MDEV-5148: Server crashes in print_explain on killing EXPLAIN EXTENDED

- Make mysql_select() return error when the query was killed.

mysql-test/r/show_explain.result

@@ -1166,6 +1166,22 @@ SUM(b)
 0
 set debug_dbug=@old_debug;
 DROP TABLE t1,t2;
-# End
 drop table t0;
+#
+# MDEV-5148: Server crashes in print_explain on killing EXPLAIN EXTENDED
+#
+create table t0 (a int not null);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t1 (a int, b int);
+insert into t1 select a,a from t0;
+create table t2 as select * from t1;
+set @show_explain_probe_select_id=2;
+set debug_dbug='+d,show_explain_probe_best_ext_lim_search';
+explain 
+select * from t0 
+where not exists ( select 1 from t1, t2 where t1.b=t2.b and t2.a=t0.a) and a is null;
+kill query $thr2;
+ERROR 70100: Query execution was interrupted
+drop table t0,t1,t2;
+# End
 set debug_sync='RESET';

mysql-test/t/show_explain.test

@@ -1167,9 +1167,37 @@ reap;
 
 set debug_dbug=@old_debug;
 DROP TABLE t1,t2;
+drop table t0;
+
+--echo #
+--echo # MDEV-5148: Server crashes in print_explain on killing EXPLAIN EXTENDED
+--echo #
+create table t0 (a int not null);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+create table t1 (a int, b int);
+insert into t1 select a,a from t0;
+
+create table t2 as select * from t1;
+
+set @show_explain_probe_select_id=2;
+set debug_dbug='+d,show_explain_probe_best_ext_lim_search';
+send
+explain 
+select * from t0 
+where not exists ( select 1 from t1, t2 where t1.b=t2.b and t2.a=t0.a) and a is null;
+
+connection default;
+--source include/wait_condition.inc
+evalp kill query $thr2;
+
+connection con1;
+--error ER_QUERY_INTERRUPTED
+reap;
+
+drop table t0,t1,t2;
 
 --echo # End
-drop table t0;
 connection default;
 disconnect con1;
 set debug_sync='RESET';

sql/sql_select.cc

@@ -3200,7 +3200,7 @@ mysql_select(THD *thd, Item ***rref_pointer_array,
 	     select_result *result, SELECT_LEX_UNIT *unit,
 	     SELECT_LEX *select_lex)
 {
-  bool err;
+  int err= 0;
   bool free_join= 1;
   DBUG_ENTER("mysql_select");
 
@@ -3294,7 +3294,7 @@ mysql_select(THD *thd, Item ***rref_pointer_array,
     err|= select_lex->cleanup();
     DBUG_RETURN(err || thd->is_error());
   }
-  DBUG_RETURN(join->error);
+  DBUG_RETURN(join->error ? join->error: err);
 }
 
 
@@ -7412,6 +7412,14 @@ best_extension_by_limited_search(JOIN      *join,
   DBUG_ENTER("best_extension_by_limited_search");
 
   THD *thd= join->thd;
+
+  DBUG_EXECUTE_IF("show_explain_probe_best_ext_lim_search", 
+                  if (dbug_user_var_equals_int(thd, 
+                                               "show_explain_probe_select_id", 
+                                               join->select_lex->select_number))
+                        dbug_serve_apcs(thd, 1);
+                 );
+
   if (thd->check_killed())  // Abort
     DBUG_RETURN(TRUE);