MDEV-20396 Server crashes after DELETE with SEL NULL Foreign key and a

virtual column in index

Problem:
row_ins_foreign_fill_virtual was unconditionally set virtual fields to NULL
even though the field is not a part of a foreign key
(but a part of an index)

Solution:
The new virtual value should be computed with regard to cascade updates.

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

@@ -740,3 +740,32 @@ t1	CREATE TABLE `t1` (
   KEY `v4` (`v4`)
 ) ENGINE=InnoDB DEFAULT CHARSET=latin1
 DROP TABLE t1;
+#
+# MDEV-20396 Server crashes after DELETE with SEL NULL Foreign key and a
+# virtual column in index
+#
+CREATE TABLE parent
+(
+ID int unsigned NOT NULL,
+PRIMARY KEY (ID)
+);
+CREATE TABLE child
+(
+ID int unsigned NOT NULL,
+ParentID int unsigned NULL,
+Value int unsigned NOT NULL DEFAULT 0,
+Flag int unsigned AS (Value) VIRTUAL,
+PRIMARY KEY (ID),
+KEY (ParentID, Flag),
+FOREIGN KEY (ParentID) REFERENCES parent (ID) ON DELETE SET NULL
+ON UPDATE CASCADE
+);
+INSERT INTO parent (ID) VALUES (100);
+INSERT INTO child (ID,ParentID,Value) VALUES (123123,100,1);
+DELETE FROM parent WHERE ID=100;
+select * from child;
+ID	ParentID	Value	Flag
+123123	NULL	1	1
+INSERT INTO parent (ID) VALUES (100);
+UPDATE child SET ParentID=100 WHERE ID=123123;
+DROP TABLE child, parent;

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

@@ -605,3 +605,35 @@ ALTER TABLE t1 ADD CONSTRAINT fk FOREIGN KEY (v4) REFERENCES nosuch(col);
 SHOW CREATE TABLE t1;
 # Cleanup
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-20396 Server crashes after DELETE with SEL NULL Foreign key and a
+--echo # virtual column in index
+--echo #
+CREATE TABLE parent
+(
+  ID int unsigned NOT NULL,
+  PRIMARY KEY (ID)
+);
+
+CREATE TABLE child
+(
+  ID int unsigned NOT NULL,
+  ParentID int unsigned NULL,
+  Value int unsigned NOT NULL DEFAULT 0,
+  Flag int unsigned AS (Value) VIRTUAL,
+  PRIMARY KEY (ID),
+  KEY (ParentID, Flag),
+  FOREIGN KEY (ParentID) REFERENCES parent (ID) ON DELETE SET NULL
+                                                ON UPDATE CASCADE
+);
+
+INSERT INTO parent (ID) VALUES (100);
+INSERT INTO child (ID,ParentID,Value) VALUES (123123,100,1);
+DELETE FROM parent WHERE ID=100;
+select * from child;
+INSERT INTO parent (ID) VALUES (100);
+UPDATE child SET ParentID=100 WHERE ID=123123;
+
+# Cleanup
+DROP TABLE child, parent;

storage/innobase/row/row0ins.cc

@@ -945,8 +945,6 @@ row_ins_foreign_fill_virtual(
 		&ext, cascade->heap);
 	n_diff = update->n_fields;
 
-	update->n_fields += n_v_fld;
-
 	if (index->table->vc_templ == NULL) {
 		/** This can occur when there is a cascading
 		delete or update after restart. */
@@ -979,7 +977,7 @@ row_ins_foreign_fill_virtual(
 			return DB_COMPUTE_VALUE_FAILED;
 		}
 
-		upd_field = upd_get_nth_field(update, n_diff);
+		upd_field = update->fields + n_diff;
 
 		upd_field->old_v_val = static_cast<dfield_t*>(
 				mem_heap_alloc(cascade->heap,
@@ -989,30 +987,27 @@ row_ins_foreign_fill_virtual(
 
 		upd_field_set_v_field_no(upd_field, i, index);
 
-		if (node->is_delete
-		    ? (foreign->type & DICT_FOREIGN_ON_DELETE_SET_NULL)
-		    : (foreign->type & DICT_FOREIGN_ON_UPDATE_SET_NULL)) {
-
-			dfield_set_null(&upd_field->new_val);
-		}
-
-		if (!node->is_delete
-		    && (foreign->type & DICT_FOREIGN_ON_UPDATE_CASCADE)) {
-
-			dfield_t* new_vfield = innobase_get_computed_value(
-					update->old_vrow, col, index,
-					&vc.heap, update->heap, NULL, thd,
-					mysql_table, record, NULL,
-					node->update, foreign);
+		bool set_null =
+			node->is_delete
+			? (foreign->type & DICT_FOREIGN_ON_DELETE_SET_NULL)
+			: (foreign->type & DICT_FOREIGN_ON_UPDATE_SET_NULL);
 
-			if (new_vfield == NULL) {
-				return DB_COMPUTE_VALUE_FAILED;
-			}
+		dfield_t* new_vfield = innobase_get_computed_value(
+				update->old_vrow, col, index,
+				&vc.heap, update->heap, NULL, thd,
+				mysql_table, record, NULL,
+				set_null ? update : node->update, foreign);
 
-			dfield_copy(&(upd_field->new_val), new_vfield);
+		if (new_vfield == NULL) {
+			return DB_COMPUTE_VALUE_FAILED;
 		}
 
-		n_diff++;
+		dfield_copy(&upd_field->new_val, new_vfield);
+
+		if (!dfield_datas_are_binary_equal(
+				upd_field->old_v_val,
+				&upd_field->new_val, 0))
+			n_diff++;
 	}
 
 	update->n_fields = n_diff;