELECT bt_index_parent_check('bttest_a_brin_idx');
+ROLLBACK;
+
+-- normal check outside of xact
+SELECT bt_index_check('bttest_a_idx');
+-- more expansive tests
+SELECT bt_index_check('bttest_a_idx', true);
+SELECT bt_index_parent_check('bttest_b_idx', true);
+
+BEGIN;
+SELECT bt_index_check('bttest_a_idx');
+SELECT bt_index_parent_check('bttest_b_idx');
+-- make sure we don't have any leftover locks
+SELECT * FROM pg_locks
+WHERE relation = ANY(ARRAY['bttest_a', 'bttest_a_idx', 'bttest_b', 'bttest_b_idx']::regclass[])
+    AND pid = pg_backend_pid();
+COMMIT;
+
+-- Deduplication
+TRUNCATE bttest_a;
+INSERT INTO bttest_a SELECT 42 FROM generate_series(1, 2000);
+SELECT bt_index_check('bttest_a_idx', true);
+
+-- normal check outside of xact for index with included columns
+SELECT bt_index_check('bttest_multi_idx');
+-- more expansive tests for index with included columns
+SELECT bt_index_parent_check('bttest_multi_idx', true, true);
+
+-- repeat expansive tests for index built using insertions
+TRUNCATE bttest_multi;
+INSERT INTO bttest_multi SELECT i, i%2  FROM generate_series(1, 100000) as i;
+SELECT bt_index_parent_check('bttest_multi_idx', true, true);
+
+--
+-- Test for multilevel page deletion/downlink present checks, and rootdescend
+-- checks
+--
+INSERT INTO delete_test_table SELECT i, 1, 2, 3 FROM generate_series(1,80000) i;
+ALTER TABLE delete_test_table ADD PRIMARY KEY (a,b,c,d);
+-- Delete most entries, and vacuum, deleting internal pages and creating "fast
+-- root"
+DELETE FROM delete_test_table WHERE a < 79990;
+VACUUM delete_test_table;
+SELECT bt_index_parent_check('delete_test_table_pkey', true);
+
+--
+-- BUG #15597: must not assume consistent input toasting state when forming
+-- tuple.  Bloom filter must fingerprint normalized index tuple representation.
+--
+CREATE TABLE toast_bug(buggy text);
+ALTER TABLE toast_bug ALTER COLUMN buggy SET STORAGE extended;
+CREATE INDEX toasty ON toast_bug(buggy);
+
+-- pg_attribute entry for toasty.buggy (the index) will have plain storage:
+UPDATE pg_attribute SET attstorage = 'p'
+WHERE attrelid = 'toasty'::regclass AND attname = 'buggy';
+
+-- Whereas pg_attribute entry for toast_bug.buggy (the table) still has extended storage:
+SELECT attstorage FROM pg_attribute
+WHERE attrelid = 'toast_bug'::regclass AND attname = 'buggy';
+
+-- Insert compressible heap tuple (comfortably exceeds TOAST_TUPLE_THRESHOLD):
+INSERT INTO toast_bug SELECT repeat('a', 2200);
+-- Should not get false positive report of corruption:
+SELECT bt_index_check('toasty', true);
+
+--
+-- Check that index expressions and predicates are run as the table's owner
+--
+TRUNCATE bttest_a;
+INSERT INTO bttest_a SELECT * FROM generate_series(1, 1000);
+ALTER TABLE bttest_a OWNER TO regress_bttest_role;
+-- A dummy index function checking current_user
+CREATE FUNCTION ifun(int8) RETURNS int8 AS $$
+BEGIN
+	ASSERT current_user = 'regress_bttest_role',
+		format('ifun(%s) called by %s', $1, current_user);
+	RETURN $1;
+END;
+$$ LANGUAGE plpgsql IMMUTABLE;
+
+CREATE INDEX bttest_a_expr_idx ON bttest_a ((ifun(id) + ifun(0)))
+	WHERE ifun(id + 10) > ifun(10);
+
+SELECT bt_index_check('bttest_a_expr_idx', true);
+
+-- cleanup
+DROP TABLE bttest_a;
+DROP TABLE bttest_b;
+DROP TABLE bttest_multi;
+DROP TABLE delete_test_table;
+DROP TABLE toast_bug;
+DROP FUNCTION ifun(int8);
+DROP OWNED BY regress_bttest_role; -- permissions
+DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/sql/check_heap.sql b/contrib/amcheck/sql/check_heap.sql
new file mode 100644
index 0000000..298de68
--- /dev/null
+++ b/contrib/amcheck/sql/check_heap.sql
@@ -0,0 +1,116 @@
+CREATE TABLE heaptest (a integer, b text);
+REVOKE ALL ON heaptest FROM PUBLIC;
+
+-- Check that invalid skip option is rejected
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'rope');
+
+-- Check specifying invalid block ranges when verifying an empty table
+SELECT * FROM verify_heapam(relation := 'heaptest', startblock := 0, endblock := 0);
+SELECT * FROM verify_heapam(relation := 'heaptest', startblock := 5, endblock := 8);
+
+-- Check that valid options are not rejected nor corruption reported
+-- for an empty table, and that skip enum-like parameter is case-insensitive
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'none');
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'all-frozen');
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'all-visible');
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'None');
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'All-Frozen');
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'All-Visible');
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'NONE');
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'ALL-FROZEN');
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'ALL-VISIBLE');
+
+-- Add some data so subsequent tests are not entirely trivial
+INSERT INTO heaptest (a, b)
+	(SELECT gs, repeat('x', gs)
+		FROM generate_series(1,50) gs);
+
+-- Check that valid options are not rejected nor corruption reported
+-- for a non-empty table
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'none');
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'all-frozen');
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'all-visible');
+SELECT * FROM verify_heapam(relation := 'heaptest', startblock := 0, endblock := 0);
+
+CREATE ROLE regress_heaptest_role;
+
+-- verify permissions are checked (error due to function not callable)
+SET ROLE regress_heaptest_role;
+SELECT * FROM verify_heapam(relation := 'heaptest');
+RESET ROLE;
+
+GRANT EXECUTE ON FUNCTION verify_heapam(regclass, boolean, boolean, text, bigint, bigint) TO regress_heaptest_role;
+
+-- verify permissions are now sufficient
+SET ROLE regress_heaptest_role;
+SELECT * FROM verify_heapam(relation := 'heaptest');
+RESET ROLE;
+
+-- Check specifying invalid block ranges when verifying a non-empty table.
+SELECT * FROM verify_heapam(relation := 'heaptest', startblock := 0, endblock := 10000);
+SELECT * FROM verify_heapam(relation := 'heaptest', startblock := 10000, endblock := 11000);
+
+-- Vacuum freeze to change the xids encountered in subsequent tests
+VACUUM (FREEZE, DISABLE_PAGE_SKIPPING) heaptest;
+
+-- Check that valid options are not rejected nor corruption reported
+-- for a non-empty frozen table
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'none');
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'all-frozen');
+SELECT * FROM verify_heapam(relation := 'heaptest', skip := 'all-visible');
+SELECT * FROM verify_heapam(relation := 'heaptest', startblock := 0, endblock := 0);
+
+-- Check that partitioned tables (the parent ones) which don't have visibility
+-- maps are rejected
+CREATE TABLE test_partitioned (a int, b text default repeat('x', 5000))
+			 PARTITION BY list (a);
+SELECT * FROM verify_heapam('test_partitioned',
+							startblock := NULL,
+							endblock := NULL);
+
+-- Check that valid options are not rejected nor corruption reported
+-- for an empty partition table (the child one)
+CREATE TABLE test_partition partition OF test_partitioned FOR VALUES IN (1);
+SELECT * FROM verify_heapam('test_partition',
+							startblock := NULL,
+							endblock := NULL);
+
+-- Check that valid options are not rejected nor corruption reported
+-- for a non-empty partition table (the child one)
+INSERT INTO test_partitioned (a) (SELECT 1 FROM generate_series(1,1000) gs);
+SELECT * FROM verify_heapam('test_partition',
+							startblock := NULL,
+							endblock := NULL);
+
+-- Check that indexes are rejected
+CREATE INDEX test_index ON test_partition (a);
+SELECT * FROM verify_heapam('test_index',
+							startblock := NULL,
+							endblock := NULL);
+
+-- Check that views are rejected
+CREATE VIEW test_view AS SELECT 1;
+SELECT * FROM verify_heapam('test_view',
+							startblock := NULL,
+							endblock := NULL);
+
+-- Check that sequences are rejected
+CREATE SEQUENCE test_sequence;
+SELECT * FROM verify_heapam('test_sequence',
+							startblock := NULL,
+							endblock := NULL);
+
+-- Check that foreign tables are rejected
+CREATE FOREIGN DATA WRAPPER dummy;
+CREATE SERVER dummy_server FOREIGN DATA WRAPPER dummy;
+CREATE FOREIGN TABLE test_foreign_table () SERVER dummy_server;
+SELECT * FROM verify_heapam('test_foreign_table',
+							startblock := NULL,
+							endblock := NULL);
+
+-- cleanup
+DROP TABLE heaptest;
+DROP TABLE test_partition;
+DROP TABLE test_partitioned;
+DROP OWNED BY regress_heaptest_role; -- permissions
+DROP ROLE regress_heaptest_role;
diff --git a/contrib/amcheck/t/001_verify_heapam.pl b/contrib/amcheck/t/001_verify_heapam.pl
new file mode 100644
index 0000000..64ba64d
--- /dev/null
+++ b/contrib/amcheck/t/001_verify_heapam.pl
@@ -0,0 +1,218 @@
+
+# Copyright (c) 2021, PostgreSQL Global Development Group
+
+use strict;
+use warnings;
+
+use PostgresNode;
+use TestLib;
+
+use Test::More tests => 80;
+
+my ($node, $result);
+
+#
+# Test set-up
+#
+$node = get_new_node('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum=off');
+$node->start;
+$node->safe_psql('postgres', q(CREATE EXTENSION amcheck));
+
+#
+# Check a table with data loaded but no corruption, freezing, etc.
+#
+fresh_test_table('test');
+check_all_options_uncorrupted('test', 'plain');
+
+#
+# Check a corrupt table
+#
+fresh_test_table('test');
+corrupt_first_page('test');
+detects_heap_corruption("verify_heapam('test')", "plain corrupted table");
+detects_heap_corruption(
+	"verify_heapam('test', skip := 'all-visible')",
+	"plain corrupted table skipping all-visible");
+detects_heap_corruption(
+	"verify_heapam('test', skip := 'all-frozen')",
+	"plain corrupted table skipping all-frozen");
+detects_heap_corruption(
+	"verify_heapam('test', check_toast := false)",
+	"plain corrupted table skipping toast");
+detects_heap_corruption(
+	"verify_heapam('test', startblock := 0, endblock := 0)",
+	"plain corrupted table checking only block zero");
+
+#
+# Check a corrupt table with all-frozen data
+#
+fresh_test_table('test');
+$node->safe_psql('postgres', q(VACUUM (FREEZE, DISABLE_PAGE_SKIPPING) test));
+detects_no_corruption("verify_heapam('test')",
+	"all-frozen not corrupted table");
+corrupt_first_page('test');
+detects_heap_corruption("verify_heapam('test')",
+	"all-frozen corrupted table");
+detects_no_corruption(
+	"verify_heapam('test', skip := 'all-frozen')",
+	"all-frozen corrupted table skipping all-frozen");
+
+# Returns the filesystem path for the named relation.
+sub relation_filepath
+{
+	my ($relname) = @_;
+
+	my $pgdata = $node->data_dir;
+	my $rel    = $node->safe_psql('postgres',
+		qq(SELECT pg_relation_filepath('$relname')));
+	die "path not found for relation $relname" unless defined $rel;
+	return "$pgdata/$rel";
+}
+
+# Returns the fully qualified name of the toast table for the named relation
+sub get_toast_for
+{
+	my ($relname) = @_;
+
+	return $node->safe_psql(
+		'postgres', qq(
+		SELECT 'pg_toast.' || t.relname
+			FROM pg_catalog.pg_class c, pg_catalog.pg_class t
+			WHERE c.relname = '$relname'
+			  AND c.reltoastrelid = t.oid));
+}
+
+# (Re)create and populate a test table of the given name.
+sub fresh_test_table
+{
+	my ($relname) = @_;
+
+	return $node->safe_psql(
+		'postgres', qq(
+		DROP TABLE IF EXISTS $relname CASCADE;
+		CREATE TABLE $relname (a integer, b text);
+		ALTER TABLE $relname SET (autovacuum_enabled=false);
+		ALTER TABLE $relname ALTER b SET STORAGE external;
+		INSERT INTO $relname (a, b)
+			(SELECT gs, repeat('b',gs*10) FROM generate_series(1,1000) gs);
+		BEGIN;
+		SAVEPOINT s1;
+		SELECT 1 FROM $relname WHERE a = 42 FOR UPDATE;
+		UPDATE $relname SET b = b WHERE a = 42;
+		RELEASE s1;
+		SAVEPOINT s1;
+		SELECT 1 FROM $relname WHERE a = 42 FOR UPDATE;
+		UPDATE $relname SET b = b WHERE a = 42;
+		COMMIT;
+	));
+}
+
+# Stops the test node, corrupts the first page of the named relation, and
+# restarts the node.
+sub corrupt_first_page
+{
+	my ($relname) = @_;
+	my $relpath = relation_filepath($relname);
+
+	$node->stop;
+
+	my $fh;
+	open($fh, '+<', $relpath)
+	  or BAIL_OUT("open failed: $!");
+	binmode $fh;
+
+	# Corrupt some line pointers.  The values are chosen to hit the
+	# various line-pointer-corruption checks in verify_heapam.c
+	# on both little-endian and big-endian architectures.
+	sysseek($fh, 32, 0)
+	  or BAIL_OUT("sysseek failed: $!");
+	syswrite(
+		$fh,
+		pack("L*",
+			0xAAA15550, 0xAAA0D550, 0x00010000,
+			0x00008000, 0x0000800F, 0x001e8000)
+	) or BAIL_OUT("syswrite failed: $!");
+	close($fh)
+	  or BAIL_OUT("close failed: $!");
+
+	$node->start;
+}
+
+sub detects_heap_corruption
+{
+	local $Test::Builder::Level = $Test::Builder::Level + 1;
+
+	my ($function, $testname) = @_;
+
+	detects_corruption(
+		$function,
+		$testname,
+		qr/line pointer redirection to item at offset \d+ precedes minimum offset \d+/,
+		qr/line pointer redirection to item at offset \d+ exceeds maximum offset \d+/,
+		qr/line pointer to page offset \d+ is not maximally aligned/,
+		qr/line pointer length \d+ is less than the minimum tuple header size \d+/,
+		qr/line pointer to page offset \d+ with length \d+ ends beyond maximum page offset \d+/,
+	);
+}
+
+sub detects_corruption
+{
+	local $Test::Builder::Level = $Test::Builder::Level + 1;
+
+	my ($function, $testname, @re) = @_;
+
+	my $result = $node->safe_psql('postgres', qq(SELECT * FROM $function));
+	like($result, $_, $testname) for (@re);
+}
+
+sub detects_no_corruption
+{
+	local $Test::Builder::Level = $Test::Builder::Level + 1;
+
+	my ($function, $testname) = @_;
+
+	my $result = $node->safe_psql('postgres', qq(SELECT * FROM $function));
+	is($result, '', $testname);
+}
+
+# Check various options are stable (don't abort) and do not report corruption
+# when running verify_heapam on an uncorrupted test table.
+#
+# The relname *must* be an uncorrupted table, or this will fail.
+#
+# The prefix is used to identify the test, along with the options,
+# and should be unique.
+sub check_all_options_uncorrupted
+{
+	local $Test::Builder::Level = $Test::Builder::Level + 1;
+
+	my ($relname, $prefix) = @_;
+
+	for my $stop (qw(true false))
+	{
+		for my $check_toast (qw(true false))
+		{
+			for my $skip ("'none'", "'all-frozen'", "'all-visible'")
+			{
+				for my $startblock (qw(NULL 0))
+				{
+					for my $endblock (qw(NULL 0))
+					{
+						my $opts =
+						    "on_error_stop := $stop, "
+						  . "check_toast := $check_toast, "
+						  . "skip := $skip, "
+						  . "startblock := $startblock, "
+						  . "endblock := $endblock";
+
+						detects_no_corruption(
+							"verify_heapam('$relname', $opts)",
+							"$prefix: $opts");
+					}
+				}
+			}
+		}
+	}
+}
diff --git a/contrib/amcheck/t/002_cic.pl b/contrib/amcheck/t/002_cic.pl
new file mode 100644
index 0000000..3587807
--- /dev/null
+++ b/contrib/amcheck/t/002_cic.pl
@@ -0,0 +1,65 @@
+
+# Copyright (c) 2021, PostgreSQL Global Development Group
+
+# Test CREATE INDEX CONCURRENTLY with concurrent modifications
+use strict;
+use warnings;
+
+use Config;
+use PostgresNode;
+use TestLib;
+
+use Test::More tests => 3;
+
+my ($node, $result);
+
+#
+# Test set-up
+#
+$node = get_new_node('CIC_test');
+$node->init;
+$node->append_conf('postgresql.conf',
+	'lock_timeout = ' . (1000 * $TestLib::timeout_default));
+$node->start;
+$node->safe_psql('postgres', q(CREATE EXTENSION amcheck));
+$node->safe_psql('postgres', q(CREATE TABLE tbl(i int)));
+$node->safe_psql('postgres', q(CREATE INDEX idx ON tbl(i)));
+
+#
+# Stress CIC with pgbench.
+#
+# pgbench might try to launch more than one instance of the CIC
+# transaction concurrently.  That would deadlock, so use an advisory
+# lock to ensure only one CIC runs at a time.
+#
+$node->pgbench(
+	'--no-vacuum --client=5 --transactions=100',
+	0,
+	[qr{actually processed}],
+	[qr{^$}],
+	'concurrent INSERTs and CIC',
+	{
+		'002_pgbench_concurrent_transaction' => q(
+			BEGIN;
+			INSERT INTO tbl VALUES(0);
+			COMMIT;
+		  ),
+		'002_pgbench_concurrent_transaction_savepoints' => q(
+			BEGIN;
+			SAVEPOINT s1;
+			INSERT INTO tbl VALUES(0);
+			COMMIT;
+		  ),
+		'002_pgbench_concurrent_cic' => q(
+			SELECT pg_try_advisory_lock(42)::integer AS gotlock \gset
+			\if :gotlock
+				DROP INDEX CONCURRENTLY idx;
+				CREATE INDEX CONCURRENTLY idx ON tbl(i);
+				SELECT bt_index_check('idx',true);
+				SELECT pg_advisory_unlock(42);
+			\endif
+		  )
+	});
+
+$node->stop;
+done_testing();
diff --git a/contrib/amcheck/t/003_cic_2pc.pl b/contrib/amcheck/t/003_cic_2pc.pl
new file mode 100644
index 0000000..445aaba
--- /dev/null
+++ b/contrib/amcheck/t/003_cic_2pc.pl
@@ -0,0 +1,188 @@
+
+# Copyright (c) 2021, PostgreSQL Global Development Group
+
+# Test CREATE INDEX CONCURRENTLY with concurrent prepared-xact modifications
+use strict;
+use warnings;
+
+use Config;
+use PostgresNode;
+use TestLib;
+
+use Test::More tests => 5;
+
+Test::More->builder->todo_start('filesystem bug')
+  if TestLib::has_wal_read_bug;
+
+my ($node, $result);
+
+#
+# Test set-up
+#
+$node = get_new_node('CIC_2PC_test');
+$node->init;
+$node->append_conf('postgresql.conf', 'max_prepared_transactions = 10');
+$node->append_conf('postgresql.conf',
+	'lock_timeout = ' . (1000 * $TestLib::timeout_default));
+$node->start;
+$node->safe_psql('postgres', q(CREATE EXTENSION amcheck));
+$node->safe_psql('postgres', q(CREATE TABLE tbl(i int)));
+
+
+#
+# Run 3 overlapping 2PC transactions with CIC
+#
+# We have two concurrent background psql processes: $main_h for INSERTs and
+# $cic_h for CIC.  Also, we use non-background psql for some COMMIT PREPARED
+# statements.
+#
+
+my $main_in    = '';
+my $main_out   = '';
+my $main_timer = IPC::Run::timeout($TestLib::timeout_default);
+
+my $main_h =
+  $node->background_psql('postgres', \$main_in, \$main_out,
+	$main_timer, on_error_stop => 1);
+$main_in .= q(
+BEGIN;
+INSERT INTO tbl VALUES(0);
+\echo syncpoint1
+);
+pump $main_h until $main_out =~ /syncpoint1/ || $main_timer->is_expired;
+
+my $cic_in    = '';
+my $cic_out   = '';
+my $cic_timer = IPC::Run::timeout($TestLib::timeout_default);
+my $cic_h =
+  $node->background_psql('postgres', \$cic_in, \$cic_out,
+	$cic_timer, on_error_stop => 1);
+$cic_in .= q(
+\echo start
+CREATE INDEX CONCURRENTLY idx ON tbl(i);
+);
+pump $cic_h until $cic_out =~ /start/ || $cic_timer->is_expired;
+
+$main_in .= q(
+PREPARE TRANSACTION 'a';
+);
+
+$main_in .= q(
+BEGIN;
+INSERT INTO tbl VALUES(0);
+\echo syncpoint2
+);
+pump $main_h until $main_out =~ /syncpoint2/ || $main_timer->is_expired;
+
+$node->safe_psql('postgres', q(COMMIT PREPARED 'a';));
+
+$main_in .= q(
+PREPARE TRANSACTION 'b';
+BEGIN;
+INSERT INTO tbl VALUES(0);
+\echo syncpoint3
+);
+pump $main_h until $main_out =~ /syncpoint3/ || $main_timer->is_expired;
+
+$node->safe_psql('postgres', q(COMMIT PREPARED 'b';));
+
+$main_in .= q(
+PREPARE TRANSACTION 'c';
+COMMIT PREPARED 'c';
+);
+$main_h->pump_nb;
+
+$main_h->finish;
+$cic_h->finish;
+
+$result = $node->psql('postgres', q(SELECT bt_index_check('idx',true)));
+is($result, '0', 'bt_index_check after overlapping 2PC');
+
+
+#
+# Server restart shall not change whether prepared xact blocks CIC
+#
+
+$node->safe_psql(
+	'postgres', q(
+BEGIN;
+INSERT INTO tbl VALUES(0);
+PREPARE TRANSACTION 'spans_restart';
+BEGIN;
+CREATE TABLE unused ();
+PREPARE TRANSACTION 'persists_forever';
+));
+$node->restart;
+
+my $reindex_in  = '';
+my $reindex_out = '';
+my $reindex_timer =
+  IPC::Run::timeout($TestLib::timeout_default);
+my $reindex_h =
+  $node->background_psql('postgres', \$reindex_in, \$reindex_out,
+	$reindex_timer, on_error_stop => 1);
+$reindex_in .= q(
+\echo start
+DROP INDEX CONCURRENTLY idx;
+CREATE INDEX CONCURRENTLY idx ON tbl(i);
+);
+pump $reindex_h until $reindex_out =~ /start/ || $reindex_timer->is_expired;
+
+$node->safe_psql('postgres', "COMMIT PREPARED 'spans_restart'");
+$reindex_h->finish;
+$result = $node->psql('postgres', q(SELECT bt_index_check('idx',true)));
+is($result, '0', 'bt_index_check after 2PC and restart');
+
+
+#
+# Stress CIC+2PC with pgbench
+#
+# pgbench might try to launch more than one instance of the CIC
+# transaction concurrently.  That would deadlock, so use an advisory
+# lock to ensure only one CIC runs at a time.
+
+# Fix broken index first
+$node->safe_psql('postgres', q(REINDEX TABLE tbl;));
+
+# Run pgbench.
+$node->pgbench(
+	'--no-vacuum --client=5 --transactions=100',
+	0,
+	[qr{actually processed}],
+	[qr{^$}],
+	'concurrent INSERTs w/ 2PC and CIC',
+	{
+		'003_pgbench_concurrent_2pc' => q(
+			BEGIN;
+			INSERT INTO tbl VALUES(0);
+			PREPARE TRANSACTION 'c:client_id';
+			COMMIT PREPARED 'c:client_id';
+		  ),
+		'003_pgbench_concurrent_2pc_savepoint' => q(
+			BEGIN;
+			SAVEPOINT s1;
+			INSERT INTO tbl VALUES(0);
+			PREPARE TRANSACTION 'c:client_id';
+			COMMIT PREPARED 'c:client_id';
+		  ),
+		'003_pgbench_concurrent_cic' => q(
+			SELECT pg_try_advisory_lock(42)::integer AS gotlock \gset
+			\if :gotlock
+				DROP INDEX CONCURRENTLY idx;
+				CREATE INDEX CONCURRENTLY idx ON tbl(i);
+				SELECT bt_index_check('idx',true);
+				SELECT pg_advisory_unlock(42);
+			\endif
+		  ),
+		'004_pgbench_concurrent_ric' => q(
+			SELECT pg_try_advisory_lock(42)::integer AS gotlock \gset
+			\if :gotlock
+				REINDEX INDEX CONCURRENTLY idx;
+				SELECT bt_index_check('idx',true);
+				SELECT pg_advisory_unlock(42);
+			\endif
+		  )
+	});
+
+$node->stop;
+done_testing();
diff --git a/contrib/amcheck/verify_heapam.c b/contrib/amcheck/verify_heapam.c
new file mode 100644
index 0000000..30064d0
--- /dev/null
+++ b/contrib/amcheck/verify_heapam.c
@@ -0,0 +1,1752 @@
+/*-------------------------------------------------------------------------
+ *
+ * verify_heapam.c
+ *	  Functions to check postgresql heap relations for corruption
+ *
+ * Copyright (c) 2016-2021, PostgreSQL Global Development Group
+ *
+ *	  contrib/amcheck/verify_heapam.c
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/detoast.h"
+#include "access/genam.h"
+#include "access/heapam.h"
+#include "access/heaptoast.h"
+#include "access/multixact.h"
+#include "access/toast_internals.h"
+#include "access/visibilitymap.h"
+#include "catalog/pg_am.h"
+#include "funcapi.h"
+#include "miscadmin.h"
+#include "storage/bufmgr.h"
+#include "storage/procarray.h"
+#include "utils/builtins.h"
+#include "utils/fmgroids.h"
+
+PG_FUNCTION_INFO_V1(verify_heapam);
+
+/* The number of columns in tuples returned by verify_heapam */
+#define HEAPCHECK_RELATION_COLS 4
+
+/*
+ * Despite the name, we use this for reporting problems with both XIDs and
+ * MXIDs.
+ */
+typedef enum XidBoundsViolation
+{
+	XID_INVALID,
+	XID_IN_FUTURE,
+	XID_PRECEDES_CLUSTERMIN,
+	XID_PRECEDES_RELMIN,
+	XID_BOUNDS_OK
+} XidBoundsViolation;
+
+typedef enum XidCommitStatus
+{
+	XID_COMMITTED,
+	XID_IS_CURRENT_XID,
+	XID_IN_PROGRESS,
+	XID_ABORTED
+} XidCommitStatus;
+
+typedef enum SkipPages
+{
+	SKIP_PAGES_ALL_FROZEN,
+	SKIP_PAGES_ALL_VISIBLE,
+	SKIP_PAGES_NONE
+} SkipPages;
+
+/*
+ * Struct holding information about a toasted attribute sufficient to both
+ * check the toasted attribute and, if found to be corrupt, to report where it
+ * was encountered in the main table.
+ */
+typedef struct ToastedAttribute
+{
+	struct varatt_external toast_pointer;
+	BlockNumber blkno;			/* block in main table */
+	OffsetNumber offnum;		/* offset in main table */
+	AttrNumber	attnum;			/* attribute in main table */
+} ToastedAttribute;
+
+/*
+ * Struct holding the running context information during
+ * a lifetime of a verify_heapam execution.
+ */
+typedef struct HeapCheckContext
+{
+	/*
+	 * Cached copies of values from ShmemVariableCache and computed values
+	 * from them.
+	 */
+	FullTransactionId next_fxid;	/* ShmemVariableCache->nextXid */
+	TransactionId next_xid;		/* 32-bit version of next_fxid */
+	TransactionId oldest_xid;	/* ShmemVariableCache->oldestXid */
+	FullTransactionId oldest_fxid;	/* 64-bit version of oldest_xid, computed
+									 * relative to next_fxid */
+	TransactionId safe_xmin;	/* this XID and newer ones can't become
+								 * all-visible while we're running */
+
+	/*
+	 * Cached copy of value from MultiXactState
+	 */
+	MultiXactId next_mxact;		/* MultiXactState->nextMXact */
+	MultiXactId oldest_mxact;	/* MultiXactState->oldestMultiXactId */
+
+	/*
+	 * Cached copies of the most recently checked xid and its status.
+	 */
+	TransactionId cached_xid;
+	XidCommitStatus cached_status;
+
+	/* Values concerning the heap relation being checked */
+	Relation	rel;
+	TransactionId relfrozenxid;
+	FullTransactionId relfrozenfxid;
+	TransactionId relminmxid;
+	Relation	toast_rel;
+	Relation   *toast_indexes;
+	Relation	valid_toast_index;
+	int			num_toast_indexes;
+
+	/* Values for iterating over pages in the relation */
+	BlockNumber blkno;
+	BufferAccessStrategy bstrategy;
+	Buffer		buffer;
+	Page		page;
+
+	/* Values for iterating over tuples within a page */
+	OffsetNumber offnum;
+	ItemId		itemid;
+	uint16		lp_len;
+	uint16		lp_off;
+	HeapTupleHeader tuphdr;
+	int			natts;
+
+	/* Values for iterating over attributes within the tuple */
+	uint32		offset;			/* offset in tuple data */
+	AttrNumber	attnum;
+
+	/* True if tuple's xmax makes it eligible for pruning */
+	bool		tuple_could_be_pruned;
+
+	/*
+	 * List of ToastedAttribute structs for toasted attributes which are not
+	 * eligible for pruning and should be checked
+	 */
+	List	   *toasted_attributes;
+
+	/* Whether verify_heapam has yet encountered any corrupt tuples */
+	bool		is_corrupt;
+
+	/* The descriptor and tuplestore for verify_heapam's result tuples */
+	TupleDesc	tupdesc;
+	Tuplestorestate *tupstore;
+} HeapCheckContext;
+
+/* Internal implementation */
+static void sanity_check_relation(Relation rel);
+static void check_tuple(HeapCheckContext *ctx);
+static void check_toast_tuple(HeapTuple toasttup, HeapCheckContext *ctx,
+							  ToastedAttribute *ta, int32 *expected_chunk_seq,
+							  uint32 extsize);
+
+static bool check_tuple_attribute(HeapCheckContext *ctx);
+static void check_toasted_attribute(HeapCheckContext *ctx,
+									ToastedAttribute *ta);
+
+static bool check_tuple_header(HeapCheckContext *ctx);
+static bool check_tuple_visibility(HeapCheckContext *ctx);
+
+static void report_corruption(HeapCheckContext *ctx, char *msg);
+static void report_toast_corruption(HeapCheckContext *ctx,
+									ToastedAttribute *ta, char *msg);
+static TupleDesc verify_heapam_tupdesc(void);
+static FullTransactionId FullTransactionIdFromXidAndCtx(TransactionId xid,
+														const HeapCheckContext *ctx);
+static void update_cached_xid_range(HeapCheckContext *ctx);
+static void update_cached_mxid_range(HeapCheckContext *ctx);
+static XidBoundsViolation check_mxid_in_range(MultiXactId mxid,
+											  HeapCheckContext *ctx);
+static XidBoundsViolation check_mxid_valid_in_rel(MultiXactId mxid,
+												  HeapCheckContext *ctx);
+static XidBoundsViolation get_xid_status(TransactionId xid,
+										 HeapCheckContext *ctx,
+										 XidCommitStatus *status);
+
+/*
+ * Scan and report corruption in heap pages, optionally reconciling toasted
+ * attributes with entries in the associated toast table.  Intended to be
+ * called from SQL with the following parameters:
+ *
+ *   relation:
+ *     The Oid of the heap relation to be checked.
+ *
+ *   on_error_stop:
+ *     Whether to stop at the end of the first page for which errors are
+ *     detected.  Note that multiple rows may be returned.
+ *
+ *   check_toast:
+ *     Whether to check each toasted attribute against the toast table to
+ *     verify that it can be found there.
+ *
+ *   skip:
+ *     What kinds of pages in the heap relation should be skipped.  Valid
+ *     options are "all-visible", "all-frozen", and "none".
+ *
+ * Returns to the SQL caller a set of tuples, each containing the location
+ * and a description of a corruption found in the heap.
+ *
+ * This code goes to some trouble to avoid crashing the server even if the
+ * table pages are badly corrupted, but it's probably not perfect. If
+ * check_toast is true, we'll use regular index lookups to try to fetch TOAST
+ * tuples, which can certainly cause crashes if the right kind of corruption
+ * exists in the toast table or index. No matter what parameters you pass,
+ * we can't protect against crashes that might occur trying to look up the
+ * commit status of transaction IDs (though we avoid trying to do such lookups
+ * for transaction IDs that can't legally appear in the table).
+ */
+Datum
+verify_heapam(PG_FUNCTION_ARGS)
+{
+	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
+	MemoryContext old_context;
+	bool		random_access;
+	HeapCheckContext ctx;
+	Buffer		vmbuffer = InvalidBuffer;
+	Oid			relid;
+	bool		on_error_stop;
+	bool		check_toast;
+	SkipPages	skip_option = SKIP_PAGES_NONE;
+	BlockNumber first_block;
+	BlockNumber last_block;
+	BlockNumber nblocks;
+	const char *skip;
+
+	/* Check to see if caller supports us returning a tuplestore */
+	if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("set-valued function called in context that cannot accept a set")));
+	if (!(rsinfo->allowedModes & SFRM_Materialize))
+		ereport(ERROR,
+				(errcode(ERRCODE_SYNTAX_ERROR),
+				 errmsg("materialize mode required, but it is not allowed in this context")));
+
+	/* Check supplied arguments */
+	if (PG_ARGISNULL(0))
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("relation cannot be null")));
+	relid = PG_GETARG_OID(0);
+
+	if (PG_ARGISNULL(1))
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("on_error_stop cannot be null")));
+	on_error_stop = PG_GETARG_BOOL(1);
+
+	if (PG_ARGISNULL(2))
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("check_toast cannot be null")));
+	check_toast = PG_GETARG_BOOL(2);
+
+	if (PG_ARGISNULL(3))
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("skip cannot be null")));
+	skip = text_to_cstring(PG_GETARG_TEXT_PP(3));
+	if (pg_strcasecmp(skip, "all-visible") == 0)
+		skip_option = SKIP_PAGES_ALL_VISIBLE;
+	else if (pg_strcasecmp(skip, "all-frozen") == 0)
+		skip_option = SKIP_PAGES_ALL_FROZEN;
+	else if (pg_strcasecmp(skip, "none") == 0)
+		skip_option = SKIP_PAGES_NONE;
+	else
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("invalid skip option"),
+				 errhint("Valid skip options are \"all-visible\", \"all-frozen\", and \"none\".")));
+
+	memset(&ctx, 0, sizeof(HeapCheckContext));
+	ctx.cached_xid = InvalidTransactionId;
+	ctx.toasted_attributes = NIL;
+
+	/*
+	 * Any xmin newer than the xmin of our snapshot can't become all-visible
+	 * while we're running.
+	 */
+	ctx.safe_xmin = GetTransactionSnapshot()->xmin;
+
+	/*
+	 * If we report corruption when not examining some individual attribute,
+	 * we need attnum to be reported as NULL.  Set that up before any
+	 * corruption reporting might happen.
+	 */
+	ctx.attnum = -1;
+
+	/* The tupdesc and tuplestore must be created in ecxt_per_query_memory */
+	old_context = MemoryContextSwitchTo(rsinfo->econtext->ecxt_per_query_memory);
+	random_access = (rsinfo->allowedModes & SFRM_Materialize_Random) != 0;
+	ctx.tupdesc = verify_heapam_tupdesc();
+	ctx.tupstore = tuplestore_begin_heap(random_access, false, work_mem);
+	rsinfo->returnMode = SFRM_Materialize;
+	rsinfo->setResult = ctx.tupstore;
+	rsinfo->setDesc = ctx.tupdesc;
+	MemoryContextSwitchTo(old_context);
+
+	/* Open relation, check relkind and access method */
+	ctx.rel = relation_open(relid, AccessShareLock);
+	sanity_check_relation(ctx.rel);
+
+	/*
+	 * Early exit for unlogged relations during recovery.  These will have no
+	 * relation fork, so there won't be anything to check.  We behave as if
+	 * the relation is empty.
+	 */
+	if (ctx.rel->rd_rel->relpersistence == RELPERSISTENCE_UNLOGGED &&
+		RecoveryInProgress())
+	{
+		ereport(DEBUG1,
+				(errcode(ERRCODE_READ_ONLY_SQL_TRANSACTION),
+				 errmsg("cannot verify unlogged relation \"%s\" during recovery, skipping",
+						RelationGetRelationName(ctx.rel))));
+		relation_close(ctx.rel, AccessShareLock);
+		PG_RETURN_NULL();
+	}
+
+	/* Early exit if the relation is empty */
+	nblocks = RelationGetNumberOfBlocks(ctx.rel);
+	if (!nblocks)
+	{
+		relation_close(ctx.rel, AccessShareLock);
+		PG_RETURN_NULL();
+	}
+
+	ctx.bstrategy = GetAccessStrategy(BAS_BULKREAD);
+	ctx.buffer = InvalidBuffer;
+	ctx.page = NULL;
+
+	/* Validate block numbers, or handle nulls. */
+	if (PG_ARGISNULL(4))
+		first_block = 0;
+	else
+	{
+		int64		fb = PG_GETARG_INT64(4);
+
+		if (fb < 0 || fb >= nblocks)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("starting block number must be between 0 and %u",
+							nblocks - 1)));
+		first_block = (BlockNumber) fb;
+	}
+	if (PG_ARGISNULL(5))
+		last_block = nblocks - 1;
+	else
+	{
+		int64		lb = PG_GETARG_INT64(5);
+
+		if (lb < 0 || lb >= nblocks)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("ending block number must be between 0 and %u",
+							nblocks - 1)));
+		last_block = (BlockNumber) lb;
+	}
+
+	/* Optionally open the toast relation, if any. */
+	if (ctx.rel->rd_rel->reltoastrelid && check_toast)
+	{
+		int			offset;
+
+		/* Main relation has associated toast relation */
+		ctx.toast_rel = table_open(ctx.rel->rd_rel->reltoastrelid,
+								   AccessShareLock);
+		offset = toast_open_indexes(ctx.toast_rel,
+									AccessShareLock,
+									&(ctx.toast_indexes),
+									&(ctx.num_toast_indexes));
+		ctx.valid_toast_index = ctx.toast_indexes[offset];
+	}
+	else
+	{
+		/*
+		 * Main relation has no associated toast relation, or we're
+		 * intentionally skipping it.
+		 */
+		ctx.toast_rel = NULL;
+		ctx.toast_indexes = NULL;
+		ctx.num_toast_indexes = 0;
+	}
+
+	update_cached_xid_range(&ctx);
+	update_cached_mxid_range(&ctx);
+	ctx.relfrozenxid = ctx.rel->rd_rel->relfrozenxid;
+	ctx.relfrozenfxid = FullTransactionIdFromXidAndCtx(ctx.relfrozenxid, &ctx);
+	ctx.relminmxid = ctx.rel->rd_rel->relminmxid;
+
+	if (TransactionIdIsNormal(ctx.relfrozenxid))
+		ctx.oldest_xid = ctx.relfrozenxid;
+
+	for (ctx.blkno = first_block; ctx.blkno <= last_block; ctx.blkno++)
+	{
+		OffsetNumber maxoff;
+
+		CHECK_FOR_INTERRUPTS();
+
+		/* Optionally skip over all-frozen or all-visible blocks */
+		if (skip_option != SKIP_PAGES_NONE)
+		{
+			int32		mapbits;
+
+			mapbits = (int32) visibilitymap_get_status(ctx.rel, ctx.blkno,
+													   &vmbuffer);
+			if (skip_option == SKIP_PAGES_ALL_FROZEN)
+			{
+				if ((mapbits & VISIBILITYMAP_ALL_FROZEN) != 0)
+					continue;
+			}
+
+			if (skip_option == SKIP_PAGES_ALL_VISIBLE)
+			{
+				if ((mapbits & VISIBILITYMAP_ALL_VISIBLE) != 0)
+					continue;
+			}
+		}
+
+		/* Read and lock the next page. */
+		ctx.buffer = ReadBufferExtended(ctx.rel, MAIN_FORKNUM, ctx.blkno,
+										RBM_NORMAL, ctx.bstrategy);
+		LockBuffer(ctx.buffer, BUFFER_LOCK_SHARE);
+		ctx.page = BufferGetPage(ctx.buffer);
+
+		/* Perform tuple checks */
+		maxoff = PageGetMaxOffsetNumber(ctx.page);
+		for (ctx.offnum = FirstOffsetNumber; ctx.offnum <= maxoff;
+			 ctx.offnum = OffsetNumberNext(ctx.offnum))
+		{
+			ctx.itemid = PageGetItemId(ctx.page, ctx.offnum);
+
+			/* Skip over unused/dead line pointers */
+			if (!ItemIdIsUsed(ctx.itemid) || ItemIdIsDead(ctx.itemid))
+				continue;
+
+			/*
+			 * If this line pointer has been redirected, check that it
+			 * redirects to a valid offset within the line pointer array
+			 */
+			if (ItemIdIsRedirected(ctx.itemid))
+			{
+				OffsetNumber rdoffnum = ItemIdGetRedirect(ctx.itemid);
+				ItemId		rditem;
+
+				if (rdoffnum < FirstOffsetNumber)
+				{
+					report_corruption(&ctx,
+									  psprintf("line pointer redirection to item at offset %u precedes minimum offset %u",
+											   (unsigned) rdoffnum,
+											   (unsigned) FirstOffsetNumber));
+					continue;
+				}
+				if (rdoffnum > maxoff)
+				{
+					report_corruption(&ctx,
+									  psprintf("line pointer redirection to item at offset %u exceeds maximum offset %u",
+											   (unsigned) rdoffnum,
+											   (unsigned) maxoff));
+					continue;
+				}
+				rditem = PageGetItemId(ctx.page, rdoffnum);
+				if (!ItemIdIsUsed(rditem))
+					report_corruption(&ctx,
+									  psprintf("line pointer redirection to unused item at offset %u",
+											   (unsigned) rdoffnum));
+				continue;
+			}
+
+			/* Sanity-check the line pointer's offset and length values */
+			ctx.lp_len = ItemIdGetLength(ctx.itemid);
+			ctx.lp_off = ItemIdGetOffset(ctx.itemid);
+
+			if (ctx.lp_off != MAXALIGN(ctx.lp_off))
+			{
+				report_corruption(&ctx,
+								  psprintf("line pointer to page offset %u is not maximally aligned",
+										   ctx.lp_off));
+				continue;
+			}
+			if (ctx.lp_len < MAXALIGN(SizeofHeapTupleHeader))
+			{
+				report_corruption(&ctx,
+								  psprintf("line pointer length %u is less than the minimum tuple header size %u",
+										   ctx.lp_len,
+										   (unsigned) MAXALIGN(SizeofHeapTupleHeader)));
+				continue;
+			}
+			if (ctx.lp_off + ctx.lp_len > BLCKSZ)
+			{
+				report_corruption(&ctx,
+								  psprintf("line pointer to page offset %u with length %u ends beyond maximum page offset %u",
+										   ctx.lp_off,
+										   ctx.lp_len,
+										   (unsigned) BLCKSZ));
+				continue;
+			}
+
+			/* It should be safe to examine the tuple's header, at least */
+			ctx.tuphdr = (HeapTupleHeader) PageGetItem(ctx.page, ctx.itemid);
+			ctx.natts = HeapTupleHeaderGetNatts(ctx.tuphdr);
+
+			/* Ok, ready to check this next tuple */
+			check_tuple(&ctx);
+		}
+
+		/* clean up */
+		UnlockReleaseBuffer(ctx.buffer);
+
+		/*
+		 * Check any toast pointers from the page whose lock we just released
+		 */
+		if (ctx.toasted_attributes != NIL)
+		{
+			ListCell   *cell;
+
+			foreach(cell, ctx.toasted_attributes)
+				check_toasted_attribute(&ctx, lfirst(cell));
+			list_free_deep(ctx.toasted_attributes);
+			ctx.toasted_attributes = NIL;
+		}
+
+		if (on_error_stop && ctx.is_corrupt)
+			break;
+	}
+
+	if (vmbuffer != InvalidBuffer)
+		ReleaseBuffer(vmbuffer);
+
+	/* Close the associated toast table and indexes, if any. */
+	if (ctx.toast_indexes)
+		toast_close_indexes(ctx.toast_indexes, ctx.num_toast_indexes,
+							AccessShareLock);
+	if (ctx.toast_rel)
+		table_close(ctx.toast_rel, AccessShareLock);
+
+	/* Close the main relation */
+	relation_close(ctx.rel, AccessShareLock);
+
+	PG_RETURN_NULL();
+}
+
+/*
+ * Check that a relation's relkind and access method are both supported.
+ */
+static void
+sanity_check_relation(Relation rel)
+{
+	if (rel->rd_rel->relkind != RELKIND_RELATION &&
+		rel->rd_rel->relkind != RELKIND_MATVIEW &&
+		rel->rd_rel->relkind != RELKIND_TOASTVALUE)
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("\"%s\" is not a table, materialized view, or TOAST table",
+						RelationGetRelationName(rel))));
+	if (rel->rd_rel->relam != HEAP_TABLE_AM_OID)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("only heap AM is supported")));
+}
+
+/*
+ * Shared internal implementation for report_corruption and
+ * report_toast_corruption.
+ */
+static void
+report_corruption_internal(Tuplestorestate *tupstore, TupleDesc tupdesc,
+						   BlockNumber blkno, OffsetNumber offnum,
+						   AttrNumber attnum, char *msg)
+{
+	Datum		values[HEAPCHECK_RELATION_COLS];
+	bool		nulls[HEAPCHECK_RELATION_COLS];
+	HeapTuple	tuple;
+
+	MemSet(values, 0, sizeof(values));
+	MemSet(nulls, 0, sizeof(nulls));
+	values[0] = Int64GetDatum(blkno);
+	values[1] = Int32GetDatum(offnum);
+	values[2] = Int32GetDatum(attnum);
+	nulls[2] = (attnum < 0);
+	values[3] = CStringGetTextDatum(msg);
+
+	/*
+	 * In principle, there is nothing to prevent a scan over a large, highly
+	 * corrupted table from using work_mem worth of memory building up the
+	 * tuplestore.  That's ok, but if we also leak the msg argument memory
+	 * until the end of the query, we could exceed work_mem by more than a
+	 * trivial amount.  Therefore, free the msg argument each time we are
+	 * called rather than waiting for our current memory context to be freed.
+	 */
+	pfree(msg);
+
+	tuple = heap_form_tuple(tupdesc, values, nulls);
+	tuplestore_puttuple(tupstore, tuple);
+}
+
+/*
+ * Record a single corruption found in the main table.  The values in ctx should
+ * indicate the location of the corruption, and the msg argument should contain
+ * a human-readable description of the corruption.
+ *
+ * The msg argument is pfree'd by this function.
+ */
+static void
+report_corruption(HeapCheckContext *ctx, char *msg)
+{
+	report_corruption_internal(ctx->tupstore, ctx->tupdesc, ctx->blkno,
+							   ctx->offnum, ctx->attnum, msg);
+	ctx->is_corrupt = true;
+}
+
+/*
+ * Record corruption found in the toast table.  The values in ta should
+ * indicate the location in the main table where the toast pointer was
+ * encountered, and the msg argument should contain a human-readable
+ * description of the toast table corruption.
+ *
+ * As above, the msg argument is pfree'd by this function.
+ */
+static void
+report_toast_corruption(HeapCheckContext *ctx, ToastedAttribute *ta,
+						char *msg)
+{
+	report_corruption_internal(ctx->tupstore, ctx->tupdesc, ta->blkno,
+							   ta->offnum, ta->attnum, msg);
+	ctx->is_corrupt = true;
+}
+
+/*
+ * Construct the TupleDesc used to report messages about corruptions found
+ * while scanning the heap.
+ */
+static TupleDesc
+verify_heapam_tupdesc(void)
+{
+	TupleDesc	tupdesc;
+	AttrNumber	a = 0;
+
+	tupdesc = CreateTemplateTupleDesc(HEAPCHECK_RELATION_COLS);
+	TupleDescInitEntry(tupdesc, ++a, "blkno", INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, ++a, "offnum", INT4OID, -1, 0);
+	TupleDescInitEntry(tupdesc, ++a, "attnum", INT4OID, -1, 0);
+	TupleDescInitEntry(tupdesc, ++a, "msg", TEXTOID, -1, 0);
+	Assert(a == HEAPCHECK_RELATION_COLS);
+
+	return BlessTupleDesc(tupdesc);
+}
+
+/*
+ * Check for tuple header corruption.
+ *
+ * Some kinds of corruption make it unsafe to check the tuple attributes, for
+ * example when the line pointer refers to a range of bytes outside the page.
+ * In such cases, we return false (not checkable) after recording appropriate
+ * corruption messages.
+ *
+ * Some other kinds of tuple header corruption confuse the question of where
+ * the tuple attributes begin, or how long the nulls bitmap is, etc., making it
+ * unreasonable to attempt to check attributes, even if all candidate answers
+ * to those questions would not result in reading past the end of the line
+ * pointer or page.  In such cases, like above, we record corruption messages
+ * about the header and then return false.
+ *
+ * Other kinds of tuple header corruption do not bear on the question of
+ * whether the tuple attributes can be checked, so we record corruption
+ * messages for them but we do not return false merely because we detected
+ * them.
+ *
+ * Returns whether the tuple is sufficiently sensible to undergo visibility and
+ * attribute checks.
+ */
+static bool
+check_tuple_header(HeapCheckContext *ctx)
+{
+	HeapTupleHeader tuphdr = ctx->tuphdr;
+	uint16		infomask = tuphdr->t_infomask;
+	bool		result = true;
+	unsigned	expected_hoff;
+
+	if (ctx->tuphdr->t_hoff > ctx->lp_len)
+	{
+		report_corruption(ctx,
+						  psprintf("data begins at offset %u beyond the tuple length %u",
+								   ctx->tuphdr->t_hoff, ctx->lp_len));
+		result = false;
+	}
+
+	if ((ctx->tuphdr->t_infomask & HEAP_XMAX_COMMITTED) &&
+		(ctx->tuphdr->t_infomask & HEAP_XMAX_IS_MULTI))
+	{
+		report_corruption(ctx,
+						  pstrdup("multixact should not be marked committed"));
+
+		/*
+		 * This condition is clearly wrong, but it's not enough to justify
+		 * skipping further checks, because we don't rely on this to determine
+		 * whether the tuple is visible or to interpret other relevant header
+		 * fields.
+		 */
+	}
+
+	if (infomask & HEAP_HASNULL)
+		expected_hoff = MAXALIGN(SizeofHeapTupleHeader + BITMAPLEN(ctx->natts));
+	else
+		expected_hoff = MAXALIGN(SizeofHeapTupleHeader);
+	if (ctx->tuphdr->t_hoff != expected_hoff)
+	{
+		if ((infomask & HEAP_HASNULL) && ctx->natts == 1)
+			report_corruption(ctx,
+							  psprintf("tuple data should begin at byte %u, but actually begins at byte %u (1 attribute, has nulls)",
+									   expected_hoff, ctx->tuphdr->t_hoff));
+		else if ((infomask & HEAP_HASNULL))
+			report_corruption(ctx,
+							  psprintf("tuple data should begin at byte %u, but actually begins at byte %u (%u attributes, has nulls)",
+									   expected_hoff, ctx->tuphdr->t_hoff, ctx->natts));
+		else if (ctx->natts == 1)
+			report_corruption(ctx,
+							  psprintf("tuple data should begin at byte %u, but actually begins at byte %u (1 attribute, no nulls)",
+									   expected_hoff, ctx->tuphdr->t_hoff));
+		else
+			report_corruption(ctx,
+							  psprintf("tuple data should begin at byte %u, but actually begins at byte %u (%u attributes, no nulls)",
+									   expected_hoff, ctx->tuphdr->t_hoff, ctx->natts));
+		result = false;
+	}
+
+	return result;
+}
+
+/*
+ * Checks tuple visibility so we know which further checks are safe to
+ * perform.
+ *
+ * If a tuple could have been inserted by a transaction that also added a
+ * column to the table, but which ultimately did not commit, or which has not
+ * yet committed, then the table's current TupleDesc might differ from the one
+ * used to construct this tuple, so we must not check it.
+ *
+ * As a special case, if our own transaction inserted the tuple, even if we
+ * added a column to the table, our TupleDesc should match.  We could check the
+ * tuple, but choose not to do so.
+ *
+ * If a tuple has been updated or deleted, we can still read the old tuple for
+ * corruption checking purposes, as long as we are careful about concurrent
+ * vacuums.  The main table tuple itself cannot be vacuumed away because we
+ * hold a buffer lock on the page, but if the deleting transaction is older
+ * than our transaction snapshot's xmin, then vacuum could remove the toast at
+ * any time, so we must not try to follow TOAST pointers.
+ *
+ * If xmin or xmax values are older than can be checked against clog, or appear
+ * to be in the future (possibly due to wrap-around), then we cannot make a
+ * determination about the visibility of the tuple, so we skip further checks.
+ *
+ * Returns true if the tuple itself should be checked, false otherwise.  Sets
+ * ctx->tuple_could_be_pruned if the tuple -- and thus also any associated
+ * TOAST tuples -- are eligible for pruning.
+ */
+static bool
+check_tuple_visibility(HeapCheckContext *ctx)
+{
+	TransactionId xmin;
+	TransactionId xvac;
+	TransactionId xmax;
+	XidCommitStatus xmin_status;
+	XidCommitStatus xvac_status;
+	XidCommitStatus xmax_status;
+	HeapTupleHeader tuphdr = ctx->tuphdr;
+
+	ctx->tuple_could_be_pruned = true;	/* have not yet proven otherwise */
+
+	/* If xmin is normal, it should be within valid range */
+	xmin = HeapTupleHeaderGetXmin(tuphdr);
+	switch (get_xid_status(xmin, ctx, &xmin_status))
+	{
+		case XID_INVALID:
+		case XID_BOUNDS_OK:
+			break;
+		case XID_IN_FUTURE:
+			report_corruption(ctx,
+							  psprintf("xmin %u equals or exceeds next valid transaction ID %u:%u",
+									   xmin,
+									   EpochFromFullTransactionId(ctx->next_fxid),
+									   XidFromFullTransactionId(ctx->next_fxid)));
+			return false;
+		case XID_PRECEDES_CLUSTERMIN:
+			report_corruption(ctx,
+							  psprintf("xmin %u precedes oldest valid transaction ID %u:%u",
+									   xmin,
+									   EpochFromFullTransactionId(ctx->oldest_fxid),
+									   XidFromFullTransactionId(ctx->oldest_fxid)));
+			return false;
+		case XID_PRECEDES_RELMIN:
+			report_corruption(ctx,
+							  psprintf("xmin %u precedes relation freeze threshold %u:%u",
+									   xmin,
+									   EpochFromFullTransactionId(ctx->relfrozenfxid),
+									   XidFromFullTransactionId(ctx->relfrozenfxid)));
+			return false;
+	}
+
+	/*
+	 * Has inserting transaction committed?
+	 */
+	if (!HeapTupleHeaderXminCommitted(tuphdr))
+	{
+		if (HeapTupleHeaderXminInvalid(tuphdr))
+			return false;		/* inserter aborted, don't check */
+		/* Used by pre-9.0 binary upgrades */
+		else if (tuphdr->t_infomask & HEAP_MOVED_OFF)
+		{
+			xvac = HeapTupleHeaderGetXvac(tuphdr);
+
+			switch (get_xid_status(xvac, ctx, &xvac_status))
+			{
+				case XID_INVALID:
+					report_corruption(ctx,
+									  pstrdup("old-style VACUUM FULL transaction ID for moved off tuple is invalid"));
+					return false;
+				case XID_IN_FUTURE:
+					report_corruption(ctx,
+									  psprintf("old-style VACUUM FULL transaction ID %u for moved off tuple equals or exceeds next valid transaction ID %u:%u",
+											   xvac,
+											   EpochFromFullTransactionId(ctx->next_fxid),
+											   XidFromFullTransactionId(ctx->next_fxid)));
+					return false;
+				case XID_PRECEDES_RELMIN:
+					report_corruption(ctx,
+									  psprintf("old-style VACUUM FULL transaction ID %u for moved off tuple precedes relation freeze threshold %u:%u",
+											   xvac,
+											   EpochFromFullTransactionId(ctx->relfrozenfxid),
+											   XidFromFullTransactionId(ctx->relfrozenfxid)));
+					return false;
+				case XID_PRECEDES_CLUSTERMIN:
+					report_corruption(ctx,
+									  psprintf("old-style VACUUM FULL transaction ID %u for moved off tuple precedes oldest valid transaction ID %u:%u",
+											   xvac,
+											   EpochFromFullTransactionId(ctx->oldest_fxid),
+											   XidFromFullTransactionId(ctx->oldest_fxid)));
+					return false;
+				case XID_BOUNDS_OK:
+					break;
+			}
+
+			switch (xvac_status)
+			{
+				case XID_IS_CURRENT_XID:
+					report_corruption(ctx,
+									  psprintf("old-style VACUUM FULL transaction ID %u for moved off tuple matches our current transaction ID",
+											   xvac));
+					return false;
+				case XID_IN_PROGRESS:
+					report_corruption(ctx,
+									  psprintf("old-style VACUUM FULL transaction ID %u for moved off tuple appears to be in progress",
+											   xvac));
+					return false;
+
+				case XID_COMMITTED:
+
+					/*
+					 * The tuple is dead, because the xvac transaction moved
+					 * it off and committed. It's checkable, but also
+					 * prunable.
+					 */
+					return true;
+
+				case XID_ABORTED:
+
+					/*
+					 * The original xmin must have committed, because the xvac
+					 * transaction tried to move it later. Since xvac is
+					 * aborted, whether it's still alive now depends on the
+					 * status of xmax.
+					 */
+					break;
+			}
+		}
+		/* Used by pre-9.0 binary upgrades */
+		else if (tuphdr->t_infomask & HEAP_MOVED_IN)
+		{
+			xvac = HeapTupleHeaderGetXvac(tuphdr);
+
+			switch (get_xid_status(xvac, ctx, &xvac_status))
+			{
+				case XID_INVALID:
+					report_corruption(ctx,
+									  pstrdup("old-style VACUUM FULL transaction ID for moved in tuple is invalid"));
+					return false;
+				case XID_IN_FUTURE:
+					report_corruption(ctx,
+									  psprintf("old-style VACUUM FULL transaction ID %u for moved in tuple equals or exceeds next valid transaction ID %u:%u",
+											   xvac,
+											   EpochFromFullTransactionId(ctx->next_fxid),
+											   XidFromFullTransactionId(ctx->next_fxid)));
+					return false;
+				case XID_PRECEDES_RELMIN:
+					report_corruption(ctx,
+									  psprintf("old-style VACUUM FULL transaction ID %u for moved in tuple precedes relation freeze threshold %u:%u",
+											   xvac,
+											   EpochFromFullTransactionId(ctx->relfrozenfxid),
+											   XidFromFullTransactionId(ctx->relfrozenfxid)));
+					return false;
+				case XID_PRECEDES_CLUSTERMIN:
+					report_corruption(ctx,
+									  psprintf("old-style VACUUM FULL transaction ID %u for moved in tuple precedes oldest valid transaction ID %u:%u",
+											   xvac,
+											   EpochFromFullTransactionId(ctx->oldest_fxid),
+											   XidFromFullTransactionId(ctx->oldest_fxid)));
+					return false;
+				case XID_BOUNDS_OK:
+					break;
+			}
+
+			switch (xvac_status)
+			{
+				case XID_IS_CURRENT_XID:
+					report_corruption(ctx,
+									  psprintf("old-style VACUUM FULL transaction ID %u for moved in tuple matches our current transaction ID",
+											   xvac));
+					return false;
+				case XID_IN_PROGRESS:
+					report_corruption(ctx,
+									  psprintf("old-style VACUUM FULL transaction ID %u for moved in tuple appears to be in progress",
+											   xvac));
+					return false;
+
+				case XID_COMMITTED:
+
+					/*
+					 * The original xmin must have committed, because the xvac
+					 * transaction moved it later. Whether it's still alive
+					 * now depends on the status of xmax.
+					 */
+					break;
+
+				case XID_ABORTED:
+
+					/*
+					 * The tuple is dead, because the xvac transaction moved
+					 * it off and committed. It's checkable, but also
+					 * prunable.
+					 */
+					return true;
+			}
+		}
+		else if (xmin_status != XID_COMMITTED)
+		{
+			/*
+			 * Inserting transaction is not in progress, and not committed, so
+			 * it might have changed the TupleDesc in ways we don't know
+			 * about. Thus, don't try to check the tuple structure.
+			 *
+			 * If xmin_status happens to be XID_IS_CURRENT_XID, then in theory
+			 * any such DDL changes ought to be visible to us, so perhaps we
+			 * could check anyway in that case. But, for now, let's be
+			 * conservative and treat this like any other uncommitted insert.
+			 */
+			return false;
+		}
+	}
+
+	/*
+	 * Okay, the inserter committed, so it was good at some point.  Now what
+	 * about the deleting transaction?
+	 */
+
+	if (tuphdr->t_infomask & HEAP_XMAX_IS_MULTI)
+	{
+		/*
+		 * xmax is a multixact, so sanity-check the MXID. Note that we do this
+		 * prior to checking for HEAP_XMAX_INVALID or
+		 * HEAP_XMAX_IS_LOCKED_ONLY. This might therefore complain about
+		 * things that wouldn't actually be a problem during a normal scan,
+		 * but eventually we're going to have to freeze, and that process will
+		 * ignore hint bits.
+		 *
+		 * Even if the MXID is out of range, we still know that the original
+		 * insert committed, so we can check the tuple itself. However, we
+		 * can't rule out the possibility that this tuple is dead, so don't
+		 * clear ctx->tuple_could_be_pruned. Possibly we should go ahead and
+		 * clear that flag anyway if HEAP_XMAX_INVALID is set or if
+		 * HEAP_XMAX_IS_LOCKED_ONLY is true, but for now we err on the side of
+		 * avoiding possibly-bogus complaints about missing TOAST entries.
+		 */
+		xmax = HeapTupleHeaderGetRawXmax(tuphdr);
+		switch (check_mxid_valid_in_rel(xmax, ctx))
+		{
+			case XID_INVALID:
+				report_corruption(ctx,
+								  pstrdup("multitransaction ID is invalid"));
+				return true;
+			case XID_PRECEDES_RELMIN:
+				report_corruption(ctx,
+								  psprintf("multitransaction ID %u precedes relation minimum multitransaction ID threshold %u",
+										   xmax, ctx->relminmxid));
+				return true;
+			case XID_PRECEDES_CLUSTERMIN:
+				report_corruption(ctx,
+								  psprintf("multitransaction ID %u precedes oldest valid multitransaction ID threshold %u",
+										   xmax, ctx->oldest_mxact));
+				return true;
+			case XID_IN_FUTURE:
+				report_corruption(ctx,
+								  psprintf("multitransaction ID %u equals or exceeds next valid multitransaction ID %u",
+										   xmax,
+										   ctx->next_mxact));
+				return true;
+			case XID_BOUNDS_OK:
+				break;
+		}
+	}
+
+	if (tuphdr->t_infomask & HEAP_XMAX_INVALID)
+	{
+		/*
+		 * This tuple is live.  A concurrently running transaction could
+		 * delete it before we get around to checking the toast, but any such
+		 * running transaction is surely not less than our safe_xmin, so the
+		 * toast cannot be vacuumed out from under us.
+		 */
+		ctx->tuple_could_be_pruned = false;
+		return true;
+	}
+
+	if (HEAP_XMAX_IS_LOCKED_ONLY(tuphdr->t_infomask))
+	{
+		/*
+		 * "Deleting" xact really only locked it, so the tuple is live in any
+		 * case.  As above, a concurrently running transaction could delete
+		 * it, but it cannot be vacuumed out from under us.
+		 */
+		ctx->tuple_could_be_pruned = false;
+		return true;
+	}
+
+	if (tuphdr->t_infomask & HEAP_XMAX_IS_MULTI)
+	{
+		/*
+		 * We already checked above that this multixact is within limits for
+		 * this table.  Now check the update xid from this multixact.
+		 */
+		xmax = HeapTupleGetUpdateXid(tuphdr);
+		switch (get_xid_status(xmax, ctx, &xmax_status))
+		{
+			case XID_INVALID:
+				/* not LOCKED_ONLY, so it has to have an xmax */
+				report_corruption(ctx,
+								  pstrdup("update xid is invalid"));
+				return true;
+			case XID_IN_FUTURE:
+				report_corruption(ctx,
+								  psprintf("update xid %u equals or exceeds next valid transaction ID %u:%u",
+										   xmax,
+										   EpochFromFullTransactionId(ctx->next_fxid),
+										   XidFromFullTransactionId(ctx->next_fxid)));
+				return true;
+			case XID_PRECEDES_RELMIN:
+				report_corruption(ctx,
+								  psprintf("update xid %u precedes relation freeze threshold %u:%u",
+										   xmax,
+										   EpochFromFullTransactionId(ctx->relfrozenfxid),
+										   XidFromFullTransactionId(ctx->relfrozenfxid)));
+				return true;
+			case XID_PRECEDES_CLUSTERMIN:
+				report_corruption(ctx,
+								  psprintf("update xid %u precedes oldest valid transaction ID %u:%u",
+										   xmax,
+										   EpochFromFullTransactionId(ctx->oldest_fxid),
+										   XidFromFullTransactionId(ctx->oldest_fxid)));
+				return true;
+			case XID_BOUNDS_OK:
+				break;
+		}
+
+		switch (xmax_status)
+		{
+			case XID_IS_CURRENT_XID:
+			case XID_IN_PROGRESS:
+
+				/*
+				 * The delete is in progress, so it cannot be visible to our
+				 * snapshot.
+				 */
+				ctx->tuple_could_be_pruned = false;
+				break;
+			case XID_COMMITTED:
+
+				/*
+				 * The delete committed.  Whether the toast can be vacuumed
+				 * away depends on how old the deleting transaction is.
+				 */
+				ctx->tuple_could_be_pruned = TransactionIdPrecedes(xmax,
+																   ctx->safe_xmin);
+				break;
+			case XID_ABORTED:
+
+				/*
+				 * The delete aborted or crashed.  The tuple is still live.
+				 */
+				ctx->tuple_could_be_pruned = false;
+				break;
+		}
+
+		/* Tuple itself is checkable even if it's dead. */
+		return true;
+	}
+
+	/* xmax is an XID, not a MXID. Sanity check it. */
+	xmax = HeapTupleHeaderGetRawXmax(tuphdr);
+	switch (get_xid_status(xmax, ctx, &xmax_status))
+	{
+		case XID_IN_FUTURE:
+			report_corruption(ctx,
+							  psprintf("xmax %u equals or exceeds next valid transaction ID %u:%u",
+									   xmax,
+									   EpochFromFullTransactionId(ctx->next_fxid),
+									   XidFromFullTransactionId(ctx->next_fxid)));
+			return false;		/* corrupt */
+		case XID_PRECEDES_RELMIN:
+			report_corruption(ctx,
+							  psprintf("xmax %u precedes relation freeze threshold %u:%u",
+									   xmax,
+									   EpochFromFullTransactionId(ctx->relfrozenfxid),
+									   XidFromFullTransactionId(ctx->relfrozenfxid)));
+			return false;		/* corrupt */
+		case XID_PRECEDES_CLUSTERMIN:
+			report_corruption(ctx,
+							  psprintf("xmax %u precedes oldest valid transaction ID %u:%u",
+									   xmax,
+									   EpochFromFullTransactionId(ctx->oldest_fxid),
+									   XidFromFullTransactionId(ctx->oldest_fxid)));
+			return false;		/* corrupt */
+		case XID_BOUNDS_OK:
+		case XID_INVALID:
+			break;
+	}
+
+	/*
+	 * Whether the toast can be vacuumed away depends on how old the deleting
+	 * transaction is.
+	 */
+	switch (xmax_status)
+	{
+		case XID_IS_CURRENT_XID:
+		case XID_IN_PROGRESS:
+
+			/*
+			 * The delete is in progress, so it cannot be visible to our
+			 * snapshot.
+			 */
+			ctx->tuple_could_be_pruned = false;
+			break;
+
+		case XID_COMMITTED:
+
+			/*
+			 * The delete committed.  Whether the toast can be vacuumed away
+			 * depends on how old the deleting transaction is.
+			 */
+			ctx->tuple_could_be_pruned = TransactionIdPrecedes(xmax,
+															   ctx->safe_xmin);
+			break;
+
+		case XID_ABORTED:
+
+			/*
+			 * The delete aborted or crashed.  The tuple is still live.
+			 */
+			ctx->tuple_could_be_pruned = false;
+			break;
+	}
+
+	/* Tuple itself is checkable even if it's dead. */
+	return true;
+}
+
+
+/*
+ * Check the current toast tuple against the state tracked in ctx, recording
+ * any corruption found in ctx->tupstore.
+ *
+ * This is not equivalent to running verify_heapam on the toast table itself,
+ * and is not hardened against corruption of the toast table.  Rather, when
+ * validating a toasted attribute in the main table, the sequence of toast
+ * tuples that store the toasted value are retrieved and checked in order, with
+ * each toast tuple being checked against where we are in the sequence, as well
+ * as each toast tuple having its varlena structure sanity checked.
+ *
+ * On entry, *expected_chunk_seq should be the chunk_seq value that we expect
+ * to find in toasttup. On exit, it will be updated to the value the next call
+ * to this function should expect to see.
+ */
+static void
+check_toast_tuple(HeapTuple toasttup, HeapCheckContext *ctx,
+				  ToastedAttribute *ta, int32 *expected_chunk_seq,
+				  uint32 extsize)
+{
+	int32		chunk_seq;
+	int32		last_chunk_seq = (extsize - 1) / TOAST_MAX_CHUNK_SIZE;
+	Pointer		chunk;
+	bool		isnull;
+	int32		chunksize;
+	int32		expected_size;
+
+	/* Sanity-check the sequence number. */
+	chunk_seq = DatumGetInt32(fastgetattr(toasttup, 2,
+										  ctx->toast_rel->rd_att, &isnull));
+	if (isnull)
+	{
+		report_toast_corruption(ctx, ta,
+								psprintf("toast value %u has toast chunk with null sequence number",
+										 ta->toast_pointer.va_valueid));
+		return;
+	}
+	if (chunk_seq != *expected_chunk_seq)
+	{
+		/* Either the TOAST index is corrupt, or we don't have all chunks. */
+		report_toast_corruption(ctx, ta,
+								psprintf("toast value %u index scan returned chunk %d when expecting chunk %d",
+										 ta->toast_pointer.va_valueid,
+										 chunk_seq, *expected_chunk_seq));
+	}
+	*expected_chunk_seq = chunk_seq + 1;
+
+	/* Sanity-check the chunk data. */
+	chunk = DatumGetPointer(fastgetattr(toasttup, 3,
+										ctx->toast_rel->rd_att, &isnull));
+	if (isnull)
+	{
+		report_toast_corruption(ctx, ta,
+								psprintf("toast value %u chunk %d has null data",
+										 ta->toast_pointer.va_valueid,
+										 chunk_seq));
+		return;
+	}
+	if (!VARATT_IS_EXTENDED(chunk))
+		chunksize = VARSIZE(chunk) - VARHDRSZ;
+	else if (VARATT_IS_SHORT(chunk))
+	{
+		/*
+		 * could happen due to heap_form_tuple doing its thing
+		 */
+		chunksize = VARSIZE_SHORT(chunk) - VARHDRSZ_SHORT;
+	}
+	else
+	{
+		/* should never happen */
+		uint32		header = ((varattrib_4b *) chunk)->va_4byte.va_header;
+
+		report_toast_corruption(ctx, ta,
+								psprintf("toast value %u chunk %d has invalid varlena header %0x",
+										 ta->toast_pointer.va_valueid,
+										 chunk_seq, header));
+		return;
+	}
+
+	/*
+	 * Some checks on the data we've found
+	 */
+	if (chunk_seq > last_chunk_seq)
+	{
+		report_toast_corruption(ctx, ta,
+								psprintf("toast value %u chunk %d follows last expected chunk %d",
+										 ta->toast_pointer.va_valueid,
+										 chunk_seq, last_chunk_seq));
+		return;
+	}
+
+	expected_size = chunk_seq < last_chunk_seq ? TOAST_MAX_CHUNK_SIZE
+		: extsize - (last_chunk_seq * TOAST_MAX_CHUNK_SIZE);
+
+	if (chunksize != expected_size)
+		report_toast_corruption(ctx, ta,
+								psprintf("toast value %u chunk %d has size %u, but expected size %u",
+										 ta->toast_pointer.va_valueid,
+										 chunk_seq, chunksize, expected_size));
+}
+
+/*
+ * Check the current attribute as tracked in ctx, recording any corruption
+ * found in ctx->tupstore.
+ *
+ * This function follows the logic performed by heap_deform_tuple(), and in the
+ * case of a toasted value, optionally stores the toast pointer so later it can
+ * be checked following the logic of detoast_external_attr(), checking for any
+ * conditions that would result in either of those functions Asserting or
+ * crashing the backend.  The checks performed by Asserts present in those two
+ * functions are also performed here and in check_toasted_attribute.  In cases
+ * where those two functions are a bit cavalier in their assumptions about data
+ * being correct, we perform additional checks not present in either of those
+ * two functions.  Where some condition is checked in both of those functions,
+ * we perform it here twice, as we parallel the logical flow of those two
+ * functions.  The presence of duplicate checks seems a reasonable price to pay
+ * for keeping this code tightly coupled with the code it protects.
+ *
+ * Returns true if the tuple attribute is sane enough for processing to
+ * continue on to the next attribute, false otherwise.
+ */
+static bool
+check_tuple_attribute(HeapCheckContext *ctx)
+{
+	Datum		attdatum;
+	struct varlena *attr;
+	char	   *tp;				/* pointer to the tuple data */
+	uint16		infomask;
+	Form_pg_attribute thisatt;
+	struct varatt_external toast_pointer;
+
+	infomask = ctx->tuphdr->t_infomask;
+	thisatt = TupleDescAttr(RelationGetDescr(ctx->rel), ctx->attnum);
+
+	tp = (char *) ctx->tuphdr + ctx->tuphdr->t_hoff;
+
+	if (ctx->tuphdr->t_hoff + ctx->offset > ctx->lp_len)
+	{
+		report_corruption(ctx,
+						  psprintf("attribute with length %u starts at offset %u beyond total tuple length %u",
+								   thisatt->attlen,
+								   ctx->tuphdr->t_hoff + ctx->offset,
+								   ctx->lp_len));
+		return false;
+	}
+
+	/* Skip null values */
+	if (infomask & HEAP_HASNULL && att_isnull(ctx->attnum, ctx->tuphdr->t_bits))
+		return true;
+
+	/* Skip non-varlena values, but update offset first */
+	if (thisatt->attlen != -1)
+	{
+		ctx->offset = att_align_nominal(ctx->offset, thisatt->attalign);
+		ctx->offset = att_addlength_pointer(ctx->offset, thisatt->attlen,
+											tp + ctx->offset);
+		if (ctx->tuphdr->t_hoff + ctx->offset > ctx->lp_len)
+		{
+			report_corruption(ctx,
+							  psprintf("attribute with length %u ends at offset %u beyond total tuple length %u",
+									   thisatt->attlen,
+									   ctx->tuphdr->t_hoff + ctx->offset,
+									   ctx->lp_len));
+			return false;
+		}
+		return true;
+	}
+
+	/* Ok, we're looking at a varlena attribute. */
+	ctx->offset = att_align_pointer(ctx->offset, thisatt->attalign, -1,
+									tp + ctx->offset);
+
+	/* Get the (possibly corrupt) varlena datum */
+	attdatum = fetchatt(thisatt, tp + ctx->offset);
+
+	/*
+	 * We have the datum, but we cannot decode it carelessly, as it may still
+	 * be corrupt.
+	 */
+
+	/*
+	 * Check that VARTAG_SIZE won't hit a TrapMacro on a corrupt va_tag before
+	 * risking a call into att_addlength_pointer
+	 */
+	if (VARATT_IS_EXTERNAL(tp + ctx->offset))
+	{
+		uint8		va_tag = VARTAG_EXTERNAL(tp + ctx->offset);
+
+		if (va_tag != VARTAG_ONDISK)
+		{
+			report_corruption(ctx,
+							  psprintf("toasted attribute has unexpected TOAST tag %u",
+									   va_tag));
+			/* We can't know where the next attribute begins */
+			return false;
+		}
+	}
+
+	/* Ok, should be safe now */
+	ctx->offset = att_addlength_pointer(ctx->offset, thisatt->attlen,
+										tp + ctx->offset);
+
+	if (ctx->tuphdr->t_hoff + ctx->offset > ctx->lp_len)
+	{
+		report_corruption(ctx,
+						  psprintf("attribute with length %u ends at offset %u beyond total tuple length %u",
+								   thisatt->attlen,
+								   ctx->tuphdr->t_hoff + ctx->offset,
+								   ctx->lp_len));
+
+		return false;
+	}
+
+	/*
+	 * heap_deform_tuple would be done with this attribute at this point,
+	 * having stored it in values[], and would continue to the next attribute.
+	 * We go further, because we need to check if the toast datum is corrupt.
+	 */
+
+	attr = (struct varlena *) DatumGetPointer(attdatum);
+
+	/*
+	 * Now we follow the logic of detoast_external_attr(), with the same
+	 * caveats about being paranoid about corruption.
+	 */
+
+	/* Skip values that are not external */
+	if (!VARATT_IS_EXTERNAL(attr))
+		return true;
+
+	/* It is external, and we're looking at a page on disk */
+
+	/*
+	 * Must copy attr into toast_pointer for alignment considerations
+	 */
+	VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
+
+	/* The tuple header better claim to contain toasted values */
+	if (!(infomask & HEAP_HASEXTERNAL))
+	{
+		report_corruption(ctx,
+						  psprintf("toast value %u is external but tuple header flag HEAP_HASEXTERNAL not set",
+								   toast_pointer.va_valueid));
+		return true;
+	}
+
+	/* The relation better have a toast table */
+	if (!ctx->rel->rd_rel->reltoastrelid)
+	{
+		report_corruption(ctx,
+						  psprintf("toast value %u is external but relation has no toast relation",
+								   toast_pointer.va_valueid));
+		return true;
+	}
+
+	/* If we were told to skip toast checking, then we're done. */
+	if (ctx->toast_rel == NULL)
+		return true;
+
+	/*
+	 * If this tuple is eligible to be pruned, we cannot check the toast.
+	 * Otherwise, we push a copy of the toast tuple so we can check it after
+	 * releasing the main table buffer lock.
+	 */
+	if (!ctx->tuple_could_be_pruned)
+	{
+		ToastedAttribute *ta;
+
+		ta = (ToastedAttribute *) palloc0(sizeof(ToastedAttribute));
+
+		VARATT_EXTERNAL_GET_POINTER(ta->toast_pointer, attr);
+		ta->blkno = ctx->blkno;
+		ta->offnum = ctx->offnum;
+		ta->attnum = ctx->attnum;
+		ctx->toasted_attributes = lappend(ctx->toasted_attributes, ta);
+	}
+
+	return true;
+}
+
+/*
+ * For each attribute collected in ctx->toasted_attributes, look up the value
+ * in the toast table and perform checks on it.  This function should only be
+ * called on toast pointers which cannot be vacuumed away during our
+ * processing.
+ */
+static void
+check_toasted_attribute(HeapCheckContext *ctx, ToastedAttribute *ta)
+{
+	SnapshotData SnapshotToast;
+	ScanKeyData toastkey;
+	SysScanDesc toastscan;
+	bool		found_toasttup;
+	HeapTuple	toasttup;
+	uint32		extsize;
+	int32		expected_chunk_seq = 0;
+	int32		last_chunk_seq;
+
+	extsize = VARATT_EXTERNAL_GET_EXTSIZE(ta->toast_pointer);
+	last_chunk_seq = (extsize - 1) / TOAST_MAX_CHUNK_SIZE;
+
+	/*
+	 * Setup a scan key to find chunks in toast table with matching va_valueid
+	 */
+	ScanKeyInit(&toastkey,
+				(AttrNumber) 1,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(ta->toast_pointer.va_valueid));
+
+	/*
+	 * Check if any chunks for this toasted object exist in the toast table,
+	 * accessible via the index.
+	 */
+	init_toast_snapshot(&SnapshotToast);
+	toastscan = systable_beginscan_ordered(ctx->toast_rel,
+										   ctx->valid_toast_index,
+										   &SnapshotToast, 1,
+										   &toastkey);
+	found_toasttup = false;
+	while ((toasttup =
+			systable_getnext_ordered(toastscan,
+									 ForwardScanDirection)) != NULL)
+	{
+		found_toasttup = true;
+		check_toast_tuple(toasttup, ctx, ta, &expected_chunk_seq, extsize);
+	}
+	systable_endscan_ordered(toastscan);
+
+	if (!found_toasttup)
+		report_toast_corruption(ctx, ta,
+								psprintf("toast value %u not found in toast table",
+										 ta->toast_pointer.va_valueid));
+	else if (expected_chunk_seq <= last_chunk_seq)
+		report_toast_corruption(ctx, ta,
+								psprintf("toast value %u was expected to end at chunk %d, but ended while expecting chunk %d",
+										 ta->toast_pointer.va_valueid,
+										 last_chunk_seq, expected_chunk_seq));
+}
+
+/*
+ * Check the current tuple as tracked in ctx, recording any corruption found in
+ * ctx->tupstore.
+ */
+static void
+check_tuple(HeapCheckContext *ctx)
+{
+	/*
+	 * Check various forms of tuple header corruption, and if the header is
+	 * too corrupt, do not continue with other checks.
+	 */
+	if (!check_tuple_header(ctx))
+		return;
+
+	/*
+	 * Check tuple visibility.  If the inserting transaction aborted, we
+	 * cannot assume our relation description matches the tuple structure, and
+	 * therefore cannot check it.
+	 */
+	if (!check_tuple_visibility(ctx))
+		return;
+
+	/*
+	 * The tuple is visible, so it must be compatible with the current version
+	 * of the relation descriptor. It might have fewer columns than are
+	 * present in the relation descriptor, but it cannot have more.
+	 */
+	if (RelationGetDescr(ctx->rel)->natts < ctx->natts)
+	{
+		report_corruption(ctx,
+						  psprintf("number of attributes %u exceeds maximum expected for table %u",
+								   ctx->natts,
+								   RelationGetDescr(ctx->rel)->natts));
+		return;
+	}
+
+	/*
+	 * Check each attribute unless we hit corruption that confuses what to do
+	 * next, at which point we abort further attribute checks for this tuple.
+	 * Note that we don't abort for all types of corruption, only for those
+	 * types where we don't know how to continue.  We also don't abort the
+	 * checking of toasted attributes collected from the tuple prior to
+	 * aborting.  Those will still be checked later along with other toasted
+	 * attributes collected from the page.
+	 */
+	ctx->offset = 0;
+	for (ctx->attnum = 0; ctx->attnum < ctx->natts; ctx->attnum++)
+		if (!check_tuple_attribute(ctx))
+			break;				/* cannot continue */
+
+	/* revert attnum to -1 until we again examine individual attributes */
+	ctx->attnum = -1;
+}
+
+/*
+ * Convert a TransactionId into a FullTransactionId using our cached values of
+ * the valid transaction ID range.  It is the caller's responsibility to have
+ * already updated the cached values, if necessary.
+ */
+static FullTransactionId
+FullTransactionIdFromXidAndCtx(TransactionId xid, const HeapCheckContext *ctx)
+{
+	uint32		epoch;
+
+	if (!TransactionIdIsNormal(xid))
+		return FullTransactionIdFromEpochAndXid(0, xid);
+	epoch = EpochFromFullTransactionId(ctx->next_fxid);
+	if (xid > ctx->next_xid)
+		epoch--;
+	return FullTransactionIdFromEpochAndXid(epoch, xid);
+}
+
+/*
+ * Update our cached range of valid transaction IDs.
+ */
+static void
+update_cached_xid_range(HeapCheckContext *ctx)
+{
+	/* Make cached copies */
+	LWLockAcquire(XidGenLock, LW_SHARED);
+	ctx->next_fxid = ShmemVariableCache->nextXid;
+	ctx->oldest_xid = ShmemVariableCache->oldestXid;
+	LWLockRelease(XidGenLock);
+
+	/* And compute alternate versions of the same */
+	ctx->oldest_fxid = FullTransactionIdFromXidAndCtx(ctx->oldest_xid, ctx);
+	ctx->next_xid = XidFromFullTransactionId(ctx->next_fxid);
+}
+
+/*
+ * Update our cached range of valid multitransaction IDs.
+ */
+static void
+update_cached_mxid_range(HeapCheckContext *ctx)
+{
+	ReadMultiXactIdRange(&ctx->oldest_mxact, &ctx->next_mxact);
+}
+
+/*
+ * Return whether the given FullTransactionId is within our cached valid
+ * transaction ID range.
+ */
+static inline bool
+fxid_in_cached_range(FullTransactionId fxid, const HeapCheckContext *ctx)
+{
+	return (FullTransactionIdPrecedesOrEquals(ctx->oldest_fxid, fxid) &&
+			FullTransactionIdPrecedes(fxid, ctx->next_fxid));
+}
+
+/*
+ * Checks whether a multitransaction ID is in the cached valid range, returning
+ * the nature of the range violation, if any.
+ */
+static XidBoundsViolation
+check_mxid_in_range(MultiXactId mxid, HeapCheckContext *ctx)
+{
+	if (!TransactionIdIsValid(mxid))
+		return XID_INVALID;
+	if (MultiXactIdPrecedes(mxid, ctx->relminmxid))
+		return XID_PRECEDES_RELMIN;
+	if (MultiXactIdPrecedes(mxid, ctx->oldest_mxact))
+		return XID_PRECEDES_CLUSTERMIN;
+	if (MultiXactIdPrecedesOrEquals(ctx->next_mxact, mxid))
+		return XID_IN_FUTURE;
+	return XID_BOUNDS_OK;
+}
+
+/*
+ * Checks whether the given mxid is valid to appear in the heap being checked,
+ * returning the nature of the range violation, if any.
+ *
+ * This function attempts to return quickly by caching the known valid mxid
+ * range in ctx.  Callers should already have performed the initial setup of
+ * the cache prior to the first call to this function.
+ */
+static XidBoundsViolation
+check_mxid_valid_in_rel(MultiXactId mxid, HeapCheckContext *ctx)
+{
+	XidBoundsViolation result;
+
+	result = check_mxid_in_range(mxid, ctx);
+	if (result == XID_BOUNDS_OK)
+		return XID_BOUNDS_OK;
+
+	/* The range may have advanced.  Recheck. */
+	update_cached_mxid_range(ctx);
+	return check_mxid_in_range(mxid, ctx);
+}
+
+/*
+ * Checks whether the given transaction ID is (or was recently) valid to appear
+ * in the heap being checked, or whether it is too old or too new to appear in
+ * the relation, returning information about the nature of the bounds violation.
+ *
+ * We cache the range of valid transaction IDs.  If xid is in that range, we
+ * conclude that it is valid, even though concurrent changes to the table might
+ * invalidate it under certain corrupt conditions.  (For example, if the table
+ * contains corrupt all-frozen bits, a concurrent vacuum might skip the page(s)
+ * containing the xid and then truncate clog and advance the relfrozenxid
+ * beyond xid.) Reporting the xid as valid under such conditions seems
+ * acceptable, since if we had checked it earlier in our scan it would have
+ * truly been valid at that time.
+ *
+ * If the status argument is not NULL, and if and only if the transaction ID
+ * appears to be valid in this relation, the status argument will be set with
+ * the commit status of the transaction ID.
+ */
+static XidBoundsViolation
+get_xid_status(TransactionId xid, HeapCheckContext *ctx,
+			   XidCommitStatus *status)
+{
+	FullTransactionId fxid;
+	FullTransactionId clog_horizon;
+
+	/* Quick check for special xids */
+	if (!TransactionIdIsValid(xid))
+		return XID_INVALID;
+	else if (xid == BootstrapTransactionId || xid == FrozenTransactionId)
+	{
+		if (status != NULL)
+			*status = XID_COMMITTED;
+		return XID_BOUNDS_OK;
+	}
+
+	/* Check if the xid is within bounds */
+	fxid = FullTransactionIdFromXidAndCtx(xid, ctx);
+	if (!fxid_in_cached_range(fxid, ctx))
+	{
+		/*
+		 * We may have been checking against stale values.  Update the cached
+		 * range to be sure, and since we relied on the cached range when we
+		 * performed the full xid conversion, reconvert.
+		 */
+		update_cached_xid_range(ctx);
+		fxid = FullTransactionIdFromXidAndCtx(xid, ctx);
+	}
+
+	if (FullTransactionIdPrecedesOrEquals(ctx->next_fxid, fxid))
+		return XID_IN_FUTURE;
+	if (FullTransactionIdPrecedes(fxid, ctx->oldest_fxid))
+		return XID_PRECEDES_CLUSTERMIN;
+	if (FullTransactionIdPrecedes(fxid, ctx->relfrozenfxid))
+		return XID_PRECEDES_RELMIN;
+
+	/* Early return if the caller does not request clog checking */
+	if (status == NULL)
+		return XID_BOUNDS_OK;
+
+	/* Early return if we just checked this xid in a prior call */
+	if (xid == ctx->cached_xid)
+	{
+		*status = ctx->cached_status;
+		return XID_BOUNDS_OK;
+	}
+
+	*status = XID_COMMITTED;
+	LWLockAcquire(XactTruncationLock, LW_SHARED);
+	clog_horizon =
+		FullTransactionIdFromXidAndCtx(ShmemVariableCache->oldestClogXid,
+									   ctx);
+	if (FullTransactionIdPrecedesOrEquals(clog_horizon, fxid))
+	{
+		if (TransactionIdIsCurrentTransactionId(xid))
+			*status = XID_IS_CURRENT_XID;
+		else if (TransactionIdIsInProgress(xid))
+			*status = XID_IN_PROGRESS;
+		else if (TransactionIdDidCommit(xid))
+			*status = XID_COMMITTED;
+		else
+			*status = XID_ABORTED;
+	}
+	LWLockRelease(XactTruncationLock);
+	ctx->cached_xid = xid;
+	ctx->cached_status = *status;
+	return XID_BOUNDS_OK;
+}
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
new file mode 100644
index 0000000..7f8231a
--- /dev/null
+++ b/contrib/amcheck/verify_nbtree.c
@@ -0,0 +1,3260 @@
+/*-------------------------------------------------------------------------
+ *
+ * verify_nbtree.c
+ *		Verifies the integrity of nbtree indexes based on invariants.
+ *
+ * For B-Tree indexes, verification includes checking that each page in the
+ * target index has items in logical order as reported by an insertion scankey
+ * (the insertion scankey sort-wise NULL semantics are needed for
+ * verification).
+ *
+ * When index-to-heap verification is requested, a Bloom filter is used to
+ * fingerprint all tuples in the target index, as the index is traversed to
+ * verify its structure.  A heap scan later uses Bloom filter probes to verify
+ * that every visible heap tuple has a matching index tuple.
+ *
+ *
+ * Copyright (c) 2017-2021, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  contrib/amcheck/verify_nbtree.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/htup_details.h"
+#include "access/nbtree.h"
+#include "access/table.h"
+#include "access/tableam.h"
+#include "access/transam.h"
+#include "access/xact.h"
+#include "catalog/index.h"
+#include "catalog/pg_am.h"
+#include "commands/tablecmds.h"
+#include "lib/bloomfilter.h"
+#include "miscadmin.h"
+#include "storage/lmgr.h"
+#include "storage/smgr.h"
+#include "utils/memutils.h"
+#include "utils/snapmgr.h"
+
+
+PG_MODULE_MAGIC;
+
+/*
+ * A B-Tree cannot possibly have this many levels, since there must be one
+ * block per level, which is bound by the range of BlockNumber:
+ */
+#define InvalidBtreeLevel	((uint32) InvalidBlockNumber)
+#define BTreeTupleGetNKeyAtts(itup, rel)   \
+	Min(IndexRelationGetNumberOfKeyAttributes(rel), BTreeTupleGetNAtts(itup, rel))
+
+/*
+ * State associated with verifying a B-Tree index
+ *
+ * target is the point of reference for a verification operation.
+ *
+ * Other B-Tree pages may be allocated, but those are always auxiliary (e.g.,
+ * they are current target's child pages).  Conceptually, problems are only
+ * ever found in the current target page (or for a particular heap tuple during
+ * heapallindexed verification).  Each page found by verification's left/right,
+ * top/bottom scan becomes the target exactly once.
+ */
+typedef struct BtreeCheckState
+{
+	/*
+	 * Unchanging state, established at start of verification:
+	 */
+
+	/* B-Tree Index Relation and associated heap relation */
+	Relation	rel;
+	Relation	heaprel;
+	/* rel is heapkeyspace index? */
+	bool		heapkeyspace;
+	/* ShareLock held on heap/index, rather than AccessShareLock? */
+	bool		readonly;
+	/* Also verifying heap has no unindexed tuples? */
+	bool		heapallindexed;
+	/* Also making sure non-pivot tuples can be found by new search? */
+	bool		rootdescend;
+	/* Per-page context */
+	MemoryContext targetcontext;
+	/* Buffer access strategy */
+	BufferAccessStrategy checkstrategy;
+
+	/*
+	 * Mutable state, for verification of particular page:
+	 */
+
+	/* Current target page */
+	Page		target;
+	/* Target block number */
+	BlockNumber targetblock;
+	/* Target page's LSN */
+	XLogRecPtr	targetlsn;
+
+	/*
+	 * Low key: high key of left sibling of target page.  Used only for child
+	 * verification.  So, 'lowkey' is kept only when 'readonly' is set.
+	 */
+	IndexTuple	lowkey;
+
+	/*
+	 * The rightlink and incomplete split flag of block one level down to the
+	 * target page, which was visited last time via downlink from taget page.
+	 * We use it to check for missing downlinks.
+	 */
+	BlockNumber prevrightlink;
+	bool		previncompletesplit;
+
+	/*
+	 * Mutable state, for optional heapallindexed verification:
+	 */
+
+	/* Bloom filter fingerprints B-Tree index */
+	bloom_filter *filter;
+	/* Debug counter */
+	int64		heaptuplespresent;
+} BtreeCheckState;
+
+/*
+ * Starting point for verifying an entire B-Tree index level
+ */
+typedef struct BtreeLevel
+{
+	/* Level number (0 is leaf page level). */
+	uint32		level;
+
+	/* Left most block on level.  Scan of level begins here. */
+	BlockNumber leftmost;
+
+	/* Is this level reported as "true" root level by meta page? */
+	bool		istruerootlevel;
+} BtreeLevel;
+
+PG_FUNCTION_INFO_V1(bt_index_check);
+PG_FUNCTION_INFO_V1(bt_index_parent_check);
+
+static void bt_index_check_internal(Oid indrelid, bool parentcheck,
+									bool heapallindexed, bool rootdescend);
+static inline void btree_index_checkable(Relation rel);
+static inline bool btree_index_mainfork_expected(Relation rel);
+static void bt_check_every_level(Relation rel, Relation heaprel,
+								 bool heapkeyspace, bool readonly, bool heapallindexed,
+								 bool rootdescend);
+static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
+											   BtreeLevel level);
+static void bt_recheck_sibling_links(BtreeCheckState *state,
+									 BlockNumber btpo_prev_from_target,
+									 BlockNumber leftcurrent);
+static void bt_target_page_check(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
+						   OffsetNumber downlinkoffnum);
+static void bt_child_highkey_check(BtreeCheckState *state,
+								   OffsetNumber target_downlinkoffnum,
+								   Page loaded_child,
+								   uint32 target_level);
+static void bt_downlink_missing_check(BtreeCheckState *state, bool rightsplit,
+									  BlockNumber targetblock, Page target);
+static void bt_tuple_present_callback(Relation index, ItemPointer tid,
+									  Datum *values, bool *isnull,
+									  bool tupleIsAlive, void *checkstate);
+static IndexTuple bt_normalize_tuple(BtreeCheckState *state,
+									 IndexTuple itup);
+static inline IndexTuple bt_posting_plain_tuple(IndexTuple itup, int n);
+static bool bt_rootdescend(BtreeCheckState *state, IndexTuple itup);
+static inline bool offset_is_negative_infinity(BTPageOpaque opaque,
+											   OffsetNumber offset);
+static inline bool invariant_l_offset(BtreeCheckState *state, BTScanInsert key,
+									  OffsetNumber upperbound);
+static inline bool invariant_leq_offset(BtreeCheckState *state,
+										BTScanInsert key,
+										OffsetNumber upperbound);
+static inline bool invariant_g_offset(BtreeCheckState *state, BTScanInsert key,
+									  OffsetNumber lowerbound);
+static inline bool invariant_l_nontarget_offset(BtreeCheckState *state,
+												BTScanInsert key,
+												BlockNumber nontargetblock,
+												Page nontarget,
+												OffsetNumber upperbound);
+static Page palloc_btree_page(BtreeCheckState *state, BlockNumber blocknum);
+static inline BTScanInsert bt_mkscankey_pivotsearch(Relation rel,
+													IndexTuple itup);
+static ItemId PageGetItemIdCareful(BtreeCheckState *state, BlockNumber block,
+								   Page page, OffsetNumber offset);
+static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
+													  IndexTuple itup, bool nonpivot);
+static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
+
+/*
+ * bt_index_check(index regclass, heapallindexed boolean)
+ *
+ * Verify integrity of B-Tree index.
+ *
+ * Acquires AccessShareLock on heap & index relations.  Does not consider
+ * invariants that exist between parent/child pages.  Optionally verifies
+ * that heap does not contain any unindexed or incorrectly indexed tuples.
+ */
+Datum
+bt_index_check(PG_FUNCTION_ARGS)
+{
+	Oid			indrelid = PG_GETARG_OID(0);
+	bool		heapallindexed = false;
+
+	if (PG_NARGS() == 2)
+		heapallindexed = PG_GETARG_BOOL(1);
+
+	bt_index_check_internal(indrelid, false, heapallindexed, false);
+
+	PG_RETURN_VOID();
+}
+
+/*
+ * bt_index_parent_check(index regclass, heapallindexed boolean)
+ *
+ * Verify integrity of B-Tree index.
+ *
+ * Acquires ShareLock on heap & index relations.  Verifies that downlinks in
+ * parent pages are valid lower bounds on child pages.  Optionally verifies
+ * that heap does not contain any unindexed or incorrectly indexed tuples.
+ */
+Datum
+bt_index_parent_check(PG_FUNCTION_ARGS)
+{
+	Oid			indrelid = PG_GETARG_OID(0);
+	bool		heapallindexed = false;
+	bool		rootdescend = false;
+
+	if (PG_NARGS() >= 2)
+		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		rootdescend = PG_GETARG_BOOL(2);
+
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+
+	PG_RETURN_VOID();
+}
+
+/*
+ * Helper for bt_index_[parent_]check, coordinating the bulk of the work.
+ */
+static void
+bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
+						bool rootdescend)
+{
+	Oid			heapid;
+	Relation	indrel;
+	Relation	heaprel;
+	LOCKMODE	lockmode;
+	Oid			save_userid;
+	int			save_sec_context;
+	int			save_nestlevel;
+
+	if (parentcheck)
+		lockmode = ShareLock;
+	else
+		lockmode = AccessShareLock;
+
+	/*
+	 * We must lock table before index to avoid deadlocks.  However, if the
+	 * passed indrelid isn't an index then IndexGetRelation() will fail.
+	 * Rather than emitting a not-very-helpful error message, postpone
+	 * complaining, expecting that the is-it-an-index test below will fail.
+	 *
+	 * In hot standby mode this will raise an error when parentcheck is true.
+	 */
+	heapid = IndexGetRelation(indrelid, true);
+	if (OidIsValid(heapid))
+	{
+		heaprel = table_open(heapid, lockmode);
+
+		/*
+		 * Switch to the table owner's userid, so that any index functions are
+		 * run as that user.  Also lock down security-restricted operations
+		 * and arrange to make GUC variable changes local to this command.
+		 */
+		GetUserIdAndSecContext(&save_userid, &save_sec_context);
+		SetUserIdAndSecContext(heaprel->rd_rel->relowner,
+							   save_sec_context | SECURITY_RESTRICTED_OPERATION);
+		save_nestlevel = NewGUCNestLevel();
+	}
+	else
+	{
+		heaprel = NULL;
+		/* Set these just to suppress "uninitialized variable" warnings */
+		save_userid = InvalidOid;
+		save_sec_context = -1;
+		save_nestlevel = -1;
+	}
+
+	/*
+	 * Open the target index relations separately (like relation_openrv(), but
+	 * with heap relation locked first to prevent deadlocking).  In hot
+	 * standby mode this will raise an error when parentcheck is true.
+	 *
+	 * There is no need for the usual indcheckxmin usability horizon test
+	 * here, even in the heapallindexed case, because index undergoing
+	 * verification only needs to have entries for a new transaction snapshot.
+	 * (If this is a parentcheck verification, there is no question about
+	 * committed or recently dead heap tuples lacking index entries due to
+	 * concurrent activity.)
+	 */
+	indrel = index_open(indrelid, lockmode);
+
+	/*
+	 * Since we did the IndexGetRelation call above without any lock, it's
+	 * barely possible that a race against an index drop/recreation could have
+	 * netted us the wrong table.
+	 */
+	if (heaprel == NULL || heapid != IndexGetRelation(indrelid, false))
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_TABLE),
+				 errmsg("could not open parent table of index \"%s\"",
+						RelationGetRelationName(indrel))));
+
+	/* Relation suitable for checking as B-Tree? */
+	btree_index_checkable(indrel);
+
+	if (btree_index_mainfork_expected(indrel))
+	{
+		bool		heapkeyspace,
+					allequalimage;
+
+		RelationOpenSmgr(indrel);
+		if (!smgrexists(indrel->rd_smgr, MAIN_FORKNUM))
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("index \"%s\" lacks a main relation fork",
+							RelationGetRelationName(indrel))));
+
+		/* Extract metadata from metapage, and sanitize it in passing */
+		_bt_metaversion(indrel, &heapkeyspace, &allequalimage);
+		if (allequalimage && !heapkeyspace)
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("index \"%s\" metapage has equalimage field set on unsupported nbtree version",
+							RelationGetRelationName(indrel))));
+		if (allequalimage && !_bt_allequalimage(indrel, false))
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("index \"%s\" metapage incorrectly indicates that deduplication is safe",
+							RelationGetRelationName(indrel))));
+
+		/* Check index, possibly against table it is an index on */
+		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
+							 heapallindexed, rootdescend);
+	}
+
+	/* Roll back any GUC changes executed by index functions */
+	AtEOXact_GUC(false, save_nestlevel);
+
+	/* Restore userid and security context */
+	SetUserIdAndSecContext(save_userid, save_sec_context);
+
+	/*
+	 * Release locks early. That's ok here because nothing in the called
+	 * routines will trigger shared cache invalidations to be sent, so we can
+	 * relax the usual pattern of only releasing locks after commit.
+	 */
+	index_close(indrel, lockmode);
+	if (heaprel)
+		table_close(heaprel, lockmode);
+}
+
+/*
+ * Basic checks about the suitability of a relation for checking as a B-Tree
+ * index.
+ *
+ * NB: Intentionally not checking permissions, the function is normally not
+ * callable by non-superusers. If granted, it's useful to be able to check a
+ * whole cluster.
+ */
+static inline void
+btree_index_checkable(Relation rel)
+{
+	if (rel->rd_rel->relkind != RELKIND_INDEX ||
+		rel->rd_rel->relam != BTREE_AM_OID)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("only B-Tree indexes are supported as targets for verification"),
+				 errdetail("Relation \"%s\" is not a B-Tree index.",
+						   RelationGetRelationName(rel))));
+
+	if (RELATION_IS_OTHER_TEMP(rel))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot access temporary tables of other sessions"),
+				 errdetail("Index \"%s\" is associated with temporary relation.",
+						   RelationGetRelationName(rel))));
+
+	if (!rel->rd_index->indisvalid)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot check index \"%s\"",
+						RelationGetRelationName(rel)),
+				 errdetail("Index is not valid.")));
+}
+
+/*
+ * Check if B-Tree index relation should have a file for its main relation
+ * fork.  Verification uses this to skip unlogged indexes when in hot standby
+ * mode, where there is simply nothing to verify.  We behave as if the
+ * relation is empty.
+ *
+ * NB: Caller should call btree_index_checkable() before calling here.
+ */
+static inline bool
+btree_index_mainfork_expected(Relation rel)
+{
+	if (rel->rd_rel->relpersistence != RELPERSISTENCE_UNLOGGED ||
+		!RecoveryInProgress())
+		return true;
+
+	ereport(DEBUG1,
+			(errcode(ERRCODE_READ_ONLY_SQL_TRANSACTION),
+			 errmsg("cannot verify unlogged index \"%s\" during recovery, skipping",
+					RelationGetRelationName(rel))));
+
+	return false;
+}
+
+/*
+ * Main entry point for B-Tree SQL-callable functions. Walks the B-Tree in
+ * logical order, verifying invariants as it goes.  Optionally, verification
+ * checks if the heap relation contains any tuples that are not represented in
+ * the index but should be.
+ *
+ * It is the caller's responsibility to acquire appropriate heavyweight lock on
+ * the index relation, and advise us if extra checks are safe when a ShareLock
+ * is held.  (A lock of the same type must also have been acquired on the heap
+ * relation.)
+ *
+ * A ShareLock is generally assumed to prevent any kind of physical
+ * modification to the index structure, including modifications that VACUUM may
+ * make.  This does not include setting of the LP_DEAD bit by concurrent index
+ * scans, although that is just metadata that is not able to directly affect
+ * any check performed here.  Any concurrent process that might act on the
+ * LP_DEAD bit being set (recycle space) requires a heavyweight lock that
+ * cannot be held while we hold a ShareLock.  (Besides, even if that could
+ * happen, the ad-hoc recycling when a page might otherwise split is performed
+ * per-page, and requires an exclusive buffer lock, which wouldn't cause us
+ * trouble.  _bt_delitems_vacuum() may only delete leaf items, and so the extra
+ * parent/child check cannot be affected.)
+ */
+static void
+bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
+					 bool readonly, bool heapallindexed, bool rootdescend)
+{
+	BtreeCheckState *state;
+	Page		metapage;
+	BTMetaPageData *metad;
+	uint32		previouslevel;
+	BtreeLevel	current;
+	Snapshot	snapshot = SnapshotAny;
+
+	if (!readonly)
+		elog(DEBUG1, "verifying consistency of tree structure for index \"%s\"",
+			 RelationGetRelationName(rel));
+	else
+		elog(DEBUG1, "verifying consistency of tree structure for index \"%s\" with cross-level checks",
+			 RelationGetRelationName(rel));
+
+	/*
+	 * This assertion matches the one in index_getnext_tid().  See page
+	 * recycling/"visible to everyone" notes in nbtree README.
+	 */
+	Assert(TransactionIdIsValid(RecentXmin));
+
+	/*
+	 * Initialize state for entire verification operation
+	 */
+	state = palloc0(sizeof(BtreeCheckState));
+	state->rel = rel;
+	state->heaprel = heaprel;
+	state->heapkeyspace = heapkeyspace;
+	state->readonly = readonly;
+	state->heapallindexed = heapallindexed;
+	state->rootdescend = rootdescend;
+
+	if (state->heapallindexed)
+	{
+		int64		total_pages;
+		int64		total_elems;
+		uint64		seed;
+
+		/*
+		 * Size Bloom filter based on estimated number of tuples in index,
+		 * while conservatively assuming that each block must contain at least
+		 * MaxTIDsPerBTreePage / 3 "plain" tuples -- see
+		 * bt_posting_plain_tuple() for definition, and details of how posting
+		 * list tuples are handled.
+		 */
+		total_pages = RelationGetNumberOfBlocks(rel);
+		total_elems = Max(total_pages * (MaxTIDsPerBTreePage / 3),
+						  (int64) state->rel->rd_rel->reltuples);
+		/* Random seed relies on backend srandom() call to avoid repetition */
+		seed = random();
+		/* Create Bloom filter to fingerprint index */
+		state->filter = bloom_create(total_elems, maintenance_work_mem, seed);
+		state->heaptuplespresent = 0;
+
+		/*
+		 * Register our own snapshot in !readonly case, 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());
+
+			/*
+			 * 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))));
+		}
+	}
+
+	Assert(!state->rootdescend || state->readonly);
+	if (state->rootdescend && !state->heapkeyspace)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot verify that tuples from index \"%s\" can each be found by an independent index search",
+						RelationGetRelationName(rel)),
+				 errhint("Only B-Tree version 4 indexes support rootdescend verification.")));
+
+	/* Create context for page */
+	state->targetcontext = AllocSetContextCreate(CurrentMemoryContext,
+												 "amcheck context",
+												 ALLOCSET_DEFAULT_SIZES);
+	state->checkstrategy = GetAccessStrategy(BAS_BULKREAD);
+
+	/* Get true root block from meta-page */
+	metapage = palloc_btree_page(state, BTREE_METAPAGE);
+	metad = BTPageGetMeta(metapage);
+
+	/*
+	 * Certain deletion patterns can result in "skinny" B-Tree indexes, where
+	 * the fast root and true root differ.
+	 *
+	 * Start from the true root, not the fast root, unlike conventional index
+	 * scans.  This approach is more thorough, and removes the risk of
+	 * following a stale fast root from the meta page.
+	 */
+	if (metad->btm_fastroot != metad->btm_root)
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg_internal("harmless fast root mismatch in index \"%s\"",
+								 RelationGetRelationName(rel)),
+				 errdetail_internal("Fast root block %u (level %u) differs from true root block %u (level %u).",
+									metad->btm_fastroot, metad->btm_fastlevel,
+									metad->btm_root, metad->btm_level)));
+
+	/*
+	 * Starting at the root, verify every level.  Move left to right, top to
+	 * bottom.  Note that there may be no pages other than the meta page (meta
+	 * page can indicate that root is P_NONE when the index is totally empty).
+	 */
+	previouslevel = InvalidBtreeLevel;
+	current.level = metad->btm_level;
+	current.leftmost = metad->btm_root;
+	current.istruerootlevel = true;
+	while (current.leftmost != P_NONE)
+	{
+		/*
+		 * Verify this level, and get left most page for next level down, if
+		 * not at leaf level
+		 */
+		current = bt_check_level_from_leftmost(state, current);
+
+		if (current.leftmost == InvalidBlockNumber)
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("index \"%s\" has no valid pages on level below %u or first level",
+							RelationGetRelationName(rel), previouslevel)));
+
+		previouslevel = current.level;
+	}
+
+	/*
+	 * * Check whether heap contains unindexed/malformed tuples *
+	 */
+	if (state->heapallindexed)
+	{
+		IndexInfo  *indexinfo = BuildIndexInfo(state->rel);
+		TableScanDesc scan;
+
+		/*
+		 * 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.
+		 *
+		 * Note that table_index_build_scan() calls heap_endscan() for us.
+		 */
+		scan = table_beginscan_strat(state->heaprel,	/* relation */
+									 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.
+		 *
+		 * 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
+		 * case.  We have no reason to care about a concurrent VACUUM
+		 * operation, since there isn't going to be a second scan of the heap
+		 * that needs to be sure that there was no concurrent recycling of
+		 * TIDs.
+		 */
+		indexinfo->ii_Concurrent = !state->readonly;
+
+		/*
+		 * Don't wait for uncommitted tuple xact commit/abort when index is a
+		 * unique index on a catalog (or an index used by an exclusion
+		 * constraint).  This could otherwise happen in the readonly case.
+		 */
+		indexinfo->ii_Unique = false;
+		indexinfo->ii_ExclusionOps = NULL;
+		indexinfo->ii_ExclusionProcs = NULL;
+		indexinfo->ii_ExclusionStrats = NULL;
+
+		elog(DEBUG1, "verifying that tuples from index \"%s\" are present in \"%s\"",
+			 RelationGetRelationName(state->rel),
+			 RelationGetRelationName(state->heaprel));
+
+		table_index_build_scan(state->heaprel, state->rel, indexinfo, true, false,
+							   bt_tuple_present_callback, (void *) state, scan);
+
+		ereport(DEBUG1,
+				(errmsg_internal("finished verifying presence of " INT64_FORMAT " tuples from table \"%s\" with bitset %.2f%% set",
+								 state->heaptuplespresent, RelationGetRelationName(heaprel),
+								 100.0 * bloom_prop_bits_set(state->filter))));
+
+		if (snapshot != SnapshotAny)
+			UnregisterSnapshot(snapshot);
+
+		bloom_free(state->filter);
+	}
+
+	/* Be tidy: */
+	MemoryContextDelete(state->targetcontext);
+}
+
+/*
+ * Given a left-most block at some level, move right, verifying each page
+ * individually (with more verification across pages for "readonly"
+ * callers).  Caller should pass the true root page as the leftmost initially,
+ * working their way down by passing what is returned for the last call here
+ * until level 0 (leaf page level) was reached.
+ *
+ * Returns state for next call, if any.  This includes left-most block number
+ * one level lower that should be passed on next level/call, which is set to
+ * P_NONE on last call here (when leaf level is verified).  Level numbers
+ * follow the nbtree convention: higher levels have higher numbers, because new
+ * levels are added only due to a root page split.  Note that prior to the
+ * first root page split, the root is also a leaf page, so there is always a
+ * level 0 (leaf level), and it's always the last level processed.
+ *
+ * Note on memory management:  State's per-page context is reset here, between
+ * each call to bt_target_page_check().
+ */
+static BtreeLevel
+bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
+{
+	/* State to establish early, concerning entire level */
+	BTPageOpaque opaque;
+	MemoryContext oldcontext;
+	BtreeLevel	nextleveldown;
+
+	/* Variables for iterating across level using right links */
+	BlockNumber leftcurrent = P_NONE;
+	BlockNumber current = level.leftmost;
+
+	/* Initialize return state */
+	nextleveldown.leftmost = InvalidBlockNumber;
+	nextleveldown.level = InvalidBtreeLevel;
+	nextleveldown.istruerootlevel = false;
+
+	/* Use page-level context for duration of this call */
+	oldcontext = MemoryContextSwitchTo(state->targetcontext);
+
+	elog(DEBUG1, "verifying level %u%s", level.level,
+		 level.istruerootlevel ?
+		 " (true root level)" : level.level == 0 ? " (leaf level)" : "");
+
+	state->prevrightlink = InvalidBlockNumber;
+	state->previncompletesplit = false;
+
+	do
+	{
+		/* Don't rely on CHECK_FOR_INTERRUPTS() calls at lower level */
+		CHECK_FOR_INTERRUPTS();
+
+		/* Initialize state for this iteration */
+		state->targetblock = current;
+		state->target = palloc_btree_page(state, state->targetblock);
+		state->targetlsn = PageGetLSN(state->target);
+
+		opaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+		if (P_IGNORE(opaque))
+		{
+			/*
+			 * Since there cannot be a concurrent VACUUM operation in readonly
+			 * mode, and since a page has no links within other pages
+			 * (siblings and parent) once it is marked fully deleted, it
+			 * should be impossible to land on a fully deleted page in
+			 * readonly mode. See bt_child_check() for further details.
+			 *
+			 * The bt_child_check() P_ISDELETED() check is repeated here so
+			 * that pages that are only reachable through sibling links get
+			 * checked.
+			 */
+			if (state->readonly && P_ISDELETED(opaque))
+				ereport(ERROR,
+						(errcode(ERRCODE_INDEX_CORRUPTED),
+						 errmsg("downlink or sibling link points to deleted block in index \"%s\"",
+								RelationGetRelationName(state->rel)),
+						 errdetail_internal("Block=%u left block=%u left link from block=%u.",
+											current, leftcurrent, opaque->btpo_prev)));
+
+			if (P_RIGHTMOST(opaque))
+				ereport(ERROR,
+						(errcode(ERRCODE_INDEX_CORRUPTED),
+						 errmsg("block %u fell off the end of index \"%s\"",
+								current, RelationGetRelationName(state->rel))));
+			else
+				ereport(DEBUG1,
+						(errcode(ERRCODE_NO_DATA),
+						 errmsg_internal("block %u of index \"%s\" concurrently deleted",
+										 current, RelationGetRelationName(state->rel))));
+			goto nextpage;
+		}
+		else if (nextleveldown.leftmost == InvalidBlockNumber)
+		{
+			/*
+			 * A concurrent page split could make the caller supplied leftmost
+			 * block no longer contain the leftmost page, or no longer be the
+			 * true root, but where that isn't possible due to heavyweight
+			 * locking, check that the first valid page meets caller's
+			 * expectations.
+			 */
+			if (state->readonly)
+			{
+				if (!P_LEFTMOST(opaque))
+					ereport(ERROR,
+							(errcode(ERRCODE_INDEX_CORRUPTED),
+							 errmsg("block %u is not leftmost in index \"%s\"",
+									current, RelationGetRelationName(state->rel))));
+
+				if (level.istruerootlevel && !P_ISROOT(opaque))
+					ereport(ERROR,
+							(errcode(ERRCODE_INDEX_CORRUPTED),
+							 errmsg("block %u is not true root in index \"%s\"",
+									current, RelationGetRelationName(state->rel))));
+			}
+
+			/*
+			 * Before beginning any non-trivial examination of level, prepare
+			 * state for next bt_check_level_from_leftmost() invocation for
+			 * the next level for the next level down (if any).
+			 *
+			 * There should be at least one non-ignorable page per level,
+			 * unless this is the leaf level, which is assumed by caller to be
+			 * final level.
+			 */
+			if (!P_ISLEAF(opaque))
+			{
+				IndexTuple	itup;
+				ItemId		itemid;
+
+				/* Internal page -- downlink gets leftmost on next level */
+				itemid = PageGetItemIdCareful(state, state->targetblock,
+											  state->target,
+											  P_FIRSTDATAKEY(opaque));
+				itup = (IndexTuple) PageGetItem(state->target, itemid);
+				nextleveldown.leftmost = BTreeTupleGetDownLink(itup);
+				nextleveldown.level = opaque->btpo_level - 1;
+			}
+			else
+			{
+				/*
+				 * Leaf page -- final level caller must process.
+				 *
+				 * Note that this could also be the root page, if there has
+				 * been no root page split yet.
+				 */
+				nextleveldown.leftmost = P_NONE;
+				nextleveldown.level = InvalidBtreeLevel;
+			}
+
+			/*
+			 * Finished setting up state for this call/level.  Control will
+			 * never end up back here in any future loop iteration for this
+			 * level.
+			 */
+		}
+
+		/* Sibling links should be in mutual agreement */
+		if (opaque->btpo_prev != leftcurrent)
+			bt_recheck_sibling_links(state, opaque->btpo_prev, leftcurrent);
+
+		/* Check level */
+		if (level.level != opaque->btpo_level)
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("leftmost down link for level points to block in index \"%s\" whose level is not one level down",
+							RelationGetRelationName(state->rel)),
+					 errdetail_internal("Block pointed to=%u expected level=%u level in pointed to block=%u.",
+										current, level.level, opaque->btpo_level)));
+
+		/* Verify invariants for page */
+		bt_target_page_check(state);
+
+nextpage:
+
+		/* Try to detect circular links */
+		if (current == leftcurrent || current == opaque->btpo_prev)
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("circular link chain found in block %u of index \"%s\"",
+							current, RelationGetRelationName(state->rel))));
+
+		leftcurrent = current;
+		current = opaque->btpo_next;
+
+		if (state->lowkey)
+		{
+			Assert(state->readonly);
+			pfree(state->lowkey);
+			state->lowkey = NULL;
+		}
+
+		/*
+		 * Copy current target high key as the low key of right sibling.
+		 * Allocate memory in upper level context, so it would be cleared
+		 * after reset of target context.
+		 *
+		 * We only need the low key in corner cases of checking child high
+		 * keys. We use high key only when incomplete split on the child level
+		 * falls to the boundary of pages on the target level.  See
+		 * bt_child_highkey_check() for details.  So, typically we won't end
+		 * up doing anything with low key, but it's simpler for general case
+		 * high key verification to always have it available.
+		 *
+		 * The correctness of managing low key in the case of concurrent
+		 * splits wasn't investigated yet.  Thankfully we only need low key
+		 * for readonly verification and concurrent splits won't happen.
+		 */
+		if (state->readonly && !P_RIGHTMOST(opaque))
+		{
+			IndexTuple	itup;
+			ItemId		itemid;
+
+			itemid = PageGetItemIdCareful(state, state->targetblock,
+										  state->target, P_HIKEY);
+			itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+			state->lowkey = MemoryContextAlloc(oldcontext, IndexTupleSize(itup));
+			memcpy(state->lowkey, itup, IndexTupleSize(itup));
+		}
+
+		/* Free page and associated memory for this iteration */
+		MemoryContextReset(state->targetcontext);
+	}
+	while (current != P_NONE);
+
+	if (state->lowkey)
+	{
+		Assert(state->readonly);
+		pfree(state->lowkey);
+		state->lowkey = NULL;
+	}
+
+	/* Don't change context for caller */
+	MemoryContextSwitchTo(oldcontext);
+
+	return nextleveldown;
+}
+
+/*
+ * 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
+ * right link was followed to get to the current target page, and we expect
+ * mutual agreement among leftcurrent and the current target page.  Make sure
+ * that this condition has definitely been violated in the !readonly case,
+ * where concurrent page splits are something that we need to deal with.
+ *
+ * Cross-page inconsistencies involving pages that don't agree about being
+ * siblings are known to be a particularly good indicator of corruption
+ * involving partial writes/lost updates.  The bt_right_page_check_scankey
+ * check also provides a way of detecting cross-page inconsistencies for
+ * !readonly callers, but it can only detect sibling pages that have an
+ * out-of-order keyspace, which can't catch many of the problems that we
+ * expect to catch here.
+ *
+ * The classic example of the kind of inconsistency that we can only catch
+ * with this check (when in !readonly mode) involves three sibling pages that
+ * were affected by a faulty page split at some point in the past.  The
+ * effects of the split are reflected in the original page and its new right
+ * sibling page, with a lack of any accompanying changes for the _original_
+ * right sibling page.  The original right sibling page's left link fails to
+ * point to the new right sibling page (its left link still points to the
+ * original page), even though the first phase of a page split is supposed to
+ * work as a single atomic action.  This subtle inconsistency will probably
+ * only break backwards scans in practice.
+ *
+ * Note that this is the only place where amcheck will "couple" buffer locks
+ * (and only for !readonly callers).  In general we prefer to avoid more
+ * thorough cross-page checks in !readonly mode, but it seems worth the
+ * complexity here.  Also, the performance overhead of performing lock
+ * coupling here is negligible in practice.  Control only reaches here with a
+ * non-corrupt index when there is a concurrent page split at the instant
+ * caller crossed over to target page from leftcurrent page.
+ */
+static void
+bt_recheck_sibling_links(BtreeCheckState *state,
+						 BlockNumber btpo_prev_from_target,
+						 BlockNumber leftcurrent)
+{
+	if (!state->readonly)
+	{
+		Buffer		lbuf;
+		Buffer		newtargetbuf;
+		Page		page;
+		BTPageOpaque opaque;
+		BlockNumber newtargetblock;
+
+		/* Couple locks in the usual order for nbtree:  Left to right */
+		lbuf = ReadBufferExtended(state->rel, MAIN_FORKNUM, leftcurrent,
+								  RBM_NORMAL, state->checkstrategy);
+		LockBuffer(lbuf, BT_READ);
+		_bt_checkpage(state->rel, lbuf);
+		page = BufferGetPage(lbuf);
+		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
+		if (P_ISDELETED(opaque))
+		{
+			/*
+			 * Cannot reason about concurrently deleted page -- the left link
+			 * in the page to the right is expected to point to some other
+			 * page to the left (not leftcurrent page).
+			 *
+			 * Note that we deliberately don't give up with a half-dead page.
+			 */
+			UnlockReleaseBuffer(lbuf);
+			return;
+		}
+
+		newtargetblock = opaque->btpo_next;
+		/* Avoid self-deadlock when newtargetblock == leftcurrent */
+		if (newtargetblock != leftcurrent)
+		{
+			newtargetbuf = ReadBufferExtended(state->rel, MAIN_FORKNUM,
+											  newtargetblock, RBM_NORMAL,
+											  state->checkstrategy);
+			LockBuffer(newtargetbuf, BT_READ);
+			_bt_checkpage(state->rel, newtargetbuf);
+			page = BufferGetPage(newtargetbuf);
+			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
+			/* btpo_prev_from_target may have changed; update it */
+			btpo_prev_from_target = opaque->btpo_prev;
+		}
+		else
+		{
+			/*
+			 * leftcurrent right sibling points back to leftcurrent block.
+			 * Index is corrupt.  Easiest way to handle this is to pretend
+			 * that we actually read from a distinct page that has an invalid
+			 * block number in its btpo_prev.
+			 */
+			newtargetbuf = InvalidBuffer;
+			btpo_prev_from_target = InvalidBlockNumber;
+		}
+
+		/*
+		 * No need to check P_ISDELETED here, since new target block cannot be
+		 * marked deleted as long as we hold a lock on lbuf
+		 */
+		if (BufferIsValid(newtargetbuf))
+			UnlockReleaseBuffer(newtargetbuf);
+		UnlockReleaseBuffer(lbuf);
+
+		if (btpo_prev_from_target == leftcurrent)
+		{
+			/* Report split in left sibling, not target (or new target) */
+			ereport(DEBUG1,
+					(errcode(ERRCODE_INTERNAL_ERROR),
+					 errmsg_internal("harmless concurrent page split detected in index \"%s\"",
+									 RelationGetRelationName(state->rel)),
+					 errdetail_internal("Block=%u new right sibling=%u original right sibling=%u.",
+										leftcurrent, newtargetblock,
+										state->targetblock)));
+			return;
+		}
+
+		/*
+		 * Index is corrupt.  Make sure that we report correct target page.
+		 *
+		 * This could have changed in cases where there was a concurrent page
+		 * split, as well as index corruption (at least in theory).  Note that
+		 * btpo_prev_from_target was already updated above.
+		 */
+		state->targetblock = newtargetblock;
+	}
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("left link/right link pair in index \"%s\" not in agreement",
+					RelationGetRelationName(state->rel)),
+			 errdetail_internal("Block=%u left block=%u left link from block=%u.",
+								state->targetblock, leftcurrent,
+								btpo_prev_from_target)));
+}
+
+/*
+ * Function performs the following checks on target page, or pages ancillary to
+ * target page:
+ *
+ * - That every "real" data item is less than or equal to the high key, which
+ *	 is an upper bound on the items on the page.  Data items should be
+ *	 strictly less than the high key when the page is an internal page.
+ *
+ * - That within the page, every data item is strictly less than the item
+ *	 immediately to its right, if any (i.e., that the items are in order
+ *	 within the page, so that the binary searches performed by index scans are
+ *	 sane).
+ *
+ * - That the last data item stored on the page is strictly less than the
+ *	 first data item on the page to the right (when such a first item is
+ *	 available).
+ *
+ * - Various checks on the structure of tuples themselves.  For example, check
+ *	 that non-pivot tuples have no truncated attributes.
+ *
+ * Furthermore, when state passed shows ShareLock held, function also checks:
+ *
+ * - That all child pages respect strict lower bound from parent's pivot
+ *	 tuple.
+ *
+ * - That downlink to block was encountered in parent where that's expected.
+ *
+ * - That high keys of child pages matches corresponding pivot keys in parent.
+ *
+ * This is also where heapallindexed callers use their Bloom filter to
+ * fingerprint IndexTuples for later table_index_build_scan() verification.
+ *
+ * Note:  Memory allocated in this routine is expected to be released by caller
+ * resetting state->targetcontext.
+ */
+static void
+bt_target_page_check(BtreeCheckState *state)
+{
+	OffsetNumber offset;
+	OffsetNumber max;
+	BTPageOpaque topaque;
+
+	topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+	max = PageGetMaxOffsetNumber(state->target);
+
+	elog(DEBUG2, "verifying %u items on %s block %u", max,
+		 P_ISLEAF(topaque) ? "leaf" : "internal", state->targetblock);
+
+	/*
+	 * Check the number of attributes in high key. Note, rightmost page
+	 * doesn't contain a high key, so nothing to check
+	 */
+	if (!P_RIGHTMOST(topaque))
+	{
+		ItemId		itemid;
+		IndexTuple	itup;
+
+		/* Verify line pointer before checking tuple */
+		itemid = PageGetItemIdCareful(state, state->targetblock,
+									  state->target, P_HIKEY);
+		if (!_bt_check_natts(state->rel, state->heapkeyspace, state->target,
+							 P_HIKEY))
+		{
+			itup = (IndexTuple) PageGetItem(state->target, itemid);
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("wrong number of high key index tuple attributes in index \"%s\"",
+							RelationGetRelationName(state->rel)),
+					 errdetail_internal("Index block=%u natts=%u block type=%s page lsn=%X/%X.",
+										state->targetblock,
+										BTreeTupleGetNAtts(itup, state->rel),
+										P_ISLEAF(topaque) ? "heap" : "index",
+										LSN_FORMAT_ARGS(state->targetlsn))));
+		}
+	}
+
+	/*
+	 * Loop over page items, starting from first non-highkey item, not high
+	 * key (if any).  Most tests are not performed for the "negative infinity"
+	 * real item (if any).
+	 */
+	for (offset = P_FIRSTDATAKEY(topaque);
+		 offset <= max;
+		 offset = OffsetNumberNext(offset))
+	{
+		ItemId		itemid;
+		IndexTuple	itup;
+		size_t		tupsize;
+		BTScanInsert skey;
+		bool		lowersizelimit;
+		ItemPointer scantid;
+
+		CHECK_FOR_INTERRUPTS();
+
+		itemid = PageGetItemIdCareful(state, state->targetblock,
+									  state->target, offset);
+		itup = (IndexTuple) PageGetItem(state->target, itemid);
+		tupsize = IndexTupleSize(itup);
+
+		/*
+		 * lp_len should match the IndexTuple reported length exactly, since
+		 * lp_len is completely redundant in indexes, and both sources of
+		 * tuple length are MAXALIGN()'d.  nbtree does not use lp_len all that
+		 * frequently, and is surprisingly tolerant of corrupt lp_len fields.
+		 */
+		if (tupsize != ItemIdGetLength(itemid))
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("index tuple size does not equal lp_len in index \"%s\"",
+							RelationGetRelationName(state->rel)),
+					 errdetail_internal("Index tid=(%u,%u) tuple size=%zu lp_len=%u page lsn=%X/%X.",
+										state->targetblock, offset,
+										tupsize, ItemIdGetLength(itemid),
+										LSN_FORMAT_ARGS(state->targetlsn)),
+					 errhint("This could be a torn page problem.")));
+
+		/* Check the number of index tuple attributes */
+		if (!_bt_check_natts(state->rel, state->heapkeyspace, state->target,
+							 offset))
+		{
+			ItemPointer tid;
+			char	   *itid,
+					   *htid;
+
+			itid = psprintf("(%u,%u)", state->targetblock, offset);
+			tid = BTreeTupleGetPointsToTID(itup);
+			htid = psprintf("(%u,%u)",
+							ItemPointerGetBlockNumberNoCheck(tid),
+							ItemPointerGetOffsetNumberNoCheck(tid));
+
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("wrong number of index tuple attributes in index \"%s\"",
+							RelationGetRelationName(state->rel)),
+					 errdetail_internal("Index tid=%s natts=%u points to %s tid=%s page lsn=%X/%X.",
+										itid,
+										BTreeTupleGetNAtts(itup, state->rel),
+										P_ISLEAF(topaque) ? "heap" : "index",
+										htid,
+										LSN_FORMAT_ARGS(state->targetlsn))));
+		}
+
+		/*
+		 * Don't try to generate scankey using "negative infinity" item on
+		 * internal pages. They are always truncated to zero attributes.
+		 */
+		if (offset_is_negative_infinity(topaque, offset))
+		{
+			/*
+			 * We don't call bt_child_check() for "negative infinity" items.
+			 * But if we're performing downlink connectivity check, we do it
+			 * for every item including "negative infinity" one.
+			 */
+			if (!P_ISLEAF(topaque) && state->readonly)
+			{
+				bt_child_highkey_check(state,
+									   offset,
+									   NULL,
+									   topaque->btpo_level);
+			}
+			continue;
+		}
+
+		/*
+		 * Readonly callers may optionally verify that non-pivot tuples can
+		 * each be found by an independent search that starts from the root.
+		 * Note that we deliberately don't do individual searches for each
+		 * TID, since the posting list itself is validated by other checks.
+		 */
+		if (state->rootdescend && P_ISLEAF(topaque) &&
+			!bt_rootdescend(state, itup))
+		{
+			ItemPointer tid = BTreeTupleGetPointsToTID(itup);
+			char	   *itid,
+					   *htid;
+
+			itid = psprintf("(%u,%u)", state->targetblock, offset);
+			htid = psprintf("(%u,%u)", ItemPointerGetBlockNumber(tid),
+							ItemPointerGetOffsetNumber(tid));
+
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("could not find tuple using search from root page in index \"%s\"",
+							RelationGetRelationName(state->rel)),
+					 errdetail_internal("Index tid=%s points to heap tid=%s page lsn=%X/%X.",
+										itid, htid,
+										LSN_FORMAT_ARGS(state->targetlsn))));
+		}
+
+		/*
+		 * If tuple is a posting list tuple, make sure posting list TIDs are
+		 * in order
+		 */
+		if (BTreeTupleIsPosting(itup))
+		{
+			ItemPointerData last;
+			ItemPointer current;
+
+			ItemPointerCopy(BTreeTupleGetHeapTID(itup), &last);
+
+			for (int i = 1; i < BTreeTupleGetNPosting(itup); i++)
+			{
+
+				current = BTreeTupleGetPostingN(itup, i);
+
+				if (ItemPointerCompare(current, &last) <= 0)
+				{
+					char	   *itid = psprintf("(%u,%u)", state->targetblock, offset);
+
+					ereport(ERROR,
+							(errcode(ERRCODE_INDEX_CORRUPTED),
+							 errmsg_internal("posting list contains misplaced TID in index \"%s\"",
+											 RelationGetRelationName(state->rel)),
+							 errdetail_internal("Index tid=%s posting list offset=%d page lsn=%X/%X.",
+												itid, i,
+												LSN_FORMAT_ARGS(state->targetlsn))));
+				}
+
+				ItemPointerCopy(current, &last);
+			}
+		}
+
+		/* Build insertion scankey for current page offset */
+		skey = bt_mkscankey_pivotsearch(state->rel, itup);
+
+		/*
+		 * Make sure tuple size does not exceed the relevant BTREE_VERSION
+		 * specific limit.
+		 *
+		 * BTREE_VERSION 4 (which introduced heapkeyspace rules) requisitioned
+		 * a small amount of space from BTMaxItemSize() in order to ensure
+		 * that suffix truncation always has enough space to add an explicit
+		 * heap TID back to a tuple -- we pessimistically assume that every
+		 * newly inserted tuple will eventually need to have a heap TID
+		 * appended during a future leaf page split, when the tuple becomes
+		 * the basis of the new high key (pivot tuple) for the leaf page.
+		 *
+		 * Since the reclaimed space is reserved for that purpose, we must not
+		 * enforce the slightly lower limit when the extra space has been used
+		 * as intended.  In other words, there is only a cross-version
+		 * difference in the limit on tuple size within leaf pages.
+		 *
+		 * Still, we're particular about the details within BTREE_VERSION 4
+		 * internal pages.  Pivot tuples may only use the extra space for its
+		 * designated purpose.  Enforce the lower limit for pivot tuples when
+		 * an explicit heap TID isn't actually present. (In all other cases
+		 * suffix truncation is guaranteed to generate a pivot tuple that's no
+		 * larger than the firstright tuple provided to it by its caller.)
+		 */
+		lowersizelimit = skey->heapkeyspace &&
+			(P_ISLEAF(topaque) || BTreeTupleGetHeapTID(itup) == NULL);
+		if (tupsize > (lowersizelimit ? BTMaxItemSize(state->target) :
+					   BTMaxItemSizeNoHeapTid(state->target)))
+		{
+			ItemPointer tid = BTreeTupleGetPointsToTID(itup);
+			char	   *itid,
+					   *htid;
+
+			itid = psprintf("(%u,%u)", state->targetblock, offset);
+			htid = psprintf("(%u,%u)",
+							ItemPointerGetBlockNumberNoCheck(tid),
+							ItemPointerGetOffsetNumberNoCheck(tid));
+
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("index row size %zu exceeds maximum for index \"%s\"",
+							tupsize, RelationGetRelationName(state->rel)),
+					 errdetail_internal("Index tid=%s points to %s tid=%s page lsn=%X/%X.",
+										itid,
+										P_ISLEAF(topaque) ? "heap" : "index",
+										htid,
+										LSN_FORMAT_ARGS(state->targetlsn))));
+		}
+
+		/* Fingerprint leaf page tuples (those that point to the heap) */
+		if (state->heapallindexed && P_ISLEAF(topaque) && !ItemIdIsDead(itemid))
+		{
+			IndexTuple	norm;
+
+			if (BTreeTupleIsPosting(itup))
+			{
+				/* Fingerprint all elements as distinct "plain" tuples */
+				for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+				{
+					IndexTuple	logtuple;
+
+					logtuple = bt_posting_plain_tuple(itup, i);
+					norm = bt_normalize_tuple(state, logtuple);
+					bloom_add_element(state->filter, (unsigned char *) norm,
+									  IndexTupleSize(norm));
+					/* Be tidy */
+					if (norm != logtuple)
+						pfree(norm);
+					pfree(logtuple);
+				}
+			}
+			else
+			{
+				norm = bt_normalize_tuple(state, itup);
+				bloom_add_element(state->filter, (unsigned char *) norm,
+								  IndexTupleSize(norm));
+				/* Be tidy */
+				if (norm != itup)
+					pfree(norm);
+			}
+		}
+
+		/*
+		 * * High key check *
+		 *
+		 * If there is a high key (if this is not the rightmost page on its
+		 * entire level), check that high key actually is upper bound on all
+		 * page items.  If this is a posting list tuple, we'll need to set
+		 * scantid to be highest TID in posting list.
+		 *
+		 * We prefer to check all items against high key rather than checking
+		 * just the last and trusting that the operator class obeys the
+		 * transitive law (which implies that all previous items also
+		 * respected the high key invariant if they pass the item order
+		 * check).
+		 *
+		 * Ideally, we'd compare every item in the index against every other
+		 * item in the index, and not trust opclass obedience of the
+		 * transitive law to bridge the gap between children and their
+		 * grandparents (as well as great-grandparents, and so on).  We don't
+		 * go to those lengths because that would be prohibitively expensive,
+		 * and probably not markedly more effective in practice.
+		 *
+		 * On the leaf level, we check that the key is <= the highkey.
+		 * However, on non-leaf levels we check that the key is < the highkey,
+		 * because the high key is "just another separator" rather than a copy
+		 * of some existing key item; we expect it to be unique among all keys
+		 * on the same level.  (Suffix truncation will sometimes produce a
+		 * leaf highkey that is an untruncated copy of the lastleft item, but
+		 * never any other item, which necessitates weakening the leaf level
+		 * check to <=.)
+		 *
+		 * Full explanation for why a highkey is never truly a copy of another
+		 * item from the same level on internal levels:
+		 *
+		 * While the new left page's high key is copied from the first offset
+		 * on the right page during an internal page split, that's not the
+		 * full story.  In effect, internal pages are split in the middle of
+		 * the firstright tuple, not between the would-be lastleft and
+		 * firstright tuples: the firstright key ends up on the left side as
+		 * left's new highkey, and the firstright downlink ends up on the
+		 * right side as right's new "negative infinity" item.  The negative
+		 * infinity tuple is truncated to zero attributes, so we're only left
+		 * with the downlink.  In other words, the copying is just an
+		 * implementation detail of splitting in the middle of a (pivot)
+		 * tuple. (See also: "Notes About Data Representation" in the nbtree
+		 * README.)
+		 */
+		scantid = skey->scantid;
+		if (state->heapkeyspace && BTreeTupleIsPosting(itup))
+			skey->scantid = BTreeTupleGetMaxHeapTID(itup);
+
+		if (!P_RIGHTMOST(topaque) &&
+			!(P_ISLEAF(topaque) ? invariant_leq_offset(state, skey, P_HIKEY) :
+			  invariant_l_offset(state, skey, P_HIKEY)))
+		{
+			ItemPointer tid = BTreeTupleGetPointsToTID(itup);
+			char	   *itid,
+					   *htid;
+
+			itid = psprintf("(%u,%u)", state->targetblock, offset);
+			htid = psprintf("(%u,%u)",
+							ItemPointerGetBlockNumberNoCheck(tid),
+							ItemPointerGetOffsetNumberNoCheck(tid));
+
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("high key invariant violated for index \"%s\"",
+							RelationGetRelationName(state->rel)),
+					 errdetail_internal("Index tid=%s points to %s tid=%s page lsn=%X/%X.",
+										itid,
+										P_ISLEAF(topaque) ? "heap" : "index",
+										htid,
+										LSN_FORMAT_ARGS(state->targetlsn))));
+		}
+		/* Reset, in case scantid was set to (itup) posting tuple's max TID */
+		skey->scantid = scantid;
+
+		/*
+		 * * Item order check *
+		 *
+		 * Check that items are stored on page in logical order, by checking
+		 * current item is strictly less than next item (if any).
+		 */
+		if (OffsetNumberNext(offset) <= max &&
+			!invariant_l_offset(state, skey, OffsetNumberNext(offset)))
+		{
+			ItemPointer tid;
+			char	   *itid,
+					   *htid,
+					   *nitid,
+					   *nhtid;
+
+			itid = psprintf("(%u,%u)", state->targetblock, offset);
+			tid = BTreeTupleGetPointsToTID(itup);
+			htid = psprintf("(%u,%u)",
+							ItemPointerGetBlockNumberNoCheck(tid),
+							ItemPointerGetOffsetNumberNoCheck(tid));
+			nitid = psprintf("(%u,%u)", state->targetblock,
+							 OffsetNumberNext(offset));
+
+			/* Reuse itup to get pointed-to heap location of second item */
+			itemid = PageGetItemIdCareful(state, state->targetblock,
+										  state->target,
+										  OffsetNumberNext(offset));
+			itup = (IndexTuple) PageGetItem(state->target, itemid);
+			tid = BTreeTupleGetPointsToTID(itup);
+			nhtid = psprintf("(%u,%u)",
+							 ItemPointerGetBlockNumberNoCheck(tid),
+							 ItemPointerGetOffsetNumberNoCheck(tid));
+
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("item order invariant violated for index \"%s\"",
+							RelationGetRelationName(state->rel)),
+					 errdetail_internal("Lower index tid=%s (points to %s tid=%s) "
+										"higher index tid=%s (points to %s tid=%s) "
+										"page lsn=%X/%X.",
+										itid,
+										P_ISLEAF(topaque) ? "heap" : "index",
+										htid,
+										nitid,
+										P_ISLEAF(topaque) ? "heap" : "index",
+										nhtid,
+										LSN_FORMAT_ARGS(state->targetlsn))));
+		}
+
+		/*
+		 * * Last item check *
+		 *
+		 * Check last item against next/right page's first data item's when
+		 * last item on page is reached.  This additional check will detect
+		 * transposed pages iff the supposed right sibling page happens to
+		 * belong before target in the key space.  (Otherwise, a subsequent
+		 * heap verification will probably detect the problem.)
+		 *
+		 * This check is similar to the item order check that will have
+		 * already been performed for every other "real" item on target page
+		 * when last item is checked.  The difference is that the next item
+		 * (the item that is compared to target's last item) needs to come
+		 * from the next/sibling page.  There may not be such an item
+		 * available from sibling for various reasons, though (e.g., target is
+		 * the rightmost page on level).
+		 */
+		else if (offset == max)
+		{
+			BTScanInsert rightkey;
+
+			/* Get item in next/right page */
+			rightkey = bt_right_page_check_scankey(state);
+
+			if (rightkey &&
+				!invariant_g_offset(state, rightkey, max))
+			{
+				/*
+				 * As explained at length in bt_right_page_check_scankey(),
+				 * there is a known !readonly race that could account for
+				 * apparent violation of invariant, which we must check for
+				 * before actually proceeding with raising error.  Our canary
+				 * condition is that target page was deleted.
+				 */
+				if (!state->readonly)
+				{
+					/* Get fresh copy of target page */
+					state->target = palloc_btree_page(state, state->targetblock);
+					/* Note that we deliberately do not update target LSN */
+					topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+					/*
+					 * All !readonly checks now performed; just return
+					 */
+					if (P_IGNORE(topaque))
+						return;
+				}
+
+				ereport(ERROR,
+						(errcode(ERRCODE_INDEX_CORRUPTED),
+						 errmsg("cross page item order invariant violated for index \"%s\"",
+								RelationGetRelationName(state->rel)),
+						 errdetail_internal("Last item on page tid=(%u,%u) page lsn=%X/%X.",
+											state->targetblock, offset,
+											LSN_FORMAT_ARGS(state->targetlsn))));
+			}
+		}
+
+		/*
+		 * * Downlink check *
+		 *
+		 * Additional check of child items iff this is an internal page and
+		 * caller holds a ShareLock.  This happens for every downlink (item)
+		 * in target excluding the negative-infinity downlink (again, this is
+		 * because it has no useful value to compare).
+		 */
+		if (!P_ISLEAF(topaque) && state->readonly)
+			bt_child_check(state, skey, offset);
+	}
+
+	/*
+	 * Special case bt_child_highkey_check() call
+	 *
+	 * We don't pass a real downlink, but we've to finish the level
+	 * processing. If condition is satisfied, we've already processed all the
+	 * downlinks from the target level.  But there still might be pages to the
+	 * right of the child page pointer to by our rightmost downlink.  And they
+	 * might have missing downlinks.  This final call checks for them.
+	 */
+	if (!P_ISLEAF(topaque) && P_RIGHTMOST(topaque) && state->readonly)
+	{
+		bt_child_highkey_check(state, InvalidOffsetNumber,
+							   NULL, topaque->btpo_level);
+	}
+}
+
+/*
+ * Return a scankey for an item on page to right of current target (or the
+ * first non-ignorable page), sufficient to check ordering invariant on last
+ * item in current target page.  Returned scankey relies on local memory
+ * allocated for the child page, which caller cannot pfree().  Caller's memory
+ * context should be reset between calls here.
+ *
+ * This is the first data item, and so all adjacent items are checked against
+ * their immediate sibling item (which may be on a sibling page, or even a
+ * "cousin" page at parent boundaries where target's rightlink points to page
+ * with different parent page).  If no such valid item is available, return
+ * NULL instead.
+ *
+ * Note that !readonly callers must reverify that target page has not
+ * been concurrently deleted.
+ */
+static BTScanInsert
+bt_right_page_check_scankey(BtreeCheckState *state)
+{
+	BTPageOpaque opaque;
+	ItemId		rightitem;
+	IndexTuple	firstitup;
+	BlockNumber targetnext;
+	Page		rightpage;
+	OffsetNumber nline;
+
+	/* Determine target's next block number */
+	opaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+	/* If target is already rightmost, no right sibling; nothing to do here */
+	if (P_RIGHTMOST(opaque))
+		return NULL;
+
+	/*
+	 * General notes on concurrent page splits and page deletion:
+	 *
+	 * Routines like _bt_search() don't require *any* page split interlock
+	 * when descending the tree, including something very light like a buffer
+	 * pin. That's why it's okay that we don't either.  This avoidance of any
+	 * need to "couple" buffer locks is the raison d' etre of the Lehman & Yao
+	 * algorithm, in fact.
+	 *
+	 * That leaves deletion.  A deleted page won't actually be recycled by
+	 * VACUUM early enough for us to fail to at least follow its right link
+	 * (or left link, or downlink) and find its sibling, because recycling
+	 * does not occur until no possible index scan could land on the page.
+	 * Index scans can follow links with nothing more than their snapshot as
+	 * an interlock and be sure of at least that much.  (See page
+	 * recycling/"visible to everyone" notes in nbtree README.)
+	 *
+	 * Furthermore, it's okay if we follow a rightlink and find a half-dead or
+	 * dead (ignorable) page one or more times.  There will either be a
+	 * further right link to follow that leads to a live page before too long
+	 * (before passing by parent's rightmost child), or we will find the end
+	 * of the entire level instead (possible when parent page is itself the
+	 * rightmost on its level).
+	 */
+	targetnext = opaque->btpo_next;
+	for (;;)
+	{
+		CHECK_FOR_INTERRUPTS();
+
+		rightpage = palloc_btree_page(state, targetnext);
+		opaque = (BTPageOpaque) PageGetSpecialPointer(rightpage);
+
+		if (!P_IGNORE(opaque) || P_RIGHTMOST(opaque))
+			break;
+
+		/*
+		 * We landed on a deleted or half-dead sibling page.  Step right until
+		 * we locate a live sibling page.
+		 */
+		ereport(DEBUG2,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg_internal("level %u sibling page in block %u of index \"%s\" was found deleted or half dead",
+								 opaque->btpo_level, targetnext, RelationGetRelationName(state->rel)),
+				 errdetail_internal("Deleted page found when building scankey from right sibling.")));
+
+		targetnext = opaque->btpo_next;
+
+		/* Be slightly more pro-active in freeing this memory, just in case */
+		pfree(rightpage);
+	}
+
+	/*
+	 * No ShareLock held case -- why it's safe to proceed.
+	 *
+	 * Problem:
+	 *
+	 * We must avoid false positive reports of corruption when caller treats
+	 * item returned here as an upper bound on target's last item.  In
+	 * general, false positives are disallowed.  Avoiding them here when
+	 * caller is !readonly is subtle.
+	 *
+	 * A concurrent page deletion by VACUUM of the target page can result in
+	 * the insertion of items on to this right sibling page that would
+	 * previously have been inserted on our target page.  There might have
+	 * been insertions that followed the target's downlink after it was made
+	 * to point to right sibling instead of target by page deletion's first
+	 * phase. The inserters insert items that would belong on target page.
+	 * This race is very tight, but it's possible.  This is our only problem.
+	 *
+	 * Non-problems:
+	 *
+	 * We are not hindered by a concurrent page split of the target; we'll
+	 * never land on the second half of the page anyway.  A concurrent split
+	 * of the right page will also not matter, because the first data item
+	 * remains the same within the left half, which we'll reliably land on. If
+	 * we had to skip over ignorable/deleted pages, it cannot matter because
+	 * their key space has already been atomically merged with the first
+	 * non-ignorable page we eventually find (doesn't matter whether the page
+	 * we eventually find is a true sibling or a cousin of target, which we go
+	 * into below).
+	 *
+	 * Solution:
+	 *
+	 * Caller knows that it should reverify that target is not ignorable
+	 * (half-dead or deleted) when cross-page sibling item comparison appears
+	 * to indicate corruption (invariant fails).  This detects the single race
+	 * condition that exists for caller.  This is correct because the
+	 * continued existence of target block as non-ignorable (not half-dead or
+	 * deleted) implies that target page was not merged into from the right by
+	 * deletion; the key space at or after target never moved left.  Target's
+	 * parent either has the same downlink to target as before, or a <
+	 * downlink due to deletion at the left of target.  Target either has the
+	 * same highkey as before, or a highkey < before when there is a page
+	 * split. (The rightmost concurrently-split-from-target-page page will
+	 * still have the same highkey as target was originally found to have,
+	 * which for our purposes is equivalent to target's highkey itself never
+	 * changing, since we reliably skip over
+	 * concurrently-split-from-target-page pages.)
+	 *
+	 * In simpler terms, we allow that the key space of the target may expand
+	 * left (the key space can move left on the left side of target only), but
+	 * the target key space cannot expand right and get ahead of us without
+	 * our detecting it.  The key space of the target cannot shrink, unless it
+	 * shrinks to zero due to the deletion of the original page, our canary
+	 * condition.  (To be very precise, we're a bit stricter than that because
+	 * it might just have been that the target page split and only the
+	 * original target page was deleted.  We can be more strict, just not more
+	 * lax.)
+	 *
+	 * Top level tree walk caller moves on to next page (makes it the new
+	 * target) following recovery from this race.  (cf.  The rationale for
+	 * child/downlink verification needing a ShareLock within
+	 * bt_child_check(), where page deletion is also the main source of
+	 * trouble.)
+	 *
+	 * Note that it doesn't matter if right sibling page here is actually a
+	 * cousin page, because in order for the key space to be readjusted in a
+	 * way that causes us issues in next level up (guiding problematic
+	 * concurrent insertions to the cousin from the grandparent rather than to
+	 * the sibling from the parent), there'd have to be page deletion of
+	 * target's parent page (affecting target's parent's downlink in target's
+	 * grandparent page).  Internal page deletion only occurs when there are
+	 * no child pages (they were all fully deleted), and caller is checking
+	 * that the target's parent has at least one non-deleted (so
+	 * non-ignorable) child: the target page.  (Note that the first phase of
+	 * deletion atomically marks the page to be deleted half-dead/ignorable at
+	 * the same time downlink in its parent is removed, so caller will
+	 * definitely not fail to detect that this happened.)
+	 *
+	 * This trick is inspired by the method backward scans use for dealing
+	 * with concurrent page splits; concurrent page deletion is a problem that
+	 * similarly receives special consideration sometimes (it's possible that
+	 * the backwards scan will re-read its "original" block after failing to
+	 * find a right-link to it, having already moved in the opposite direction
+	 * (right/"forwards") a few times to try to locate one).  Just like us,
+	 * that happens only to determine if there was a concurrent page deletion
+	 * of a reference page, and just like us if there was a page deletion of
+	 * that reference page it means we can move on from caring about the
+	 * reference page.  See the nbtree README for a full description of how
+	 * that works.
+	 */
+	nline = PageGetMaxOffsetNumber(rightpage);
+
+	/*
+	 * Get first data item, if any
+	 */
+	if (P_ISLEAF(opaque) && nline >= P_FIRSTDATAKEY(opaque))
+	{
+		/* Return first data item (if any) */
+		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
+										 P_FIRSTDATAKEY(opaque));
+	}
+	else if (!P_ISLEAF(opaque) &&
+			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
+	{
+		/*
+		 * Return first item after the internal page's "negative infinity"
+		 * item
+		 */
+		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
+										 OffsetNumberNext(P_FIRSTDATAKEY(opaque)));
+	}
+	else
+	{
+		/*
+		 * No first item.  Page is probably empty leaf page, but it's also
+		 * possible that it's an internal page with only a negative infinity
+		 * item.
+		 */
+		ereport(DEBUG2,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg_internal("%s block %u of index \"%s\" has no first data item",
+								 P_ISLEAF(opaque) ? "leaf" : "internal", targetnext,
+								 RelationGetRelationName(state->rel))));
+		return NULL;
+	}
+
+	/*
+	 * Return first real item scankey.  Note that this relies on right page
+	 * memory remaining allocated.
+	 */
+	firstitup = (IndexTuple) PageGetItem(rightpage, rightitem);
+	return bt_mkscankey_pivotsearch(state->rel, firstitup);
+}
+
+/*
+ * Check if two tuples are binary identical except the block number.  So,
+ * this function is capable to compare pivot keys on different levels.
+ */
+static bool
+bt_pivot_tuple_identical(bool heapkeyspace, IndexTuple itup1, IndexTuple itup2)
+{
+	if (IndexTupleSize(itup1) != IndexTupleSize(itup2))
+		return false;
+
+	if (heapkeyspace)
+	{
+		/*
+		 * Offset number will contain important information in heapkeyspace
+		 * indexes: the number of attributes left in the pivot tuple following
+		 * suffix truncation.  Don't skip over it (compare it too).
+		 */
+		if (memcmp(&itup1->t_tid.ip_posid, &itup2->t_tid.ip_posid,
+				   IndexTupleSize(itup1) -
+				   offsetof(ItemPointerData, ip_posid)) != 0)
+			return false;
+	}
+	else
+	{
+		/*
+		 * Cannot rely on offset number field having consistent value across
+		 * levels on pg_upgrade'd !heapkeyspace indexes.  Compare contents of
+		 * tuple starting from just after item pointer (i.e. after block
+		 * number and offset number).
+		 */
+		if (memcmp(&itup1->t_info, &itup2->t_info,
+				   IndexTupleSize(itup1) -
+				   offsetof(IndexTupleData, t_info)) != 0)
+			return false;
+	}
+
+	return true;
+}
+
+/*---
+ * Check high keys on the child level.  Traverse rightlinks from previous
+ * downlink to the current one.  Check that there are no intermediate pages
+ * with missing downlinks.
+ *
+ * If 'loaded_child' is given, it's assumed to be the page pointed to by the
+ * downlink referenced by 'downlinkoffnum' of the target page.
+ *
+ * Basically this function is called for each target downlink and checks two
+ * invariants:
+ *
+ * 1) You can reach the next child from previous one via rightlinks;
+ * 2) Each child high key have matching pivot key on target level.
+ *
+ * Consider the sample tree picture.
+ *
+ *               1
+ *           /       \
+ *        2     <->     3
+ *      /   \        /     \
+ *    4  <>  5  <> 6 <> 7 <> 8
+ *
+ * This function will be called for blocks 4, 5, 6 and 8.  Consider what is
+ * happening for each function call.
+ *
+ * - The function call for block 4 initializes data structure and matches high
+ *   key of block 4 to downlink's pivot key of block 2.
+ * - The high key of block 5 is matched to the high key of block 2.
+ * - The block 6 has an incomplete split flag set, so its high key isn't
+ *   matched to anything.
+ * - The function call for block 8 checks that block 8 can be found while
+ *   following rightlinks from block 6.  The high key of block 7 will be
+ *   matched to downlink's pivot key in block 3.
+ *
+ * There is also final call of this function, which checks that there is no
+ * missing downlinks for children to the right of the child referenced by
+ * rightmost downlink in target level.
+ */
+static void
+bt_child_highkey_check(BtreeCheckState *state,
+					   OffsetNumber target_downlinkoffnum,
+					   Page loaded_child,
+					   uint32 target_level)
+{
+	BlockNumber blkno = state->prevrightlink;
+	Page		page;
+	BTPageOpaque opaque;
+	bool		rightsplit = state->previncompletesplit;
+	bool		first = true;
+	ItemId		itemid;
+	IndexTuple	itup;
+	BlockNumber downlink;
+
+	if (OffsetNumberIsValid(target_downlinkoffnum))
+	{
+		itemid = PageGetItemIdCareful(state, state->targetblock,
+									  state->target, target_downlinkoffnum);
+		itup = (IndexTuple) PageGetItem(state->target, itemid);
+		downlink = BTreeTupleGetDownLink(itup);
+	}
+	else
+	{
+		downlink = P_NONE;
+	}
+
+	/*
+	 * If no previous rightlink is memorized for current level just below
+	 * target page's level, we are about to start from the leftmost page. We
+	 * can't follow rightlinks from previous page, because there is no
+	 * previous page.  But we still can match high key.
+	 *
+	 * So we initialize variables for the loop above like there is previous
+	 * page referencing current child.  Also we imply previous page to not
+	 * have incomplete split flag, that would make us require downlink for
+	 * current child.  That's correct, because leftmost page on the level
+	 * should always have parent downlink.
+	 */
+	if (!BlockNumberIsValid(blkno))
+	{
+		blkno = downlink;
+		rightsplit = false;
+	}
+
+	/* Move to the right on the child level */
+	while (true)
+	{
+		/*
+		 * Did we traverse the whole tree level and this is check for pages to
+		 * the right of rightmost downlink?
+		 */
+		if (blkno == P_NONE && downlink == P_NONE)
+		{
+			state->prevrightlink = InvalidBlockNumber;
+			state->previncompletesplit = false;
+			return;
+		}
+
+		/* Did we traverse the whole tree level and don't find next downlink? */
+		if (blkno == P_NONE)
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("can't traverse from downlink %u to downlink %u of index \"%s\"",
+							state->prevrightlink, downlink,
+							RelationGetRelationName(state->rel))));
+
+		/* Load page contents */
+		if (blkno == downlink && loaded_child)
+			page = loaded_child;
+		else
+			page = palloc_btree_page(state, blkno);
+
+		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
+
+		/* The first page we visit at the level should be leftmost */
+		if (first && !BlockNumberIsValid(state->prevrightlink) && !P_LEFTMOST(opaque))
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("the first child of leftmost target page is not leftmost of its level in index \"%s\"",
+							RelationGetRelationName(state->rel)),
+					 errdetail_internal("Target block=%u child block=%u target page lsn=%X/%X.",
+										state->targetblock, blkno,
+										LSN_FORMAT_ARGS(state->targetlsn))));
+
+		/* Do level sanity check */
+		if ((!P_ISDELETED(opaque) || P_HAS_FULLXID(opaque)) &&
+			opaque->btpo_level != target_level - 1)
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("block found while following rightlinks from child of index \"%s\" has invalid level",
+							RelationGetRelationName(state->rel)),
+					 errdetail_internal("Block pointed to=%u expected level=%u level in pointed to block=%u.",
+										blkno, target_level - 1, opaque->btpo_level)));
+
+		/* Try to detect circular links */
+		if ((!first && blkno == state->prevrightlink) || blkno == opaque->btpo_prev)
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("circular link chain found in block %u of index \"%s\"",
+							blkno, RelationGetRelationName(state->rel))));
+
+		if (blkno != downlink && !P_IGNORE(opaque))
+		{
+			/* blkno probably has missing parent downlink */
+			bt_downlink_missing_check(state, rightsplit, blkno, page);
+		}
+
+		rightsplit = P_INCOMPLETE_SPLIT(opaque);
+
+		/*
+		 * If we visit page with high key, check that it is equal to the
+		 * target key next to corresponding downlink.
+		 */
+		if (!rightsplit && !P_RIGHTMOST(opaque))
+		{
+			BTPageOpaque topaque;
+			IndexTuple	highkey;
+			OffsetNumber pivotkey_offset;
+
+			/* Get high key */
+			itemid = PageGetItemIdCareful(state, blkno, page, P_HIKEY);
+			highkey = (IndexTuple) PageGetItem(page, itemid);
+
+			/*
+			 * There might be two situations when we examine high key.  If
+			 * current child page is referenced by given target downlink, we
+			 * should look to the next offset number for matching key from
+			 * target page.
+			 *
+			 * Alternatively, we're following rightlinks somewhere in the
+			 * middle between page referenced by previous target's downlink
+			 * and the page referenced by current target's downlink.  If
+			 * current child page hasn't incomplete split flag set, then its
+			 * high key should match to the target's key of current offset
+			 * number. This happens when a previous call here (to
+			 * bt_child_highkey_check()) found an incomplete split, and we
+			 * reach a right sibling page without a downlink -- the right
+			 * sibling page's high key still needs to be matched to a
+			 * separator key on the parent/target level.
+			 *
+			 * Don't apply OffsetNumberNext() to target_downlinkoffnum when we
+			 * already had to step right on the child level. Our traversal of
+			 * the child level must try to move in perfect lockstep behind (to
+			 * the left of) the target/parent level traversal.
+			 */
+			if (blkno == downlink)
+				pivotkey_offset = OffsetNumberNext(target_downlinkoffnum);
+			else
+				pivotkey_offset = target_downlinkoffnum;
+
+			topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+			if (!offset_is_negative_infinity(topaque, pivotkey_offset))
+			{
+				/*
+				 * If we're looking for the next pivot tuple in target page,
+				 * but there is no more pivot tuples, then we should match to
+				 * high key instead.
+				 */
+				if (pivotkey_offset > PageGetMaxOffsetNumber(state->target))
+				{
+					if (P_RIGHTMOST(topaque))
+						ereport(ERROR,
+								(errcode(ERRCODE_INDEX_CORRUPTED),
+								 errmsg("child high key is greater than rightmost pivot key on target level in index \"%s\"",
+										RelationGetRelationName(state->rel)),
+								 errdetail_internal("Target block=%u child block=%u target page lsn=%X/%X.",
+													state->targetblock, blkno,
+													LSN_FORMAT_ARGS(state->targetlsn))));
+					pivotkey_offset = P_HIKEY;
+				}
+				itemid = PageGetItemIdCareful(state, state->targetblock,
+											  state->target, pivotkey_offset);
+				itup = (IndexTuple) PageGetItem(state->target, itemid);
+			}
+			else
+			{
+				/*
+				 * We cannot try to match child's high key to a negative
+				 * infinity key in target, since there is nothing to compare.
+				 * However, it's still possible to match child's high key
+				 * outside of target page.  The reason why we're are is that
+				 * bt_child_highkey_check() was previously called for the
+				 * cousin page of 'loaded_child', which is incomplete split.
+				 * So, now we traverse to the right of that cousin page and
+				 * current child level page under consideration still belongs
+				 * to the subtree of target's left sibling.  Thus, we need to
+				 * match child's high key to it's left uncle page high key.
+				 * Thankfully we saved it, it's called a "low key" of target
+				 * page.
+				 */
+				if (!state->lowkey)
+					ereport(ERROR,
+							(errcode(ERRCODE_INDEX_CORRUPTED),
+							 errmsg("can't find left sibling high key in index \"%s\"",
+									RelationGetRelationName(state->rel)),
+							 errdetail_internal("Target block=%u child block=%u target page lsn=%X/%X.",
+												state->targetblock, blkno,
+												LSN_FORMAT_ARGS(state->targetlsn))));
+				itup = state->lowkey;
+			}
+
+			if (!bt_pivot_tuple_identical(state->heapkeyspace, highkey, itup))
+			{
+				ereport(ERROR,
+						(errcode(ERRCODE_INDEX_CORRUPTED),
+						 errmsg("mismatch between parent key and child high key in index \"%s\"",
+								RelationGetRelationName(state->rel)),
+						 errdetail_internal("Target block=%u child block=%u target page lsn=%X/%X.",
+											state->targetblock, blkno,
+											LSN_FORMAT_ARGS(state->targetlsn))));
+			}
+		}
+
+		/* Exit if we already found next downlink */
+		if (blkno == downlink)
+		{
+			state->prevrightlink = opaque->btpo_next;
+			state->previncompletesplit = rightsplit;
+			return;
+		}
+
+		/* Traverse to the next page using rightlink */
+		blkno = opaque->btpo_next;
+
+		/* Free page contents if it's allocated by us */
+		if (page != loaded_child)
+			pfree(page);
+		first = false;
+	}
+}
+
+/*
+ * Checks one of target's downlink against its child page.
+ *
+ * Conceptually, the target page continues to be what is checked here.  The
+ * target block is still blamed in the event of finding an invariant violation.
+ * The downlink insertion into the target is probably where any problem raised
+ * here arises, and there is no such thing as a parent link, so doing the
+ * verification this way around is much more practical.
+ *
+ * This function visits child page and it's sequentially called for each
+ * downlink of target page.  Assuming this we also check downlink connectivity
+ * here in order to save child page visits.
+ */
+static void
+bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
+			   OffsetNumber downlinkoffnum)
+{
+	ItemId		itemid;
+	IndexTuple	itup;
+	BlockNumber childblock;
+	OffsetNumber offset;
+	OffsetNumber maxoffset;
+	Page		child;
+	BTPageOpaque copaque;
+	BTPageOpaque topaque;
+
+	itemid = PageGetItemIdCareful(state, state->targetblock,
+								  state->target, downlinkoffnum);
+	itup = (IndexTuple) PageGetItem(state->target, itemid);
+	childblock = BTreeTupleGetDownLink(itup);
+
+	/*
+	 * Caller must have ShareLock on target relation, because of
+	 * considerations around page deletion by VACUUM.
+	 *
+	 * NB: In general, page deletion deletes the right sibling's downlink, not
+	 * the downlink of the page being deleted; the deleted page's downlink is
+	 * reused for its sibling.  The key space is thereby consolidated between
+	 * the deleted page and its right sibling.  (We cannot delete a parent
+	 * page's rightmost child unless it is the last child page, and we intend
+	 * to also delete the parent itself.)
+	 *
+	 * If this verification happened without a ShareLock, the following race
+	 * condition could cause false positives:
+	 *
+	 * In general, concurrent page deletion might occur, including deletion of
+	 * the left sibling of the child page that is examined here.  If such a
+	 * page deletion were to occur, closely followed by an insertion into the
+	 * newly expanded key space of the child, a window for the false positive
+	 * opens up: the stale parent/target downlink originally followed to get
+	 * to the child legitimately ceases to be a lower bound on all items in
+	 * the page, since the key space was concurrently expanded "left".
+	 * (Insertion followed the "new" downlink for the child, not our now-stale
+	 * downlink, which was concurrently physically removed in target/parent as
+	 * part of deletion's first phase.)
+	 *
+	 * While we use various techniques elsewhere to perform cross-page
+	 * verification for !readonly callers, a similar trick seems difficult
+	 * here.  The tricks used by bt_recheck_sibling_links and by
+	 * bt_right_page_check_scankey both involve verification of a same-level,
+	 * cross-sibling invariant.  Cross-level invariants are far more squishy,
+	 * though.  The nbtree REDO routines do not actually couple buffer locks
+	 * across levels during page splits, so making any cross-level check work
+	 * reliably in !readonly mode may be impossible.
+	 */
+	Assert(state->readonly);
+
+	/*
+	 * Verify child page has the downlink key from target page (its parent) as
+	 * a lower bound; downlink must be strictly less than all keys on the
+	 * page.
+	 *
+	 * Check all items, rather than checking just the first and trusting that
+	 * the operator class obeys the transitive law.
+	 */
+	topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+	child = palloc_btree_page(state, childblock);
+	copaque = (BTPageOpaque) PageGetSpecialPointer(child);
+	maxoffset = PageGetMaxOffsetNumber(child);
+
+	/*
+	 * Since we've already loaded the child block, combine this check with
+	 * check for downlink connectivity.
+	 */
+	bt_child_highkey_check(state, downlinkoffnum,
+						   child, topaque->btpo_level);
+
+	/*
+	 * Since there cannot be a concurrent VACUUM operation in readonly mode,
+	 * and since a page has no links within other pages (siblings and parent)
+	 * once it is marked fully deleted, it should be impossible to land on a
+	 * fully deleted page.
+	 *
+	 * It does not quite make sense to enforce that the page cannot even be
+	 * half-dead, despite the fact the downlink is modified at the same stage
+	 * that the child leaf page is marked half-dead.  That's incorrect because
+	 * there may occasionally be multiple downlinks from a chain of pages
+	 * undergoing deletion, where multiple successive calls are made to
+	 * _bt_unlink_halfdead_page() by VACUUM before it can finally safely mark
+	 * the leaf page as fully dead.  While _bt_mark_page_halfdead() usually
+	 * removes the downlink to the leaf page that is marked half-dead, that's
+	 * not guaranteed, so it's possible we'll land on a half-dead page with a
+	 * downlink due to an interrupted multi-level page deletion.
+	 *
+	 * We go ahead with our checks if the child page is half-dead.  It's safe
+	 * to do so because we do not test the child's high key, so it does not
+	 * matter that the original high key will have been replaced by a dummy
+	 * truncated high key within _bt_mark_page_halfdead().  All other page
+	 * items are left intact on a half-dead page, so there is still something
+	 * to test.
+	 */
+	if (P_ISDELETED(copaque))
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg("downlink to deleted page found in index \"%s\"",
+						RelationGetRelationName(state->rel)),
+				 errdetail_internal("Parent block=%u child block=%u parent page lsn=%X/%X.",
+									state->targetblock, childblock,
+									LSN_FORMAT_ARGS(state->targetlsn))));
+
+	for (offset = P_FIRSTDATAKEY(copaque);
+		 offset <= maxoffset;
+		 offset = OffsetNumberNext(offset))
+	{
+		/*
+		 * Skip comparison of target page key against "negative infinity"
+		 * item, if any.  Checking it would indicate that it's not a strict
+		 * lower bound, but that's only because of the hard-coding for
+		 * negative infinity items within _bt_compare().
+		 *
+		 * If nbtree didn't truncate negative infinity tuples during internal
+		 * page splits then we'd expect child's negative infinity key to be
+		 * equal to the scankey/downlink from target/parent (it would be a
+		 * "low key" in this hypothetical scenario, and so it would still need
+		 * to be treated as a special case here).
+		 *
+		 * Negative infinity items can be thought of as a strict lower bound
+		 * that works transitively, with the last non-negative-infinity pivot
+		 * followed during a descent from the root as its "true" strict lower
+		 * bound.  Only a small number of negative infinity items are truly
+		 * negative infinity; those that are the first items of leftmost
+		 * internal pages.  In more general terms, a negative infinity item is
+		 * only negative infinity with respect to the subtree that the page is
+		 * at the root of.
+		 *
+		 * See also: bt_rootdescend(), which can even detect transitive
+		 * inconsistencies on cousin leaf pages.
+		 */
+		if (offset_is_negative_infinity(copaque, offset))
+			continue;
+
+		if (!invariant_l_nontarget_offset(state, targetkey, childblock, child,
+										  offset))
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("down-link lower bound invariant violated for index \"%s\"",
+							RelationGetRelationName(state->rel)),
+					 errdetail_internal("Parent block=%u child index tid=(%u,%u) parent page lsn=%X/%X.",
+										state->targetblock, childblock, offset,
+										LSN_FORMAT_ARGS(state->targetlsn))));
+	}
+
+	pfree(child);
+}
+
+/*
+ * Checks if page is missing a downlink that it should have.
+ *
+ * A page that lacks a downlink/parent may indicate corruption.  However, we
+ * must account for the fact that a missing downlink can occasionally be
+ * encountered in a non-corrupt index.  This can be due to an interrupted page
+ * split, or an interrupted multi-level page deletion (i.e. there was a hard
+ * crash or an error during a page split, or while VACUUM was deleting a
+ * multi-level chain of pages).
+ *
+ * Note that this can only be called in readonly mode, so there is no need to
+ * be concerned about concurrent page splits or page deletions.
+ */
+static void
+bt_downlink_missing_check(BtreeCheckState *state, bool rightsplit,
+						  BlockNumber blkno, Page page)
+{
+	BTPageOpaque opaque = (BTPageOpaque) PageGetSpecialPointer(page);
+	ItemId		itemid;
+	IndexTuple	itup;
+	Page		child;
+	BTPageOpaque copaque;
+	uint32		level;
+	BlockNumber childblk;
+	XLogRecPtr	pagelsn;
+
+	Assert(state->readonly);
+	Assert(!P_IGNORE(opaque));
+
+	/* No next level up with downlinks to fingerprint from the true root */
+	if (P_ISROOT(opaque))
+		return;
+
+	pagelsn = PageGetLSN(page);
+
+	/*
+	 * Incomplete (interrupted) page splits can account for the lack of a
+	 * downlink.  Some inserting transaction should eventually complete the
+	 * page split in passing, when it notices that the left sibling page is
+	 * P_INCOMPLETE_SPLIT().
+	 *
+	 * In general, VACUUM is not prepared for there to be no downlink to a
+	 * page that it deletes.  This is the main reason why the lack of a
+	 * downlink can be reported as corruption here.  It's not obvious that an
+	 * invalid missing downlink can result in wrong answers to queries,
+	 * though, since index scans that land on the child may end up
+	 * consistently moving right. The handling of concurrent page splits (and
+	 * page deletions) within _bt_moveright() cannot distinguish
+	 * inconsistencies that last for a moment from inconsistencies that are
+	 * permanent and irrecoverable.
+	 *
+	 * VACUUM isn't even prepared to delete pages that have no downlink due to
+	 * an incomplete page split, but it can detect and reason about that case
+	 * by design, so it shouldn't be taken to indicate corruption.  See
+	 * _bt_pagedel() for full details.
+	 */
+	if (rightsplit)
+	{
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg_internal("harmless interrupted page split detected in index \"%s\"",
+								 RelationGetRelationName(state->rel)),
+				 errdetail_internal("Block=%u level=%u left sibling=%u page lsn=%X/%X.",
+									blkno, opaque->btpo_level,
+									opaque->btpo_prev,
+									LSN_FORMAT_ARGS(pagelsn))));
+		return;
+	}
+
+	/*
+	 * Page under check is probably the "top parent" of a multi-level page
+	 * deletion.  We'll need to descend the subtree to make sure that
+	 * descendant pages are consistent with that, though.
+	 *
+	 * If the page (which must be non-ignorable) is a leaf page, then clearly
+	 * it can't be the top parent.  The lack of a downlink is probably a
+	 * symptom of a broad problem that could just as easily cause
+	 * inconsistencies anywhere else.
+	 */
+	if (P_ISLEAF(opaque))
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg("leaf index block lacks downlink in index \"%s\"",
+						RelationGetRelationName(state->rel)),
+				 errdetail_internal("Block=%u page lsn=%X/%X.",
+									blkno,
+									LSN_FORMAT_ARGS(pagelsn))));
+
+	/* Descend from the given page, which is an internal page */
+	elog(DEBUG1, "checking for interrupted multi-level deletion due to missing downlink in index \"%s\"",
+		 RelationGetRelationName(state->rel));
+
+	level = opaque->btpo_level;
+	itemid = PageGetItemIdCareful(state, blkno, page, P_FIRSTDATAKEY(opaque));
+	itup = (IndexTuple) PageGetItem(page, itemid);
+	childblk = BTreeTupleGetDownLink(itup);
+	for (;;)
+	{
+		CHECK_FOR_INTERRUPTS();
+
+		child = palloc_btree_page(state, childblk);
+		copaque = (BTPageOpaque) PageGetSpecialPointer(child);
+
+		if (P_ISLEAF(copaque))
+			break;
+
+		/* Do an extra sanity check in passing on internal pages */
+		if (copaque->btpo_level != level - 1)
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg_internal("downlink points to block in index \"%s\" whose level is not one level down",
+									 RelationGetRelationName(state->rel)),
+					 errdetail_internal("Top parent/under check block=%u block pointed to=%u expected level=%u level in pointed to block=%u.",
+										blkno, childblk,
+										level - 1, copaque->btpo_level)));
+
+		level = copaque->btpo_level;
+		itemid = PageGetItemIdCareful(state, childblk, child,
+									  P_FIRSTDATAKEY(copaque));
+		itup = (IndexTuple) PageGetItem(child, itemid);
+		childblk = BTreeTupleGetDownLink(itup);
+		/* Be slightly more pro-active in freeing this memory, just in case */
+		pfree(child);
+	}
+
+	/*
+	 * Since there cannot be a concurrent VACUUM operation in readonly mode,
+	 * and since a page has no links within other pages (siblings and parent)
+	 * once it is marked fully deleted, it should be impossible to land on a
+	 * fully deleted page.  See bt_child_check() for further details.
+	 *
+	 * The bt_child_check() P_ISDELETED() check is repeated here because
+	 * bt_child_check() does not visit pages reachable through negative
+	 * infinity items.  Besides, bt_child_check() is unwilling to descend
+	 * multiple levels.  (The similar bt_child_check() P_ISDELETED() check
+	 * within bt_check_level_from_leftmost() won't reach the page either,
+	 * since the leaf's live siblings should have their sibling links updated
+	 * to bypass the deletion target page when it is marked fully dead.)
+	 *
+	 * If this error is raised, it might be due to a previous multi-level page
+	 * deletion that failed to realize that it wasn't yet safe to mark the
+	 * leaf page as fully dead.  A "dangling downlink" will still remain when
+	 * this happens.  The fact that the dangling downlink's page (the leaf's
+	 * parent/ancestor page) lacked a downlink is incidental.
+	 */
+	if (P_ISDELETED(copaque))
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg_internal("downlink to deleted leaf page found in index \"%s\"",
+								 RelationGetRelationName(state->rel)),
+				 errdetail_internal("Top parent/target block=%u leaf block=%u top parent/under check lsn=%X/%X.",
+									blkno, childblk,
+									LSN_FORMAT_ARGS(pagelsn))));
+
+	/*
+	 * Iff leaf page is half-dead, its high key top parent link should point
+	 * to what VACUUM considered to be the top parent page at the instant it
+	 * was interrupted.  Provided the high key link actually points to the
+	 * page under check, the missing downlink we detected is consistent with
+	 * there having been an interrupted multi-level page deletion.  This means
+	 * that the subtree with the page under check at its root (a page deletion
+	 * chain) is in a consistent state, enabling VACUUM to resume deleting the
+	 * entire chain the next time it encounters the half-dead leaf page.
+	 */
+	if (P_ISHALFDEAD(copaque) && !P_RIGHTMOST(copaque))
+	{
+		itemid = PageGetItemIdCareful(state, childblk, child, P_HIKEY);
+		itup = (IndexTuple) PageGetItem(child, itemid);
+		if (BTreeTupleGetTopParent(itup) == blkno)
+			return;
+	}
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("internal index block lacks downlink in index \"%s\"",
+					RelationGetRelationName(state->rel)),
+			 errdetail_internal("Block=%u level=%u page lsn=%X/%X.",
+								blkno, opaque->btpo_level,
+								LSN_FORMAT_ARGS(pagelsn))));
+}
+
+/*
+ * Per-tuple callback from table_index_build_scan, used to determine if index has
+ * all the entries that definitely should have been observed in leaf pages of
+ * the target index (that is, all IndexTuples that were fingerprinted by our
+ * Bloom filter).  All heapallindexed checks occur here.
+ *
+ * The redundancy between an index and the table it indexes provides a good
+ * opportunity to detect corruption, especially corruption within the table.
+ * The high level principle behind the verification performed here is that any
+ * IndexTuple that should be in an index following a fresh CREATE INDEX (based
+ * on the same index definition) should also have been in the original,
+ * existing index, which should have used exactly the same representation
+ *
+ * Since the overall structure of the index has already been verified, the most
+ * likely explanation for error here is a corrupt heap page (could be logical
+ * or physical corruption).  Index corruption may still be detected here,
+ * though.  Only readonly callers will have verified that left links and right
+ * links are in agreement, and so it's possible that a leaf page transposition
+ * within index is actually the source of corruption detected here (for
+ * !readonly callers).  The checks performed only for readonly callers might
+ * more accurately frame the problem as a cross-page invariant issue (this
+ * could even be due to recovery not replaying all WAL records).  The !readonly
+ * ERROR message raised here includes a HINT about retrying with readonly
+ * verification, just in case it's a cross-page invariant issue, though that
+ * isn't particularly likely.
+ *
+ * table_index_build_scan() expects to be able to find the root tuple when a
+ * heap-only tuple (the live tuple at the end of some HOT chain) needs to be
+ * indexed, in order to replace the actual tuple's TID with the root tuple's
+ * TID (which is what we're actually passed back here).  The index build heap
+ * scan code will raise an error when a tuple that claims to be the root of the
+ * heap-only tuple's HOT chain cannot be located.  This catches cases where the
+ * original root item offset/root tuple for a HOT chain indicates (for whatever
+ * reason) that the entire HOT chain is dead, despite the fact that the latest
+ * heap-only tuple should be indexed.  When this happens, sequential scans may
+ * always give correct answers, and all indexes may be considered structurally
+ * consistent (i.e. the nbtree structural checks would not detect corruption).
+ * It may be the case that only index scans give wrong answers, and yet heap or
+ * SLRU corruption is the real culprit.  (While it's true that LP_DEAD bit
+ * setting will probably also leave the index in a corrupt state before too
+ * long, the problem is nonetheless that there is heap corruption.)
+ *
+ * Heap-only tuple handling within table_index_build_scan() works in a way that
+ * helps us to detect index tuples that contain the wrong values (values that
+ * don't match the latest tuple in the HOT chain).  This can happen when there
+ * is no superseding index tuple due to a faulty assessment of HOT safety,
+ * perhaps during the original CREATE INDEX.  Because the latest tuple's
+ * contents are used with the root TID, an error will be raised when a tuple
+ * with the same TID but non-matching attribute values is passed back to us.
+ * Faulty assessment of HOT-safety was behind at least two distinct CREATE
+ * INDEX CONCURRENTLY bugs that made it into stable releases, one of which was
+ * undetected for many years.  In short, the same principle that allows a
+ * REINDEX to repair corruption when there was an (undetected) broken HOT chain
+ * also allows us to detect the corruption in many cases.
+ */
+static void
+bt_tuple_present_callback(Relation index, ItemPointer tid, Datum *values,
+						  bool *isnull, bool tupleIsAlive, void *checkstate)
+{
+	BtreeCheckState *state = (BtreeCheckState *) checkstate;
+	IndexTuple	itup,
+				norm;
+
+	Assert(state->heapallindexed);
+
+	/* Generate a normalized index tuple for fingerprinting */
+	itup = index_form_tuple(RelationGetDescr(index), values, isnull);
+	itup->t_tid = *tid;
+	norm = bt_normalize_tuple(state, itup);
+
+	/* Probe Bloom filter -- tuple should be present */
+	if (bloom_lacks_element(state->filter, (unsigned char *) norm,
+							IndexTupleSize(norm)))
+		ereport(ERROR,
+				(errcode(ERRCODE_DATA_CORRUPTED),
+				 errmsg("heap tuple (%u,%u) from table \"%s\" lacks matching index tuple within index \"%s\"",
+						ItemPointerGetBlockNumber(&(itup->t_tid)),
+						ItemPointerGetOffsetNumber(&(itup->t_tid)),
+						RelationGetRelationName(state->heaprel),
+						RelationGetRelationName(state->rel)),
+				 !state->readonly
+				 ? errhint("Retrying verification using the function bt_index_parent_check() might provide a more specific error.")
+				 : 0));
+
+	state->heaptuplespresent++;
+	pfree(itup);
+	/* Cannot leak memory here */
+	if (norm != itup)
+		pfree(norm);
+}
+
+/*
+ * Normalize an index tuple for fingerprinting.
+ *
+ * In general, index tuple formation is assumed to be deterministic by
+ * heapallindexed verification, and IndexTuples are assumed immutable.  While
+ * the LP_DEAD bit is mutable in leaf pages, that's ItemId metadata, which is
+ * not fingerprinted.  Normalization is required to compensate for corner
+ * cases where the determinism assumption doesn't quite work.
+ *
+ * There is currently one such case: index_form_tuple() does not try to hide
+ * the source TOAST state of input datums.  The executor applies TOAST
+ * compression for heap tuples based on different criteria to the compression
+ * applied within btinsert()'s call to index_form_tuple(): it sometimes
+ * compresses more aggressively, resulting in compressed heap tuple datums but
+ * uncompressed corresponding index tuple datums.  A subsequent heapallindexed
+ * verification will get a logically equivalent though bitwise unequal tuple
+ * from index_form_tuple().  False positive heapallindexed corruption reports
+ * could occur without normalizing away the inconsistency.
+ *
+ * Returned tuple is often caller's own original tuple.  Otherwise, it is a
+ * new representation of caller's original index tuple, palloc()'d in caller's
+ * memory context.
+ *
+ * Note: This routine is not concerned with distinctions about the
+ * representation of tuples beyond those that might break heapallindexed
+ * verification.  In particular, it won't try to normalize opclass-equal
+ * datums with potentially distinct representations (e.g., btree/numeric_ops
+ * index datums will not get their display scale normalized-away here).
+ * Caller does normalization for non-pivot tuples that have a posting list,
+ * since dummy CREATE INDEX callback code generates new tuples with the same
+ * normalized representation.
+ */
+static IndexTuple
+bt_normalize_tuple(BtreeCheckState *state, IndexTuple itup)
+{
+	TupleDesc	tupleDescriptor = RelationGetDescr(state->rel);
+	Datum		normalized[INDEX_MAX_KEYS];
+	bool		isnull[INDEX_MAX_KEYS];
+	bool		toast_free[INDEX_MAX_KEYS];
+	bool		formnewtup = false;
+	IndexTuple	reformed;
+	int			i;
+
+	/* Caller should only pass "logical" non-pivot tuples here */
+	Assert(!BTreeTupleIsPosting(itup) && !BTreeTupleIsPivot(itup));
+
+	/* Easy case: It's immediately clear that tuple has no varlena datums */
+	if (!IndexTupleHasVarwidths(itup))
+		return itup;
+
+	for (i = 0; i < tupleDescriptor->natts; i++)
+	{
+		Form_pg_attribute att;
+
+		att = TupleDescAttr(tupleDescriptor, i);
+
+		/* Assume untoasted/already normalized datum initially */
+		toast_free[i] = false;
+		normalized[i] = index_getattr(itup, att->attnum,
+									  tupleDescriptor,
+									  &isnull[i]);
+		if (att->attbyval || att->attlen != -1 || isnull[i])
+			continue;
+
+		/*
+		 * Callers always pass a tuple that could safely be inserted into the
+		 * index without further processing, so an external varlena header
+		 * should never be encountered here
+		 */
+		if (VARATT_IS_EXTERNAL(DatumGetPointer(normalized[i])))
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("external varlena datum in tuple that references heap row (%u,%u) in index \"%s\"",
+							ItemPointerGetBlockNumber(&(itup->t_tid)),
+							ItemPointerGetOffsetNumber(&(itup->t_tid)),
+							RelationGetRelationName(state->rel))));
+		else if (VARATT_IS_COMPRESSED(DatumGetPointer(normalized[i])))
+		{
+			formnewtup = true;
+			normalized[i] = PointerGetDatum(PG_DETOAST_DATUM(normalized[i]));
+			toast_free[i] = true;
+		}
+	}
+
+	/* Easier case: Tuple has varlena datums, none of which are compressed */
+	if (!formnewtup)
+		return itup;
+
+	/*
+	 * Hard case: Tuple had compressed varlena datums that necessitate
+	 * creating normalized version of the tuple from uncompressed input datums
+	 * (normalized input datums).  This is rather naive, but shouldn't be
+	 * necessary too often.
+	 *
+	 * Note that we rely on deterministic index_form_tuple() TOAST compression
+	 * of normalized input.
+	 */
+	reformed = index_form_tuple(tupleDescriptor, normalized, isnull);
+	reformed->t_tid = itup->t_tid;
+
+	/* Cannot leak memory here */
+	for (i = 0; i < tupleDescriptor->natts; i++)
+		if (toast_free[i])
+			pfree(DatumGetPointer(normalized[i]));
+
+	return reformed;
+}
+
+/*
+ * Produce palloc()'d "plain" tuple for nth posting list entry/TID.
+ *
+ * In general, deduplication is not supposed to change the logical contents of
+ * an index.  Multiple index tuples are merged together into one equivalent
+ * posting list index tuple when convenient.
+ *
+ * heapallindexed verification must normalize-away this variation in
+ * representation by converting posting list tuples into two or more "plain"
+ * tuples.  Each tuple must be fingerprinted separately -- there must be one
+ * tuple for each corresponding Bloom filter probe during the heap scan.
+ *
+ * Note: Caller still needs to call bt_normalize_tuple() with returned tuple.
+ */
+static inline IndexTuple
+bt_posting_plain_tuple(IndexTuple itup, int n)
+{
+	Assert(BTreeTupleIsPosting(itup));
+
+	/* Returns non-posting-list tuple */
+	return _bt_form_posting(itup, BTreeTupleGetPostingN(itup, n), 1);
+}
+
+/*
+ * Search for itup in index, starting from fast root page.  itup must be a
+ * non-pivot tuple.  This is only supported with heapkeyspace indexes, since
+ * we rely on having fully unique keys to find a match with only a single
+ * visit to a leaf page, barring an interrupted page split, where we may have
+ * to move right.  (A concurrent page split is impossible because caller must
+ * be readonly caller.)
+ *
+ * This routine can detect very subtle transitive consistency issues across
+ * more than one level of the tree.  Leaf pages all have a high key (even the
+ * rightmost page has a conceptual positive infinity high key), but not a low
+ * key.  Their downlink in parent is a lower bound, which along with the high
+ * key is almost enough to detect every possible inconsistency.  A downlink
+ * separator key value won't always be available from parent, though, because
+ * the first items of internal pages are negative infinity items, truncated
+ * down to zero attributes during internal page splits.  While it's true that
+ * bt_child_check() and the high key check can detect most imaginable key
+ * space problems, there are remaining problems it won't detect with non-pivot
+ * tuples in cousin leaf pages.  Starting a search from the root for every
+ * existing leaf tuple detects small inconsistencies in upper levels of the
+ * tree that cannot be detected any other way.  (Besides all this, this is
+ * probably also useful as a direct test of the code used by index scans
+ * themselves.)
+ */
+static bool
+bt_rootdescend(BtreeCheckState *state, IndexTuple itup)
+{
+	BTScanInsert key;
+	BTStack		stack;
+	Buffer		lbuf;
+	bool		exists;
+
+	key = _bt_mkscankey(state->rel, itup);
+	Assert(key->heapkeyspace && key->scantid != NULL);
+
+	/*
+	 * Search from root.
+	 *
+	 * Ideally, we would arrange to only move right within _bt_search() when
+	 * an interrupted page split is detected (i.e. when the incomplete split
+	 * bit is found to be set), but for now we accept the possibility that
+	 * that could conceal an inconsistency.
+	 */
+	Assert(state->readonly && state->rootdescend);
+	exists = false;
+	stack = _bt_search(state->rel, key, &lbuf, BT_READ, NULL);
+
+	if (BufferIsValid(lbuf))
+	{
+		BTInsertStateData insertstate;
+		OffsetNumber offnum;
+		Page		page;
+
+		insertstate.itup = itup;
+		insertstate.itemsz = MAXALIGN(IndexTupleSize(itup));
+		insertstate.itup_key = key;
+		insertstate.postingoff = 0;
+		insertstate.bounds_valid = false;
+		insertstate.buf = lbuf;
+
+		/* Get matching tuple on leaf page */
+		offnum = _bt_binsrch_insert(state->rel, &insertstate);
+		/* Compare first >= matching item on leaf page, if any */
+		page = BufferGetPage(lbuf);
+		/* Should match on first heap TID when tuple has a posting list */
+		if (offnum <= PageGetMaxOffsetNumber(page) &&
+			insertstate.postingoff <= 0 &&
+			_bt_compare(state->rel, key, page, offnum) == 0)
+			exists = true;
+		_bt_relbuf(state->rel, lbuf);
+	}
+
+	_bt_freestack(stack);
+	pfree(key);
+
+	return exists;
+}
+
+/*
+ * Is particular offset within page (whose special state is passed by caller)
+ * the page negative-infinity item?
+ *
+ * As noted in comments above _bt_compare(), there is special handling of the
+ * first data item as a "negative infinity" item.  The hard-coding within
+ * _bt_compare() makes comparing this item for the purposes of verification
+ * pointless at best, since the IndexTuple only contains a valid TID (a
+ * reference TID to child page).
+ */
+static inline bool
+offset_is_negative_infinity(BTPageOpaque opaque, OffsetNumber offset)
+{
+	/*
+	 * For internal pages only, the first item after high key, if any, is
+	 * negative infinity item.  Internal pages always have a negative infinity
+	 * item, whereas leaf pages never have one.  This implies that negative
+	 * infinity item is either first or second line item, or there is none
+	 * within page.
+	 *
+	 * Negative infinity items are a special case among pivot tuples.  They
+	 * always have zero attributes, while all other pivot tuples always have
+	 * nkeyatts attributes.
+	 *
+	 * Right-most pages don't have a high key, but could be said to
+	 * conceptually have a "positive infinity" high key.  Thus, there is a
+	 * symmetry between down link items in parent pages, and high keys in
+	 * children.  Together, they represent the part of the key space that
+	 * belongs to each page in the index.  For example, all children of the
+	 * root page will have negative infinity as a lower bound from root
+	 * negative infinity downlink, and positive infinity as an upper bound
+	 * (implicitly, from "imaginary" positive infinity high key in root).
+	 */
+	return !P_ISLEAF(opaque) && offset == P_FIRSTDATAKEY(opaque);
+}
+
+/*
+ * Does the invariant hold that the key is strictly less than a given upper
+ * bound offset item?
+ *
+ * Verifies line pointer on behalf of caller.
+ *
+ * If this function returns false, convention is that caller throws error due
+ * to corruption.
+ */
+static inline bool
+invariant_l_offset(BtreeCheckState *state, BTScanInsert key,
+				   OffsetNumber upperbound)
+{
+	ItemId		itemid;
+	int32		cmp;
+
+	Assert(key->pivotsearch);
+
+	/* Verify line pointer before checking tuple */
+	itemid = PageGetItemIdCareful(state, state->targetblock, state->target,
+								  upperbound);
+	/* pg_upgrade'd indexes may legally have equal sibling tuples */
+	if (!key->heapkeyspace)
+		return invariant_leq_offset(state, key, upperbound);
+
+	cmp = _bt_compare(state->rel, key, state->target, upperbound);
+
+	/*
+	 * _bt_compare() is capable of determining that a scankey with a
+	 * filled-out attribute is greater than pivot tuples where the comparison
+	 * is resolved at a truncated attribute (value of attribute in pivot is
+	 * minus infinity).  However, it is not capable of determining that a
+	 * scankey is _less than_ a tuple on the basis of a comparison resolved at
+	 * _scankey_ minus infinity attribute.  Complete an extra step to simulate
+	 * having minus infinity values for omitted scankey attribute(s).
+	 */
+	if (cmp == 0)
+	{
+		BTPageOpaque topaque;
+		IndexTuple	ritup;
+		int			uppnkeyatts;
+		ItemPointer rheaptid;
+		bool		nonpivot;
+
+		ritup = (IndexTuple) PageGetItem(state->target, itemid);
+		topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+		nonpivot = P_ISLEAF(topaque) && upperbound >= P_FIRSTDATAKEY(topaque);
+
+		/* Get number of keys + heap TID for item to the right */
+		uppnkeyatts = BTreeTupleGetNKeyAtts(ritup, state->rel);
+		rheaptid = BTreeTupleGetHeapTIDCareful(state, ritup, nonpivot);
+
+		/* Heap TID is tiebreaker key attribute */
+		if (key->keysz == uppnkeyatts)
+			return key->scantid == NULL && rheaptid != NULL;
+
+		return key->keysz < uppnkeyatts;
+	}
+
+	return cmp < 0;
+}
+
+/*
+ * Does the invariant hold that the key is less than or equal to a given upper
+ * bound offset item?
+ *
+ * Caller should have verified that upperbound's line pointer is consistent
+ * using PageGetItemIdCareful() call.
+ *
+ * If this function returns false, convention is that caller throws error due
+ * to corruption.
+ */
+static inline bool
+invariant_leq_offset(BtreeCheckState *state, BTScanInsert key,
+					 OffsetNumber upperbound)
+{
+	int32		cmp;
+
+	Assert(key->pivotsearch);
+
+	cmp = _bt_compare(state->rel, key, state->target, upperbound);
+
+	return cmp <= 0;
+}
+
+/*
+ * Does the invariant hold that the key is strictly greater than a given lower
+ * bound offset item?
+ *
+ * Caller should have verified that lowerbound's line pointer is consistent
+ * using PageGetItemIdCareful() call.
+ *
+ * If this function returns false, convention is that caller throws error due
+ * to corruption.
+ */
+static inline bool
+invariant_g_offset(BtreeCheckState *state, BTScanInsert key,
+				   OffsetNumber lowerbound)
+{
+	int32		cmp;
+
+	Assert(key->pivotsearch);
+
+	cmp = _bt_compare(state->rel, key, state->target, lowerbound);
+
+	/* pg_upgrade'd indexes may legally have equal sibling tuples */
+	if (!key->heapkeyspace)
+		return cmp >= 0;
+
+	/*
+	 * No need to consider the possibility that scankey has attributes that we
+	 * need to force to be interpreted as negative infinity.  _bt_compare() is
+	 * able to determine that scankey is greater than negative infinity.  The
+	 * distinction between "==" and "<" isn't interesting here, since
+	 * corruption is indicated either way.
+	 */
+	return cmp > 0;
+}
+
+/*
+ * Does the invariant hold that the key is strictly less than a given upper
+ * bound offset item, with the offset relating to a caller-supplied page that
+ * is not the current target page?
+ *
+ * Caller's non-target page is a child page of the target, checked as part of
+ * checking a property of the target page (i.e. the key comes from the
+ * target).  Verifies line pointer on behalf of caller.
+ *
+ * If this function returns false, convention is that caller throws error due
+ * to corruption.
+ */
+static inline bool
+invariant_l_nontarget_offset(BtreeCheckState *state, BTScanInsert key,
+							 BlockNumber nontargetblock, Page nontarget,
+							 OffsetNumber upperbound)
+{
+	ItemId		itemid;
+	int32		cmp;
+
+	Assert(key->pivotsearch);
+
+	/* Verify line pointer before checking tuple */
+	itemid = PageGetItemIdCareful(state, nontargetblock, nontarget,
+								  upperbound);
+	cmp = _bt_compare(state->rel, key, nontarget, upperbound);
+
+	/* pg_upgrade'd indexes may legally have equal sibling tuples */
+	if (!key->heapkeyspace)
+		return cmp <= 0;
+
+	/* See invariant_l_offset() for an explanation of this extra step */
+	if (cmp == 0)
+	{
+		IndexTuple	child;
+		int			uppnkeyatts;
+		ItemPointer childheaptid;
+		BTPageOpaque copaque;
+		bool		nonpivot;
+
+		child = (IndexTuple) PageGetItem(nontarget, itemid);
+		copaque = (BTPageOpaque) PageGetSpecialPointer(nontarget);
+		nonpivot = P_ISLEAF(copaque) && upperbound >= P_FIRSTDATAKEY(copaque);
+
+		/* Get number of keys + heap TID for child/non-target item */
+		uppnkeyatts = BTreeTupleGetNKeyAtts(child, state->rel);
+		childheaptid = BTreeTupleGetHeapTIDCareful(state, child, nonpivot);
+
+		/* Heap TID is tiebreaker key attribute */
+		if (key->keysz == uppnkeyatts)
+			return key->scantid == NULL && childheaptid != NULL;
+
+		return key->keysz < uppnkeyatts;
+	}
+
+	return cmp < 0;
+}
+
+/*
+ * Given a block number of a B-Tree page, return page in palloc()'d memory.
+ * While at it, perform some basic checks of the page.
+ *
+ * There is never an attempt to get a consistent view of multiple pages using
+ * multiple concurrent buffer locks; in general, we only acquire a single pin
+ * and buffer lock at a time, which is often all that the nbtree code requires.
+ * (Actually, bt_recheck_sibling_links couples buffer locks, which is the only
+ * exception to this general rule.)
+ *
+ * Operating on a copy of the page is useful because it prevents control
+ * getting stuck in an uninterruptible state when an underlying operator class
+ * misbehaves.
+ */
+static Page
+palloc_btree_page(BtreeCheckState *state, BlockNumber blocknum)
+{
+	Buffer		buffer;
+	Page		page;
+	BTPageOpaque opaque;
+	OffsetNumber maxoffset;
+
+	page = palloc(BLCKSZ);
+
+	/*
+	 * We copy the page into local storage to avoid holding pin on the buffer
+	 * longer than we must.
+	 */
+	buffer = ReadBufferExtended(state->rel, MAIN_FORKNUM, blocknum, RBM_NORMAL,
+								state->checkstrategy);
+	LockBuffer(buffer, BT_READ);
+
+	/*
+	 * Perform the same basic sanity checking that nbtree itself performs for
+	 * every page:
+	 */
+	_bt_checkpage(state->rel, buffer);
+
+	/* Only use copy of page in palloc()'d memory */
+	memcpy(page, BufferGetPage(buffer), BLCKSZ);
+	UnlockReleaseBuffer(buffer);
+
+	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
+
+	if (P_ISMETA(opaque) && blocknum != BTREE_METAPAGE)
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg("invalid meta page found at block %u in index \"%s\"",
+						blocknum, RelationGetRelationName(state->rel))));
+
+	/* Check page from block that ought to be meta page */
+	if (blocknum == BTREE_METAPAGE)
+	{
+		BTMetaPageData *metad = BTPageGetMeta(page);
+
+		if (!P_ISMETA(opaque) ||
+			metad->btm_magic != BTREE_MAGIC)
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("index \"%s\" meta page is corrupt",
+							RelationGetRelationName(state->rel))));
+
+		if (metad->btm_version < BTREE_MIN_VERSION ||
+			metad->btm_version > BTREE_VERSION)
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("version mismatch in index \"%s\": file version %d, "
+							"current version %d, minimum supported version %d",
+							RelationGetRelationName(state->rel),
+							metad->btm_version, BTREE_VERSION,
+							BTREE_MIN_VERSION)));
+
+		/* Finished with metapage checks */
+		return page;
+	}
+
+	/*
+	 * Deleted pages that still use the old 32-bit XID representation have no
+	 * sane "level" field because they type pun the field, but all other pages
+	 * (including pages deleted on Postgres 14+) have a valid value.
+	 */
+	if (!P_ISDELETED(opaque) || P_HAS_FULLXID(opaque))
+	{
+		/* Okay, no reason not to trust btpo_level field from page */
+
+		if (P_ISLEAF(opaque) && opaque->btpo_level != 0)
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg_internal("invalid leaf page level %u for block %u in index \"%s\"",
+									 opaque->btpo_level, blocknum,
+									 RelationGetRelationName(state->rel))));
+
+		if (!P_ISLEAF(opaque) && opaque->btpo_level == 0)
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg_internal("invalid internal page level 0 for block %u in index \"%s\"",
+									 blocknum,
+									 RelationGetRelationName(state->rel))));
+	}
+
+	/*
+	 * Sanity checks for number of items on page.
+	 *
+	 * As noted at the beginning of _bt_binsrch(), an internal page must have
+	 * children, since there must always be a negative infinity downlink
+	 * (there may also be a highkey).  In the case of non-rightmost leaf
+	 * pages, there must be at least a highkey.  The exceptions are deleted
+	 * pages, which contain no items.
+	 *
+	 * This is correct when pages are half-dead, since internal pages are
+	 * never half-dead, and leaf pages must have a high key when half-dead
+	 * (the rightmost page can never be deleted).  It's also correct with
+	 * fully deleted pages: _bt_unlink_halfdead_page() doesn't change anything
+	 * about the target page other than setting the page as fully dead, and
+	 * setting its xact field.  In particular, it doesn't change the sibling
+	 * links in the deletion target itself, since they're required when index
+	 * scans land on the deletion target, and then need to move right (or need
+	 * to move left, in the case of backward index scans).
+	 */
+	maxoffset = PageGetMaxOffsetNumber(page);
+	if (maxoffset > MaxIndexTuplesPerPage)
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg("Number of items on block %u of index \"%s\" exceeds MaxIndexTuplesPerPage (%u)",
+						blocknum, RelationGetRelationName(state->rel),
+						MaxIndexTuplesPerPage)));
+
+	if (!P_ISLEAF(opaque) && !P_ISDELETED(opaque) && maxoffset < P_FIRSTDATAKEY(opaque))
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg("internal block %u in index \"%s\" lacks high key and/or at least one downlink",
+						blocknum, RelationGetRelationName(state->rel))));
+
+	if (P_ISLEAF(opaque) && !P_ISDELETED(opaque) && !P_RIGHTMOST(opaque) && maxoffset < P_HIKEY)
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg("non-rightmost leaf block %u in index \"%s\" lacks high key item",
+						blocknum, RelationGetRelationName(state->rel))));
+
+	/*
+	 * In general, internal pages are never marked half-dead, except on
+	 * versions of Postgres prior to 9.4, where it can be valid transient
+	 * state.  This state is nonetheless treated as corruption by VACUUM on
+	 * from version 9.4 on, so do the same here.  See _bt_pagedel() for full
+	 * details.
+	 */
+	if (!P_ISLEAF(opaque) && P_ISHALFDEAD(opaque))
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg("internal page block %u in index \"%s\" is half-dead",
+						blocknum, RelationGetRelationName(state->rel)),
+				 errhint("This can be caused by an interrupted VACUUM in version 9.3 or older, before upgrade. Please REINDEX it.")));
+
+	/*
+	 * Check that internal pages have no garbage items, and that no page has
+	 * an invalid combination of deletion-related page level flags
+	 */
+	if (!P_ISLEAF(opaque) && P_HAS_GARBAGE(opaque))
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg_internal("internal page block %u in index \"%s\" has garbage items",
+								 blocknum, RelationGetRelationName(state->rel))));
+
+	if (P_HAS_FULLXID(opaque) && !P_ISDELETED(opaque))
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg_internal("full transaction id page flag appears in non-deleted block %u in index \"%s\"",
+								 blocknum, RelationGetRelationName(state->rel))));
+
+	if (P_ISDELETED(opaque) && P_ISHALFDEAD(opaque))
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg_internal("deleted page block %u in index \"%s\" is half-dead",
+								 blocknum, RelationGetRelationName(state->rel))));
+
+	return page;
+}
+
+/*
+ * _bt_mkscankey() wrapper that automatically prevents insertion scankey from
+ * being considered greater than the pivot tuple that its values originated
+ * from (or some other identical pivot tuple) in the common case where there
+ * are truncated/minus infinity attributes.  Without this extra step, there
+ * are forms of corruption that amcheck could theoretically fail to report.
+ *
+ * For example, invariant_g_offset() might miss a cross-page invariant failure
+ * on an internal level if the scankey built from the first item on the
+ * target's right sibling page happened to be equal to (not greater than) the
+ * last item on target page.  The !pivotsearch tiebreaker in _bt_compare()
+ * might otherwise cause amcheck to assume (rather than actually verify) that
+ * the scankey is greater.
+ */
+static inline BTScanInsert
+bt_mkscankey_pivotsearch(Relation rel, IndexTuple itup)
+{
+	BTScanInsert skey;
+
+	skey = _bt_mkscankey(rel, itup);
+	skey->pivotsearch = true;
+
+	return skey;
+}
+
+/*
+ * PageGetItemId() wrapper that validates returned line pointer.
+ *
+ * Buffer page/page item access macros generally trust that line pointers are
+ * not corrupt, which might cause problems for verification itself.  For
+ * example, there is no bounds checking in PageGetItem().  Passing it a
+ * corrupt line pointer can cause it to return a tuple/pointer that is unsafe
+ * to dereference.
+ *
+ * Validating line pointers before tuples avoids undefined behavior and
+ * assertion failures with corrupt indexes, making the verification process
+ * more robust and predictable.
+ */
+static ItemId
+PageGetItemIdCareful(BtreeCheckState *state, BlockNumber block, Page page,
+					 OffsetNumber offset)
+{
+	ItemId		itemid = PageGetItemId(page, offset);
+
+	if (ItemIdGetOffset(itemid) + ItemIdGetLength(itemid) >
+		BLCKSZ - MAXALIGN(sizeof(BTPageOpaqueData)))
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg("line pointer points past end of tuple space in index \"%s\"",
+						RelationGetRelationName(state->rel)),
+				 errdetail_internal("Index tid=(%u,%u) lp_off=%u, lp_len=%u lp_flags=%u.",
+									block, offset, ItemIdGetOffset(itemid),
+									ItemIdGetLength(itemid),
+									ItemIdGetFlags(itemid))));
+
+	/*
+	 * Verify that line pointer isn't LP_REDIRECT or LP_UNUSED, since nbtree
+	 * never uses either.  Verify that line pointer has storage, too, since
+	 * even LP_DEAD items should within nbtree.
+	 */
+	if (ItemIdIsRedirected(itemid) || !ItemIdIsUsed(itemid) ||
+		ItemIdGetLength(itemid) == 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg("invalid line pointer storage in index \"%s\"",
+						RelationGetRelationName(state->rel)),
+				 errdetail_internal("Index tid=(%u,%u) lp_off=%u, lp_len=%u lp_flags=%u.",
+									block, offset, ItemIdGetOffset(itemid),
+									ItemIdGetLength(itemid),
+									ItemIdGetFlags(itemid))));
+
+	return itemid;
+}
+
+/*
+ * BTreeTupleGetHeapTID() wrapper that enforces that a heap TID is present in
+ * cases where that is mandatory (i.e. for non-pivot tuples)
+ */
+static inline ItemPointer
+BTreeTupleGetHeapTIDCareful(BtreeCheckState *state, IndexTuple itup,
+							bool nonpivot)
+{
+	ItemPointer htid;
+
+	/*
+	 * Caller determines whether this is supposed to be a pivot or non-pivot
+	 * tuple using page type and item offset number.  Verify that tuple
+	 * metadata agrees with this.
+	 */
+	Assert(state->heapkeyspace);
+	if (BTreeTupleIsPivot(itup) && nonpivot)
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg_internal("block %u or its right sibling block or child block in index \"%s\" has unexpected pivot tuple",
+								 state->targetblock,
+								 RelationGetRelationName(state->rel))));
+
+	if (!BTreeTupleIsPivot(itup) && !nonpivot)
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg_internal("block %u or its right sibling block or child block in index \"%s\" has unexpected non-pivot tuple",
+								 state->targetblock,
+								 RelationGetRelationName(state->rel))));
+
+	htid = BTreeTupleGetHeapTID(itup);
+	if (!ItemPointerIsValid(htid) && nonpivot)
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				 errmsg("block %u or its right sibling block or child block in index \"%s\" contains non-pivot tuple that lacks a heap TID",
+						state->targetblock,
+						RelationGetRelationName(state->rel))));
+
+	return htid;
+}
+
+/*
+ * Return the "pointed to" TID for itup, which is used to generate a
+ * descriptive error message.  itup must be a "data item" tuple (it wouldn't
+ * make much sense to call here with a high key tuple, since there won't be a
+ * valid downlink/block number to display).
+ *
+ * Returns either a heap TID (which will be the first heap TID in posting list
+ * if itup is posting list tuple), or a TID that contains downlink block
+ * number, plus some encoded metadata (e.g., the number of attributes present
+ * in itup).
+ */
+static inline ItemPointer
+BTreeTupleGetPointsToTID(IndexTuple itup)
+{
+	/*
+	 * Rely on the assumption that !heapkeyspace internal page data items will
+	 * correctly return TID with downlink here -- BTreeTupleGetHeapTID() won't
+	 * recognize it as a pivot tuple, but everything still works out because
+	 * the t_tid field is still returned
+	 */
+	if (!BTreeTupleIsPivot(itup))
+		return BTreeTupleGetHeapTID(itup);
+
+	/* Pivot tuple returns TID with downlink block (heapkeyspace variant) */
+	return &itup->t_tid;
+}