Fix for MDEV-8321, MDEV-6223

EXPLAIN INSERT ... SELECT tried to use SELECT's execution path. This
caused a collection of problems:
- SELECT_DESCRIBE flag was not put into select_lex->options, which
  means it was not in JOIN::select_options either (except for the first
  member of the UNION).
- This caused UNION members to be executed. They would attempt to write
  join output rows to the output.
- (Actual cause of the crash) second join sibling would call
  result->send_eof() when finished execution. Then,
  Explain_query::print_explain would attempt to write to query output
  again, and cause an assertion due to non-empty query output.

mysql-test/r/explain_non_select.result

@@ -247,3 +247,29 @@ CHECK TABLE t1;
 Table	Op	Msg_type	Msg_text
 test.t1	check	status	OK
 DROP TABLE t1, t2;
+#
+# MDEV-8321: Assertion `! is_set()' failed in Diagnostics_area::set_eof_status on EXPLAIN INSERT ... UNION
+#
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (b INT);
+CREATE TABLE t3 (c INT);
+INSERT INTO t1 VALUES (1),(2);
+INSERT INTO t2 VALUES (3),(4);
+INSERT INTO t3 VALUES (5),(6);
+EXPLAIN INSERT INTO t1 SELECT * FROM t2 UNION SELECT * FROM t3;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	
+2	UNION	t3	ALL	NULL	NULL	NULL	NULL	2	
+NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	
+drop table t1,t2,t3;
+#
+# MDEV-6223: Assertion `! is_set()' fails in Diagnostics_area::set_eof_status on EXPLAIN INSERT executed as a PS
+#
+CREATE TABLE t1 (a INT) ENGINE = MyISAM;
+CREATE TABLE t2 (b INT) ENGINE = MyISAM;
+INSERT INTO t2 VALUES (1),(2);
+PREPARE stmt FROM 'EXPLAIN INSERT INTO t1 SELECT * FROM t2';
+EXECUTE stmt;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	2	
+drop table t1,t2;

mysql-test/t/explain_non_select.test

@@ -224,3 +224,29 @@ CHECK TABLE t1;
 
 DROP TABLE t1, t2;
 
+--echo #
+--echo # MDEV-8321: Assertion `! is_set()' failed in Diagnostics_area::set_eof_status on EXPLAIN INSERT ... UNION
+--echo #
+
+CREATE TABLE t1 (a INT);
+CREATE TABLE t2 (b INT);
+CREATE TABLE t3 (c INT);
+
+# Data is not necessary, tables can be empty as well
+INSERT INTO t1 VALUES (1),(2);
+INSERT INTO t2 VALUES (3),(4);
+INSERT INTO t3 VALUES (5),(6);
+
+EXPLAIN INSERT INTO t1 SELECT * FROM t2 UNION SELECT * FROM t3;
+drop table t1,t2,t3;
+
+--echo #
+--echo # MDEV-6223: Assertion `! is_set()' fails in Diagnostics_area::set_eof_status on EXPLAIN INSERT executed as a PS
+--echo #
+CREATE TABLE t1 (a INT) ENGINE = MyISAM;
+CREATE TABLE t2 (b INT) ENGINE = MyISAM;
+INSERT INTO t2 VALUES (1),(2);
+PREPARE stmt FROM 'EXPLAIN INSERT INTO t1 SELECT * FROM t2';
+EXECUTE stmt;
+drop table t1,t2;
+

sql/sql_parse.cc

@@ -3533,7 +3533,10 @@ case SQLCOM_PREPARE:
                                                  lex->duplicates,
                                                  lex->ignore)))
       {
-	res= handle_select(thd, lex, sel_result, OPTION_SETUP_TABLES_DONE);
+        if (explain)
+          res= mysql_explain_union(thd, &thd->lex->unit, sel_result);
+        else
+          res= handle_select(thd, lex, sel_result, OPTION_SETUP_TABLES_DONE);
         /*
           Invalidate the table in the query cache if something changed
           after unlocking when changes become visible.

sql/sql_union.cc

@@ -286,8 +286,6 @@ bool st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
   {
     if (!(tmp_result= union_result= new select_union))
       goto err;
-    if (describe)
-      tmp_result= sel_result;
   }
   else
     tmp_result= sel_result;