Adding upstream version 14.5.upstream/14.5upstream

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

1 files changed, 3260 insertions, 0 deletions

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;
+}