MDEV-8299: MyISAM or Aria table gets corrupted after EXPLAIN INSERT and INSERT

[EXPLAIN] INSERT .. SELECT creates a select_insert object.
select_insert calls handler->start_bulk_insert() during
initialization.

For MyISAM/Aria this requires that a matching call to
handler->end_bulk_insert() call is made.

Regular INSERT .. SELECT accomplishes this by calling either
select_result->send_eof() or select_result->abort_result_set().

EXPLAIN INSERT ... SELECT didn't call either, which resulted in
improper de-initializaiton of handler object. Make it call
abort_result_set(), which invokes handler->end_bulk_insert().

mysql-test/r/explain_non_select.result

@@ -232,3 +232,18 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	user	index	NULL	PRIMARY	420	NULL	4	Using index
 DROP TABLE t1;
 DROP VIEW v1;
+#
+# MDEV-8299: MyISAM or Aria table gets corrupted after EXPLAIN INSERT and INSERT
+#
+CREATE TABLE t1 (pk INT NOT NULL AUTO_INCREMENT PRIMARY KEY, i INT, KEY(i)) ENGINE=MyISAM;
+INSERT INTO t1 (i) VALUES (100),(200);
+CREATE TABLE t2 (j INT) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (10),(20);
+EXPLAIN INSERT INTO t1 (i) SELECT j FROM t2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	2	
+INSERT INTO t1 (i) VALUES (300);
+CHECK TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	check	status	OK
+DROP TABLE t1, t2;

mysql-test/t/explain_non_select.test

@@ -208,3 +208,19 @@ INSERT INTO t1 VALUES (1),(2);
 EXPLAIN UPDATE v1, mysql.user SET v1.a = v1.a + 1;
 DROP TABLE t1;
 DROP VIEW v1;
+ 
+--echo #
+--echo # MDEV-8299: MyISAM or Aria table gets corrupted after EXPLAIN INSERT and INSERT
+--echo #
+CREATE TABLE t1 (pk INT NOT NULL AUTO_INCREMENT PRIMARY KEY, i INT, KEY(i)) ENGINE=MyISAM;
+INSERT INTO t1 (i) VALUES (100),(200);
+
+CREATE TABLE t2 (j INT) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (10),(20);
+
+EXPLAIN INSERT INTO t1 (i) SELECT j FROM t2;
+INSERT INTO t1 (i) VALUES (300);
+CHECK TABLE t1;
+
+DROP TABLE t1, t2;
+

sql/sql_parse.cc

@@ -3549,6 +3549,16 @@ end_with_restore_list:
           query_cache_invalidate3(thd, first_table, 1);
           first_table->next_local= save_table;
         }
+        if (explain)
+        {
+          /*
+            sel_result needs to be cleaned up properly.
+            INSERT... SELECT statement will call either send_eof() or
+            abort_result_set(). EXPLAIN doesn't call either, so we need
+            to cleanup manually.
+          */
+          sel_result->abort_result_set();
+        }
         delete sel_result;
       }