MDEV-323: SHOW EXPLAIN: Plan produced by SHOW EXPLAIN loses 'UNION RESULT' line

- Make SHOW EXPLAIN code correctly print fake_select_lex: both in the case where 
  it has not yet been executed, and when it has been executed.

mysql-test/r/show_explain.result

@@ -625,4 +625,38 @@ a + 1
 set debug_dbug='';
 DROP VIEW v1;
 DROP TABLE t1;
+#
+# MDEV-323: SHOW EXPLAIN: Plan produced by SHOW EXPLAIN loses 
+#            'UNION RESULT' line on the way without saying that the plan was already deleted
+#
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (4),(6);
+EXPLAIN 
+SELECT a FROM t1 WHERE a IN ( SELECT 1+SLEEP(0.01) UNION SELECT 2 );
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+3	DEPENDENT UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	
+set debug_dbug='d,show_explain_probe_union_read';
+SELECT a FROM t1 WHERE a IN ( SELECT 1+SLEEP(0.01) UNION SELECT 2 );
+show explain for $thr2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+3	DEPENDENT UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	
+Warnings:
+Note	1003	SELECT a FROM t1 WHERE a IN ( SELECT 1+SLEEP(0.01) UNION SELECT 2 )
+show explain for $thr2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+3	DEPENDENT UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	
+Warnings:
+Note	1003	SELECT a FROM t1 WHERE a IN ( SELECT 1+SLEEP(0.01) UNION SELECT 2 )
+a
+set debug_dbug='';
+DROP TABLE t1;
 drop table t0;

mysql-test/t/show_explain.test

@@ -634,6 +634,33 @@ DROP VIEW v1;
 DROP TABLE t1;
 
 
+--echo #
+--echo # MDEV-323: SHOW EXPLAIN: Plan produced by SHOW EXPLAIN loses 
+--echo #            'UNION RESULT' line on the way without saying that the plan was already deleted
+--echo #
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (4),(6);
+
+EXPLAIN 
+SELECT a FROM t1 WHERE a IN ( SELECT 1+SLEEP(0.01) UNION SELECT 2 );
+
+set debug_dbug='d,show_explain_probe_union_read';
+send
+SELECT a FROM t1 WHERE a IN ( SELECT 1+SLEEP(0.01) UNION SELECT 2 );
+
+connection default;
+--source include/wait_condition.inc
+evalp show explain for $thr2;
+
+--source include/wait_condition.inc
+evalp show explain for $thr2;
+
+connection con1;
+reap;
+
+set debug_dbug='';
+
+DROP TABLE t1;
 
 ## TODO: Test this: have several SHOW EXPLAIN requests be queued up for a
 ##       thread and served together.

sql/sql_lex.cc

@@ -4165,10 +4165,8 @@ int st_select_lex_unit::print_explain(select_result_sink *output,
       break;
   }
 
-  /* 
-    Note: it could be that fake_select_lex->join == NULL still at this point
-  */
-  if (fake_select_lex && !fake_select_lex->join)
+  /* Note: fake_select_lex->join may be NULL or non-NULL at this point */
+  if (fake_select_lex)
   {
     res= print_fake_select_lex_join(output, TRUE /* on the fly */,
                                     fake_select_lex, explain_flags);

sql/sql_union.cc

@@ -720,6 +720,8 @@ bool st_select_lex_unit::exec()
     }
   }
 
+  DBUG_EXECUTE_IF("show_explain_probe_union_read", 
+                   dbug_serve_apcs(thd, 1););
   /* Send result to 'result' */
   saved_error= TRUE;
   {