Bug #39918 memory (heap) engine crashing with b-tree index and DELETE

         with seg fault

Multiple-table DELETE from a table joined to itself may cause
server crash. This was originally discovered with MEMORY engine,
but may affect other engines with different symptoms.

The problem was that the server violated SE API by performing
parallel table scan in one handler and removing records in
another (delete on the fly optimization).

mysql-test/r/heap_btree.result

@@ -336,4 +336,11 @@ a	b
 NULL	NULL
 NULL	1
 drop table t1;
+#
+# bug#39918 - memory (heap) engine crashing while executing self join with delete
+#
+CREATE TABLE t1(a INT, KEY USING BTREE (a)) ENGINE=MEMORY;
+INSERT INTO t1 VALUES(1),(1);
+DELETE a1 FROM t1 AS a1, t1 AS a2 WHERE a1.a=a2.a;
+DROP TABLE t1;
 End of 5.0 tests

mysql-test/t/heap_btree.test

@@ -253,5 +253,13 @@ insert into t1 values (1, 1), (3, 3), (2, 2), (NULL, 1), (NULL, NULL), (0, 0);
 select * from t1 where a is null;
 drop table t1;
 
+-- echo #
+-- echo # bug#39918 - memory (heap) engine crashing while executing self join with delete
+-- echo #
+
+CREATE TABLE t1(a INT, KEY USING BTREE (a)) ENGINE=MEMORY;
+INSERT INTO t1 VALUES(1),(1);
+DELETE a1 FROM t1 AS a1, t1 AS a2 WHERE a1.a=a2.a;
+DROP TABLE t1;
 --echo End of 5.0 tests
 

sql/sql_delete.cc

@@ -503,6 +503,11 @@ int mysql_multi_delete_prepare(THD *thd)
       }
     }
   }
+  /*
+    Reset the exclude flag to false so it doesn't interfare
+    with further calls to unique_table
+  */
+  lex->select_lex.exclude_from_table_unique_test= FALSE;
   DBUG_RETURN(FALSE);
 }
 
@@ -538,11 +543,24 @@ multi_delete::initialize_tables(JOIN *join)
     DBUG_RETURN(1);
 
   table_map tables_to_delete_from=0;
+  delete_while_scanning= 1;
   for (walk= delete_tables; walk; walk= walk->next_local)
+  {
     tables_to_delete_from|= walk->table->map;
+    if (delete_while_scanning &&
+        unique_table(thd, walk, join->tables_list, false))
+    {
+      /*
+        If the table we are going to delete from appears
+        in join, we need to defer delete. So the delete
+        doesn't interfers with the scaning of results.
+      */
+      delete_while_scanning= 0;
+    }
+  }
+
 
   walk= delete_tables;
-  delete_while_scanning= 1;
   for (JOIN_TAB *tab=join->join_tab, *end=join->join_tab+join->tables;
        tab < end;
        tab++)