MDEV-21509 Possible hang during purge of history, or rollback

WL#6326 in MariaDB 10.2.2 introduced a potential hang on purge or rollback
when an index tree is being shrunk by multiple levels.

This fix is based on
mysql/mysql-server@f2c58526300c0d84837effa26d37cbd5d2694967
with the main difference that our version of the test case uses
DEBUG_SYNC instrumentation on ROLLBACK, not on purge.

btr_cur_will_modify_tree(): Simplify the check further.
This is the actual bug fix.

row_undo_mod_remove_clust_low(), row_undo_mod_clust(): Add DEBUG_SYNC
instrumentation for the test case.

mysql-test/suite/innodb/r/innodb_bug30113362.result

+SET GLOBAL innodb_adaptive_hash_index = false;
+SET GLOBAL innodb_stats_persistent = false;
+connect  purge_control,localhost,root,,;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+connect  con2,localhost,root,,;
+CREATE TABLE t1 (
+a00 CHAR(255) NOT NULL DEFAULT 'a',
+a01 CHAR(255) NOT NULL DEFAULT 'a',
+a02 CHAR(255) NOT NULL DEFAULT 'a',
+b INT NOT NULL DEFAULT 0,
+CONSTRAINT pkey PRIMARY KEY(a00, a01, a02)
+) charset latin1 ENGINE = InnoDB COMMENT='MERGE_THRESHOLD=45';
+SET GLOBAL innodb_limit_optimistic_insert_debug = 3;
+CREATE PROCEDURE data_load_t1()
+BEGIN
+DECLARE c1 INT DEFAULT 97;
+DECLARE c2 INT DEFAULT 97;
+DECLARE c3 INT DEFAULT 97;
+WHILE c1 < 102 DO
+WHILE c2 < 123 DO
+WHILE c3 < 123 DO
+INSERT INTO t1 (a00) VALUES (CHAR(c1,c2,c3));
+SET c3 = c3 + 1;
+END WHILE;
+SET c3 = 97;
+SET c2 = c2 + 1;
+END WHILE;
+SET c2 = 97;
+SET c1 = c1 + 1;
+END WHILE;
+END |
+call data_load_t1();
+DROP PROCEDURE data_load_t1;
+ANALYZE TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+SELECT CLUST_INDEX_SIZE FROM information_schema.INNODB_SYS_TABLESTATS WHERE NAME = 'test/t1';
+CLUST_INDEX_SIZE
+1856
+connection con2;
+DELETE FROM t1 WHERE a00 = 'cnm';
+COMMIT;
+BEGIN;
+INSERT INTO t1 SET a00 = 'cnm';
+connection purge_control;
+COMMIT;
+connection con2;
+SET GLOBAL innodb_limit_optimistic_insert_debug = 0;
+ROLLBACK;
+# Test start
+connection purge_control;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+connection con2;
+DELETE FROM t1 WHERE a00 = 'bii';
+COMMIT;
+BEGIN;
+INSERT INTO t1 SET a00 = 'bii';
+SET DEBUG_SYNC = 'rollback_undo_pk SIGNAL roll1_wait WAIT_FOR roll2';
+SET DEBUG_SYNC = 'rollback_purge_clust SIGNAL rollback_waiting WAIT_FOR resume';
+ROLLBACK;
+connection purge_control;
+SET DEBUG_SYNC = 'now WAIT_FOR roll1_wait';
+COMMIT;
+SET DEBUG_SYNC = 'now SIGNAL roll2';
+connect  con1,localhost,root,,;
+SET DEBUG_SYNC = 'now WAIT_FOR rollback_waiting';
+SET DEBUG_SYNC = 'rw_s_lock_waiting SIGNAL lockwait1';
+SELECT a00 FROM t1 WHERE a00 = 'bii';
+connection default;
+SET DEBUG_SYNC = 'now WAIT_FOR lockwait1';
+SET DEBUG_SYNC = 'now SIGNAL resume';
+connection con1;
+a00
+connection con2;
+connection default;
+ANALYZE TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+SELECT CLUST_INDEX_SIZE FROM information_schema.INNODB_SYS_TABLESTATS WHERE NAME = 'test/t1';
+CLUST_INDEX_SIZE
+1856
+DELETE FROM t1 WHERE a00 = 'dpn';
+COMMIT;
+INSERT INTO t1 SET a00 = 'dpn';
+ROLLBACK;
+ALTER TABLE t1 COMMENT='MERGE_THRESHOLD=35';
+connection purge_control;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+connection con2;
+DELETE FROM t1 WHERE a00 = 'cnd';
+COMMIT;
+BEGIN;
+INSERT INTO t1 SET a00 = 'cnd';
+SET DEBUG_SYNC = 'rollback_undo_pk SIGNAL roll1_wait WAIT_FOR roll2';
+SET DEBUG_SYNC = 'rollback_purge_clust SIGNAL rollback_waiting WAIT_FOR resume EXECUTE 2';
+ROLLBACK;
+connection purge_control;
+SET DEBUG_SYNC = 'now WAIT_FOR roll1_wait';
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+SET DEBUG_SYNC = 'now SIGNAL roll2';
+connection con1;
+SET DEBUG_SYNC = 'now WAIT_FOR rollback_waiting TIMEOUT 1';
+SET DEBUG_SYNC = 'now SIGNAL resume';
+SET DEBUG_SYNC = 'now WAIT_FOR rollback_waiting TIMEOUT 1';
+disconnect purge_control;
+connection default;
+SET DEBUG_SYNC = 'now SIGNAL resume';
+disconnect con1;
+connection con2;
+disconnect con2;
+connection default;
+ANALYZE TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+SELECT CLUST_INDEX_SIZE FROM information_schema.INNODB_SYS_TABLESTATS WHERE NAME = 'test/t1';
+CLUST_INDEX_SIZE
+1856
+SET DEBUG_SYNC = 'RESET';
+DROP TABLE t1;

mysql-test/suite/innodb/r/innodb_wl6326.result

@@ -299,10 +299,10 @@ SELECT CLUST_INDEX_SIZE FROM information_schema.INNODB_SYS_TABLESTATS WHERE NAME
 CLUST_INDEX_SIZE
 30
 SET DEBUG_SYNC = 'RESET';
-INSERT INTO t1 (a00) VALUES ('cva');
+INSERT INTO t1 (a00) VALUES ('coa');
 connection con1;
 SET DEBUG_SYNC = 'before_insert_pessimitic_row_ins_clust SIGNAL reached WAIT_FOR continue';
-INSERT INTO t1 (a00) VALUES ('cvb');
+INSERT INTO t1 (a00) VALUES ('cob');
 connection con2;
 SET DEBUG_SYNC = 'now WAIT_FOR reached';
 SET DEBUG_SYNC = 'rw_s_lock_waiting SIGNAL lockwait1';

mysql-test/suite/innodb/t/innodb_bug30113362.opt

+--innodb-sys-tablestats

mysql-test/suite/innodb/t/innodb_bug30113362.test

+#
+# Test for Bug#30113362 : BTR_CUR_WILL_MODIFY_TREE() IS INSUFFICIENT FOR HIGHER TREE LEVEL
+#
+
+--source include/have_innodb.inc
+--source include/have_debug.inc
+--source include/have_debug_sync.inc
+--source include/have_innodb_16k.inc
+
+--disable_query_log
+SET @old_innodb_limit_optimistic_insert_debug = @@innodb_limit_optimistic_insert_debug;
+SET @old_innodb_adaptive_hash_index = @@innodb_adaptive_hash_index;
+SET @old_innodb_stats_persistent = @@innodb_stats_persistent;
+--enable_query_log
+
+# Save the initial number of concurrent sessions
+--source include/count_sessions.inc
+
+SET GLOBAL innodb_adaptive_hash_index = false;
+SET GLOBAL innodb_stats_persistent = false;
+
+connect (purge_control,localhost,root,,);
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+
+--connect (con2,localhost,root,,)
+
+CREATE TABLE t1 (
+  a00 CHAR(255) NOT NULL DEFAULT 'a',
+  a01 CHAR(255) NOT NULL DEFAULT 'a',
+  a02 CHAR(255) NOT NULL DEFAULT 'a',
+  b INT NOT NULL DEFAULT 0,
+  CONSTRAINT pkey PRIMARY KEY(a00, a01, a02)
+) charset latin1 ENGINE = InnoDB COMMENT='MERGE_THRESHOLD=45';
+
+#
+# Prepare primary key index tree to be used for this test.
+#
+
+SET GLOBAL innodb_limit_optimistic_insert_debug = 3;
+
+delimiter |;
+CREATE PROCEDURE data_load_t1()
+BEGIN
+  DECLARE c1 INT DEFAULT 97;
+  DECLARE c2 INT DEFAULT 97;
+  DECLARE c3 INT DEFAULT 97;
+
+  WHILE c1 < 102 DO
+    WHILE c2 < 123 DO
+      WHILE c3 < 123 DO
+        INSERT INTO t1 (a00) VALUES (CHAR(c1,c2,c3));
+        SET c3 = c3 + 1;
+      END WHILE;
+      SET c3 = 97;
+      SET c2 = c2 + 1;
+    END WHILE;
+    SET c2 = 97;
+    SET c1 = c1 + 1;
+  END WHILE;
+END |
+delimiter ;|
+call data_load_t1();
+DROP PROCEDURE data_load_t1;
+
+# all node pages are sparse (max 3 node_ptrs)
+ANALYZE TABLE t1;
+SELECT CLUST_INDEX_SIZE FROM information_schema.INNODB_SYS_TABLESTATS WHERE NAME = 'test/t1';
+
+connection con2;
+DELETE FROM t1 WHERE a00 = 'cnm';
+COMMIT;
+BEGIN;
+INSERT INTO t1 SET a00 = 'cnm';
+# causes "domino falling" merges to upper level
+connection purge_control;
+COMMIT;
+connection con2;
+SET GLOBAL innodb_limit_optimistic_insert_debug = 0;
+ROLLBACK;
+
+# at this moment, in the tree,
+# ...
+# level 4: ...(ast,avw,ayz)(bcc,bff,bii,bll,boo,brr,buu,bxx,cba,ced,cqp,cts)(cwv,czy,ddb)...
+# ...
+
+--echo # Test start
+
+# (1) Similar case to the first reported corefile at bug#30113362
+#     - Deleting 'bii' causes "domino falling" merges and the node_ptr becomes left_most of level 4.
+#       So, the operation needs upper level pages' X-latch, though doesn't cause merge more.
+
+connection purge_control;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+connection con2;
+DELETE FROM t1 WHERE a00 = 'bii';
+COMMIT;
+BEGIN;
+INSERT INTO t1 SET a00 = 'bii';
+SET DEBUG_SYNC = 'rollback_undo_pk SIGNAL roll1_wait WAIT_FOR roll2';
+SET DEBUG_SYNC = 'rollback_purge_clust SIGNAL rollback_waiting WAIT_FOR resume';
+send ROLLBACK;
+
+connection purge_control;
+SET DEBUG_SYNC = 'now WAIT_FOR roll1_wait';
+COMMIT;
+SET DEBUG_SYNC = 'now SIGNAL roll2';
+
+connect (con1,localhost,root,,);
+SET DEBUG_SYNC = 'now WAIT_FOR rollback_waiting';
+SET DEBUG_SYNC = 'rw_s_lock_waiting SIGNAL lockwait1';
+send SELECT a00 FROM t1 WHERE a00 = 'bii';
+
+connection default;
+SET DEBUG_SYNC = 'now WAIT_FOR lockwait1';
+# bug#30113362 caused deadlock
+SET DEBUG_SYNC = 'now SIGNAL resume';
+
+connection con1;
+reap;
+connection con2;
+reap;
+connection default;
+
+ANALYZE TABLE t1;
+SELECT CLUST_INDEX_SIZE FROM information_schema.INNODB_SYS_TABLESTATS WHERE NAME = 'test/t1';
+
+# (2) Confirm blocking domain caused by DELETE modify_tree for tall index tree
+
+# at this moment, in the tree,
+# ...
+# level 4: ...(ajk,amn,apq)(ast,avw,ayz,bll,boo,brr,buu,bxx,cba,ced,cqp,cts)(cwv,czy,ddb)(dge,djh,dmk)(dpn,dsq,dvt)(dyw,ebz,efc)...
+# ...
+
+# makes >17 records in level4 [(2^(4-1))*2 + 1]. (causes never left_most records)
+DELETE FROM t1 WHERE a00 = 'dpn';
+COMMIT;
+INSERT INTO t1 SET a00 = 'dpn';
+ROLLBACK;
+
+# at this moment, in the tree,
+# (* before "]" and after "[" records are treated as left_most possible records)
+# ...
+# level 4: ...(ajk,amn,apq)(ast,avw,ayz,bll,boo,brr,buu,bxx],cba,ced,[cqp,cts,cwv,czy,ddb,dge,dsq,dvt)(dyw,ebz,efc)...
+# level 3: ...(cba,ccb,cdc)(ced,cfe,cgf,chg],cih,cji,[ckj,clk,con,cpo)(cqp,crq,csr)...
+# level 2: ...(ckj,cks,clb)(clk,clt],cmc,cml,cmu,cnd,[cnv,coe)(con,cow,cpf)...
+# level 1: ...(cmu,cmx,cna)(cnd],cng,cnj,cnp,[cns)(cnv,cny,cob)...
+# level 0: ...(cnd,cne,cnf)(cng,cnh,cni)(cnj,cnk,cnl,cnn,cno)(cnp,cnq,cnr)...
+
+# deletes just 'ced' node_ptr only from level 4. doesn't cause merge and never left_most.
+# adjusts MERGE_THRESHOLD to do so.
+ALTER TABLE t1 COMMENT='MERGE_THRESHOLD=35';
+
+connection purge_control;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+
+connection con2;
+DELETE FROM t1 WHERE a00 = 'cnd';
+COMMIT;
+BEGIN;
+INSERT INTO t1 SET a00 = 'cnd';
+SET DEBUG_SYNC = 'rollback_undo_pk SIGNAL roll1_wait WAIT_FOR roll2';
+SET DEBUG_SYNC = 'rollback_purge_clust SIGNAL rollback_waiting WAIT_FOR resume EXECUTE 2';
+send ROLLBACK;
+
+connection purge_control;
+SET DEBUG_SYNC = 'now WAIT_FOR roll1_wait';
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+SET DEBUG_SYNC = 'now SIGNAL roll2';
+
+connection con1;
+# FIXME: For some reason, we will not always receive these signals!
+--disable_warnings
+# An optimistic row_undo_mod_remove_clust_low() will fail.
+SET DEBUG_SYNC = 'now WAIT_FOR rollback_waiting TIMEOUT 1';
+SET DEBUG_SYNC = 'now SIGNAL resume';
+# Wait for the pessimistic row_undo_mod_remove_clust_low() attempt.
+SET DEBUG_SYNC = 'now WAIT_FOR rollback_waiting TIMEOUT 1';
+--enable_warnings
+disconnect purge_control;
+
+# The expectation should be...
+# level 0: (#cnd#,cne,cnf): causes merge
+# level 1: (#cnd#],cng,cnj,cnp,[cns): left_most
+# level 2: (clk,clt],cmc,cml,cmu,#cnd#,[cnv,coe): causes merge
+# level 3: (ced,cfe,cgf,chg],cih,cji,[ckj,#clk#,con,cpo): left_most possible (not cause merge)
+# level 4: (ast,avw,ayz,bll,boo,brr,buu,bxx],cba,#ced#,[cqp,cts,cwv,czy,ddb,dge,dsq,dvt): no merge, not left_most possible
+# So, the top X-latch page is at level4. (ast~dvt)
+
+# blocking domain based on whether its ancestor is latched or not.
+# (*[]: ancestor is X-latched)
+# level 0: ...(asq,asr,ass) [(ast,asu,asv)...(dyt,dyu,dyv)] (dyw,dyx,dyy)...
+
+# Not blocked searches
+## In MariaDB, both these will block, because we use different DEBUG_SYNC
+## instrumentation (in rollback, not purge) and the root page (number 3)
+## is being latched in row_undo_mod_remove_clust_low().
+## SELECT a00 FROM t1 WHERE a00 = 'ass';
+## SELECT a00 FROM t1 WHERE a00 = 'dyx';
+
+## SET DEBUG_SYNC = 'rw_s_lock_waiting SIGNAL lockwait1';
+## send SELECT a00 FROM t1 WHERE a00 = 'ast';
+
+## connection con2;
+## SET DEBUG_SYNC = 'rw_s_lock_waiting SIGNAL lockwait2';
+## send SELECT a00 FROM t1 WHERE a00 = 'dyw';
+
+connection default;
+## SET DEBUG_SYNC = 'now WAIT_FOR lockwait1';
+## SET DEBUG_SYNC = 'now WAIT_FOR lockwait2';
+SET DEBUG_SYNC = 'now SIGNAL resume';
+
+## connection con1;
+## reap;
+disconnect con1;
+
+connection con2;
+reap;
+disconnect con2;
+
+connection default;
+ANALYZE TABLE t1;
+SELECT CLUST_INDEX_SIZE FROM information_schema.INNODB_SYS_TABLESTATS WHERE NAME = 'test/t1';
+
+
+# Cleanup
+SET DEBUG_SYNC = 'RESET';
+
+DROP TABLE t1;
+
+--disable_query_log
+SET GLOBAL innodb_limit_optimistic_insert_debug = @old_innodb_limit_optimistic_insert_debug;
+SET GLOBAL innodb_adaptive_hash_index = @old_innodb_adaptive_hash_index;
+SET GLOBAL innodb_stats_persistent = @old_innodb_stats_persistent;
+--enable_query_log
+
+--source include/wait_until_count_sessions.inc

mysql-test/suite/innodb/t/innodb_wl6326.test

@@ -373,23 +373,23 @@ SELECT CLUST_INDEX_SIZE FROM information_schema.INNODB_SYS_TABLESTATS WHERE NAME
 
 
 
-# (2) Insert records to leaf page (cv..) and cause modify_page
-#     - root page is X latched, because node_ptr for 'cv'
-#       is 2nd record for (co,cv,dc,dj,dq,dx,ee)
+# (2) Insert records to leaf page (co..) and cause modify_page
+#     - root page is X latched, because node_ptr for 'co'
+#       is 1st record for (co,cv,dc,dj,dq,dx,ee)
 #
 # * ordinary pessimitic insert might be done by pessistic update
 #   and we should consider possibility node_ptr to be deleted.
 
 SET DEBUG_SYNC = 'RESET';
 
-# Filling leaf page (cv..)
-INSERT INTO t1 (a00) VALUES ('cva');
+# Filling leaf page (co..)
+INSERT INTO t1 (a00) VALUES ('coa');
 
 --connection con1
 SET DEBUG_SYNC = 'before_insert_pessimitic_row_ins_clust SIGNAL reached WAIT_FOR continue';
 # Cause modify_tree
 --send
-INSERT INTO t1 (a00) VALUES ('cvb');
+INSERT INTO t1 (a00) VALUES ('cob');
 
 --connection con2
 SET DEBUG_SYNC = 'now WAIT_FOR reached';

storage/innobase/btr/btr0cur.cc

@@ -583,46 +583,66 @@ btr_cur_will_modify_tree(
 	const ulint n_recs = page_get_n_recs(page);
 
 	if (lock_intention <= BTR_INTENTION_BOTH) {
-		ulint	margin;
+		compile_time_assert(BTR_INTENTION_DELETE < BTR_INTENTION_BOTH);
+		compile_time_assert(BTR_INTENTION_BOTH < BTR_INTENTION_INSERT);
 
+		if (!page_has_siblings(page)) {
+			return true;
+		}
+
+		ulint margin = rec_size;
+
+		if (lock_intention == BTR_INTENTION_BOTH) {
+			ulint	level = btr_page_get_level(page, mtr);
+
+			/* This value is the worst expectation for the node_ptr
+			records to be deleted from this page. It is used to
+			expect whether the cursor position can be the left_most
+			record in this page or not. */
+			ulint   max_nodes_deleted = 0;
+
+			/* By modifying tree operations from the under of this
+			level, logically (2 ^ (level - 1)) opportunities to
+			deleting records in maximum even unreally rare case. */
+			if (level > 7) {
+				/* TODO: adjust this practical limit. */
+				max_nodes_deleted = 64;
+			} else if (level > 0) {
+				max_nodes_deleted = (ulint)1 << (level - 1);
+			}
 			/* check delete will cause. (BTR_INTENTION_BOTH
 			or BTR_INTENTION_DELETE) */
-		/* first, 2nd, 2nd-last and last records are 4 records */
-		if (n_recs < 5) {
-			return(true);
+			if (n_recs <= max_nodes_deleted * 2
+			    || page_rec_is_first(rec, page)) {
+				/* The cursor record can be the left most record
+				in this page. */
+				return true;
 			}
 
-		/* is first, 2nd or last record */
-		if (page_rec_is_first(rec, page)
-		    || (page_has_next(page)
-			&& (page_rec_is_last(rec, page)
-			    || page_rec_is_second_last(rec, page)))
-		    || (page_has_prev(page)
-			&& page_rec_is_second(rec, page))) {
-			return(true);
+			if (page_has_prev(page)
+			    && page_rec_distance_is_at_most(
+				    page_get_infimum_rec(page), rec,
+				    max_nodes_deleted)) {
+				return true;
+			}
+
+			if (page_has_next(page)
+			    && page_rec_distance_is_at_most(
+				    rec, page_get_supremum_rec(page),
+				    max_nodes_deleted)) {
+				return true;
 			}
 
-		if (lock_intention == BTR_INTENTION_BOTH) {
 			/* Delete at leftmost record in a page causes delete
 			& insert at its parent page. After that, the delete
 			might cause btr_compress() and delete record at its
-			parent page. Thus we should consider max 2 deletes. */
-
-			margin = rec_size * 2;
-		} else {
-			ut_ad(lock_intention == BTR_INTENTION_DELETE);
-
-			margin = rec_size;
+			parent page. Thus we should consider max deletes. */
+			margin *= max_nodes_deleted;
 		}
-		/* NOTE: call mach_read_from_4() directly to avoid assertion
-		failure. It is safe because we already have SX latch of the
-		index tree */
+
+		/* Safe because we already have SX latch of the index tree */
 		if (page_get_data_size(page)
-			< margin + BTR_CUR_PAGE_COMPRESS_LIMIT(index)
-		    || (mach_read_from_4(page + FIL_PAGE_NEXT)
-				== FIL_NULL
-			&& mach_read_from_4(page + FIL_PAGE_PREV)
-				== FIL_NULL)) {
+		    < margin + BTR_CUR_PAGE_COMPRESS_LIMIT(index)) {
 			return(true);
 		}
 	}

storage/innobase/include/page0page.h

 /*****************************************************************************
-Copyright (c) 1994, 2016, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2013, 2019, MariaDB Corporation.
+Copyright (c) 1994, 2019, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 2013, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -813,6 +813,22 @@ page_rec_is_last(
 	const page_t*	page)	/*!< in: page */
 	MY_ATTRIBUTE((warn_unused_result));
 
+/************************************************************//**
+true if distance between the records (measured in number of times we have to
+move to the next record) is at most the specified value
+@param[in]	left_rec	lefter record
+@param[in]	right_rec	righter record
+@param[in]	val		specified value to compare
+@return true if the distance is smaller than the value */
+UNIV_INLINE
+bool
+page_rec_distance_is_at_most(
+/*=========================*/
+	const rec_t*	left_rec,
+	const rec_t*	right_rec,
+	ulint		val)
+	MY_ATTRIBUTE((warn_unused_result));
+
 /************************************************************//**
 true if the record is the second last user record on a page.
 @return true if the second last user record */

storage/innobase/include/page0page.ic

 /*****************************************************************************
 
-Copyright (c) 1994, 2015, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2016, 2019, MariaDB Corporation.
+Copyright (c) 1994, 2019, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 2016, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -358,6 +358,26 @@ page_rec_is_last(
 	return(page_rec_get_next_const(rec) == page_get_supremum_rec(page));
 }
 
+/************************************************************//**
+true if distance between the records (measured in number of times we have to
+move to the next record) is at most the specified value */
+UNIV_INLINE
+bool
+page_rec_distance_is_at_most(
+/*=========================*/
+	const rec_t*	left_rec,
+	const rec_t*	right_rec,
+	ulint		val)
+{
+	for (ulint i = 0; i <= val; i++) {
+		if (left_rec == right_rec) {
+			return (true);
+		}
+		left_rec = page_rec_get_next_const(left_rec);
+	}
+	return (false);
+}
+
 /************************************************************//**
 true if the record is the second last user record on a page.
 @return true if the second last user record */

storage/innobase/row/row0umod.cc

 /*****************************************************************************
 
 Copyright (c) 1997, 2017, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, 2019, MariaDB Corporation.
+Copyright (c) 2017, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -169,12 +169,15 @@ row_undo_mod_remove_clust_low(
 	/* Find out if the record has been purged already
 	or if we can remove it. */
 
-	if (!btr_pcur_restore_position(mode, &node->pcur, mtr)
-	    || row_vers_must_preserve_del_marked(node->new_trx_id,
-						 node->table->name,
-						 mtr)) {
+	if (!btr_pcur_restore_position(mode, &node->pcur, mtr)) {
+		return DB_SUCCESS;
+	}
 
-		return(DB_SUCCESS);
+	DEBUG_SYNC_C("rollback_purge_clust");
+
+	if (row_vers_must_preserve_del_marked(node->new_trx_id,
+					      node->table->name, mtr)) {
+		return DB_SUCCESS;
 	}
 
 	btr_cur = btr_pcur_get_btr_cur(&node->pcur);
@@ -361,6 +364,7 @@ row_undo_mod_clust(
 	      == node->new_trx_id);
 
 	btr_pcur_commit_specify_mtr(pcur, &mtr);
+	DEBUG_SYNC_C("rollback_undo_pk");
 
 	if (err == DB_SUCCESS && node->rec_type == TRX_UNDO_UPD_DEL_REC) {