MDEV-12745 InnoDB Assertion `0' failed at row0umod.cc:1181

row_undo_mod_parse_undo_rec(): Relax the too strict assertion and
correct the comment.

innodb.innodb-blob: Force a flush of the redo log right before
killing the server, to ensure that the code path gets exercised.
(The bogus debug assertion failed on the rollback of the statement
UPDATE t3 SET c=REPEAT('j',3000) WHERE a=2 which did not modify
any indexes before the server was killed.)

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

@@ -97,6 +97,12 @@ SELECT info FROM information_schema.processlist
 WHERE state = 'debug sync point: before_row_upd_extern';
 info
 UPDATE t3 SET c=REPEAT('i',3000) WHERE a=2
+connect  con3,localhost,root,,;
+SET GLOBAL innodb_flush_log_at_trx_commit=1;
+DELETE FROM t1;
+ROLLBACK;
+disconnect con3;
+connection con2;
 disconnect con2;
 connection default;
 ERROR HY000: Lost connection to MySQL server during query

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

@@ -139,6 +139,14 @@ SET DEBUG_SYNC='now WAIT_FOR have_latch';
 SELECT info FROM information_schema.processlist
 WHERE state = 'debug sync point: before_row_upd_extern';
 
+connect (con3,localhost,root,,);
+SET GLOBAL innodb_flush_log_at_trx_commit=1;
+DELETE FROM t1;
+ROLLBACK;
+disconnect con3;
+
+connection con2;
+
 --let $shutdown_timeout=0
 --source include/restart_mysqld.inc
 

storage/innobase/row/row0umod.cc

@@ -1169,16 +1169,19 @@ row_undo_mod_parse_undo_rec(
 	node->new_trx_id = trx_id;
 	node->cmpl_info = cmpl_info;
 
-	if (UNIV_UNLIKELY(!row_undo_search_clust_to_pcur(node))) {
-		/* This should never occur. As long as this
-		rolling-back transaction exists, the PRIMARY KEY value
-		pointed to by the undo log record must exist.
+	if (!row_undo_search_clust_to_pcur(node)) {
+		/* As long as this rolling-back transaction exists,
+		the PRIMARY KEY value pointed to by the undo log
+		record must exist. But, it is possible that the record
+		was not modified yet (the DB_ROLL_PTR does not match
+		node->roll_ptr) and thus there is nothing to roll back.
+
 		btr_cur_upd_lock_and_undo() only writes the undo log
 		record after successfully acquiring an exclusive lock
 		on the the clustered index record. That lock will not
 		be released before the transaction is committed or
 		fully rolled back. */
-		ut_ad(0);
+		ut_ad(node->pcur.btr_cur.low_match == node->ref->n_fields);
 		goto close_table;
 	}