MDEV-28806 Assertion `flag == 1' failure in row_build_index_entry_low…

… upon concurrent ALTER and UPDATE

- During online ADD INDEX, InnoDB was incorrectly writing log for an
UPDATE that does not affect the being-created index.

mysql-test/suite/gcol/r/innodb_virtual_debug.result

@@ -124,3 +124,24 @@ CHECK TABLE t1;
 Table	Op	Msg_type	Msg_text
 test.t1	check	status	OK
 DROP TABLE t1;
+#
+# MDEV-28806 Assertion `flag == 1' failure in
+# row_build_index_entry_low upon concurrent ALTER and UPDATE
+#
+CREATE TABLE t1(a CHAR(8), b INT, c INT AS (b), KEY(a)) ENGINE=InnoDB;
+INSERT INTO t1(b) VALUES (1),(2);
+connect  con1,localhost,root,,test;
+SET DEBUG_SYNC="alter_table_inplace_before_lock_upgrade SIGNAL dml_start WAIT_FOR dml_commit";
+ALTER TABLE t1 ADD KEY ind (c);
+connection default;
+SET DEBUG_SYNC="now WAIT_FOR dml_start";
+UPDATE t1 SET a ='foo';
+SET DEBUG_SYNC="now SIGNAL dml_commit";
+connection con1;
+CHECK TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	check	status	OK
+DROP TABLE t1;
+disconnect con1;
+connection default;
+SET DEBUG_SYNC=RESET;

mysql-test/suite/gcol/t/innodb_virtual_debug.test

@@ -312,4 +312,30 @@ SELECT * FROM t1;
 CHECK TABLE t1;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-28806 Assertion `flag == 1' failure in
+--echo # row_build_index_entry_low upon concurrent ALTER and UPDATE
+--echo #
+
+CREATE TABLE t1(a CHAR(8), b INT, c INT AS (b), KEY(a)) ENGINE=InnoDB;
+INSERT INTO t1(b) VALUES (1),(2);
+
+--connect (con1,localhost,root,,test)
+SET DEBUG_SYNC="alter_table_inplace_before_lock_upgrade SIGNAL dml_start WAIT_FOR dml_commit";
+send ALTER TABLE t1 ADD KEY ind (c);
+
+--connection default
+SET DEBUG_SYNC="now WAIT_FOR dml_start";
+UPDATE t1 SET a ='foo';
+SET DEBUG_SYNC="now SIGNAL dml_commit";
+
+# Cleanup
+--connection con1
+--reap
+CHECK TABLE t1;
+DROP TABLE t1;
+--disconnect con1
+connection default;
+SET DEBUG_SYNC=RESET;
+
 --source include/wait_until_count_sessions.inc

storage/innobase/row/row0log.cc

@@ -4061,6 +4061,11 @@ void UndorecApplier::log_update(const dtuple_t &tuple,
     {
       if (is_update)
       {
+        /* Ignore the index if the update doesn't affect the index */
+        if (!row_upd_changes_ord_field_binary(index, update,
+                                              nullptr,
+                                              row, new_ext))
+          goto next_index;
         dtuple_t *old_entry= row_build_index_entry_low(
           old_row, old_ext, index, heap, ROW_BUILD_NORMAL);
 
@@ -4080,6 +4085,7 @@ void UndorecApplier::log_update(const dtuple_t &tuple,
         success= row_log_online_op(index, old_entry, 0);
       }
     }
+next_index:
     index->lock.s_unlock();
     if (!success)
     {

storage/innobase/row/row0upd.cc

@@ -1261,9 +1261,6 @@ row_upd_changes_ord_field_binary_func(
 	ulint			i;
 	const dict_index_t*	clust_index;
 
-	ut_ad(thr);
-	ut_ad(thr->graph);
-	ut_ad(thr->graph->trx);
 	ut_ad(!index->table->skip_alter_undo);
 
 	n_unique = dict_index_get_n_unique(index);
@@ -1463,9 +1460,11 @@ row_upd_changes_ord_field_binary_func(
 					trx_rollback_recovered()
 					when the server had crashed before
 					storing the field. */
-					ut_ad(thr->graph->trx->is_recovered);
-					ut_ad(thr->graph->trx
-					      == trx_roll_crash_recv_trx);
+					ut_ad(!thr
+					      || thr->graph->trx->is_recovered);
+					ut_ad(!thr
+					      || thr->graph->trx
+					         == trx_roll_crash_recv_trx);
 					return(TRUE);
 				}