MariaDB · dr-m · Apr 16, 2026
diff --git a/mysql-test/suite/innodb/r/lock_isolation.result b/mysql-test/suite/innodb/r/lock_isolation.result
@@ -280,7 +280,6 @@ connection consistent;
 SELECT * FROM t1;
 ERROR HY000: Record has changed since last read in table 't1'
 COMMIT;
-disconnect consistent;
 connection default;
 DROP TABLE t1,t2;
 #
@@ -292,4 +291,25 @@ SELECT * FROM t1 FOR UPDATE;
 a
 DROP TABLE t1;
 SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+#
+# MDEV-39263 Lost update despite innodb_snapshot_isolation
+#
+CREATE TABLE t1 (f1 INT PRIMARY KEY, val TEXT) ENGINE=InnoDB;
+connection consistent;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+SELECT * FROM t1 WHERE f1 = 1;
+f1	val
+connection default;
+SET DEBUG_SYNC="trx_after_commit_state SIGNAL committed WAIT_FOR cleanup";
+SET STATEMENT innodb_lock_wait_timeout=3 FOR INSERT INTO t1 VALUES(1,'6');
+connection consistent;
+SET DEBUG_SYNC="now WAIT_FOR committed";
+SET STATEMENT innodb_lock_wait_timeout=3 FOR
+UPDATE t1 SET val = CONCAT(val, ',', '3') WHERE f1 = 1;
+ERROR HY000: Record has changed since last read in table 't1'
+SET DEBUG_SYNC="now SIGNAL cleanup";
+disconnect consistent;
+connection default;
+SET DEBUG_SYNC="RESET";
+DROP TABLE t1;
 # End of 10.6 tests
diff --git a/mysql-test/suite/innodb/r/snapshot_isolation_race.result b/mysql-test/suite/innodb/r/snapshot_isolation_race.result
@@ -0,0 +1,37 @@
+#
+# MDEV-39263 Lost update despite innodb_snapshot_isolation
+#
+CREATE TABLE si_log(seq SERIAL, val INT UNSIGNED)
+ENGINE=InnoDB STATS_PERSISTENT=0;
+CREATE TABLE si_counter(val INT UNSIGNED) ENGINE=InnoDB STATS_PERSISTENT=0;
+INSERT INTO si_counter VALUES(0);
+CREATE PROCEDURE step()
+BEGIN NOT ATOMIC
+DECLARE EXIT HANDLER FOR 1020, 1213, 1205 BEGIN ROLLBACK; END;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+SELECT val INTO @val FROM si_counter;
+UPDATE si_counter SET val=@val+1;
+INSERT INTO si_log (val) VALUES (@val);
+DO SLEEP(0.0001);
+COMMIT;
+END|
+CREATE PROCEDURE test(c INT UNSIGNED)
+BEGIN
+WHILE c DO
+SET c=c-1;
+call step();
+START TRANSACTION;
+SELECT COUNT(*) INTO @count FROM si_log;
+SELECT SUM(val) INTO @sum FROM si_counter;
+COMMIT;
+IF (@count != @sum) THEN shutdown; END IF;
+END WHILE;
+END|
+connect stop_purge,localhost,root;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+connection default;
+SET innodb_snapshot_isolation=ON;
+disconnect stop_purge;
+DROP TABLE si_log, si_counter;
+DROP PROCEDURE test;
+DROP PROCEDURE step;
diff --git a/mysql-test/suite/innodb/t/lock_isolation.test b/mysql-test/suite/innodb/t/lock_isolation.test
@@ -291,7 +291,6 @@ COMMIT;
 SELECT * FROM t1;
 --enable_ps2_protocol
 COMMIT;
---disconnect consistent
 --connection default
 DROP TABLE t1,t2;
 
@@ -305,5 +304,36 @@ SELECT * FROM t1 FOR UPDATE;
 DROP TABLE t1;
 SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;
 
+--echo #
+--echo # MDEV-39263 Lost update despite innodb_snapshot_isolation
+--echo #
+
+CREATE TABLE t1 (f1 INT PRIMARY KEY, val TEXT) ENGINE=InnoDB;
+
+--connection consistent
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+SELECT * FROM t1 WHERE f1 = 1;
+
+# Stop autocommit INSERT execution in commit_in_memory() between commit_state()
+# and trx_sys.deregister_rw(), before locks are released
+--connection default
+SET DEBUG_SYNC="trx_after_commit_state SIGNAL committed WAIT_FOR cleanup";
+send SET STATEMENT innodb_lock_wait_timeout=3 FOR INSERT INTO t1 VALUES(1,'6');
+
+--connection consistent
+SET DEBUG_SYNC="now WAIT_FOR committed";
+--error ER_CHECKREAD
+SET STATEMENT innodb_lock_wait_timeout=3 FOR
+UPDATE t1 SET val = CONCAT(val, ',', '3') WHERE f1 = 1;
+
+SET DEBUG_SYNC="now SIGNAL cleanup";
+--disconnect consistent
+
+--connection default
+
+--reap
+SET DEBUG_SYNC="RESET";
+DROP TABLE t1;
+
 --source include/wait_until_count_sessions.inc
 --echo # End of 10.6 tests
diff --git a/mysql-test/suite/innodb/t/snapshot_isolation_race.test b/mysql-test/suite/innodb/t/snapshot_isolation_race.test
@@ -0,0 +1,58 @@
+--source include/have_innodb.inc
+
+--echo #
+--echo # MDEV-39263 Lost update despite innodb_snapshot_isolation
+--echo #
+
+CREATE TABLE si_log(seq SERIAL, val INT UNSIGNED)
+ENGINE=InnoDB STATS_PERSISTENT=0;
+
+CREATE TABLE si_counter(val INT UNSIGNED) ENGINE=InnoDB STATS_PERSISTENT=0;
+INSERT INTO si_counter VALUES(0);
+
+delimiter |;
+CREATE PROCEDURE step()
+BEGIN NOT ATOMIC
+  DECLARE EXIT HANDLER FOR 1020, 1213, 1205 BEGIN ROLLBACK; END;
+  START TRANSACTION WITH CONSISTENT SNAPSHOT;
+  SELECT val INTO @val FROM si_counter;
+  UPDATE si_counter SET val=@val+1;
+  INSERT INTO si_log (val) VALUES (@val);
+  DO SLEEP(0.0001);
+  COMMIT;
+END|
+
+CREATE PROCEDURE test(c INT UNSIGNED)
+BEGIN
+  WHILE c DO
+    SET c=c-1;
+    call step();
+    START TRANSACTION;
+      SELECT COUNT(*) INTO @count FROM si_log;
+      SELECT SUM(val) INTO @sum FROM si_counter;
+    COMMIT;
+    IF (@count != @sum) THEN shutdown; END IF;
+  END WHILE;
+END|
+delimiter ;|
+
+--connect stop_purge,localhost,root
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+--connection default
+SET innodb_snapshot_isolation=ON;
+
+--disable_query_log
+send call test(800);
+--connect con1,localhost,root
+SET innodb_snapshot_isolation=ON;
+call test(800);
+--disconnect con1
+--connection default
+--reap
+
+--enable_query_log
+--disconnect stop_purge
+
+DROP TABLE si_log, si_counter;
+DROP PROCEDURE test;
+DROP PROCEDURE step;
diff --git a/storage/innobase/lock/lock0lock.cc b/storage/innobase/lock/lock0lock.cc
@@ -6321,16 +6321,33 @@ lock_clust_rec_read_check_and_lock(
 		return DB_SUCCESS;
 	}
 
+	trx_id_t trx_id = 0;
+
 	if (heap_no > PAGE_HEAP_NO_SUPREMUM && gap_mode != LOCK_GAP
-            && trx->snapshot_isolation
+	    && trx->snapshot_isolation
 	    && trx->read_view.is_open()) {
-		trx_id_t trx_id= trx_read_trx_id(rec +
-						 row_trx_id_offset(rec, index));
-		if (!trx_sys.is_registered(trx, trx_id)
-		    && !trx->read_view.changes_visible(trx_id)
+		trx_id = trx_read_trx_id(rec + row_trx_id_offset(rec, index));
+		if (!trx->read_view.changes_visible(trx_id)
 		    && IF_WSREP(!(trx->is_wsrep()
 			&& wsrep_thd_skip_locking(trx->mysql_thd)), true)) {
-			return DB_RECORD_CHANGED;
+			/* Our record was last modified by a transaction that
+			we should not see. If that transaction has been
+			committed, we can return an error immediately,
+			without waiting for a record lock. */
+			if (!trx_sys.is_registered(trx, trx_id)) {
+				return DB_RECORD_CHANGED;
+			}
+			/* If lock_rec_lock() below returns DB_LOCK_WAIT,
+			there is a chance that the implicit lock holder will
+			be rolled back while we are waiting for a lock
+			timeout. In that case, this function would be invoked
+			again after the lock wait has been resolved.
+
+			If the lock_rec_lock() succeeds, we will have to
+			return this error. */
+		} else {
+			/* We are allowed to see this record. */
+			trx_id = 0;
 		}
 	}
 
@@ -6342,6 +6359,14 @@ lock_clust_rec_read_check_and_lock(
 
 	DEBUG_SYNC_C("after_lock_clust_rec_read_check_and_lock");
 
+	if (UNIV_UNLIKELY(trx_id != 0) && err <= DB_SUCCESS_LOCKED_REC) {
+		/* The last modifier of rec had just been committed.
+		(It cannot be rolled back, because our caller is holding
+		block->page.lock, which protects rec.)
+		We already determined that rec is too new for us. */
+		err = DB_RECORD_CHANGED;
+	}
+
 	return(err);
 }
 /*********************************************************************//**

diff --git a/storage/innobase/trx/trx0trx.cc b/storage/innobase/trx/trx0trx.cc
@@ -1403,6 +1403,7 @@ TRANSACTIONAL_INLINE inline void trx_t::commit_in_memory(const mtr_t *mtr)
         ut_ad(!l);
 #endif /* UNIV_DEBUG */
     commit_state();
+    DEBUG_SYNC_C("trx_after_commit_state");
 
     if (id)
     {