MDEV-5122: "Commands out of sync", "Malformed packet" or client hang up...

- When INSERT catches an error, it should not attempt to send parts of 
  EXPLAIN output.

mysql-test/r/explain_non_select.result

@@ -198,3 +198,24 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 3	SUBQUERY	t1	index	NULL	a	5	NULL	3	Using index
 2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
 drop table t1,t2;
+#
+# MDEV-5122: "Commands out of sync", "Malformed packet" or client hang up on unique key violation
+#
+drop table if exists t1;
+Warnings:
+Note	1051	Unknown table 't1'
+drop function if exists f1;
+create table t1 (a int, unique(a));
+create function f1(x int)
+returns int
+begin
+insert into t1 values(x),(x);
+return 10;
+end|
+select f1(100);
+ERROR 23000: Duplicate entry '100' for key 'a'
+select 'OK';
+OK
+OK
+drop function f1;
+drop table t1;

mysql-test/t/explain_non_select.test

@@ -169,3 +169,32 @@ explain insert into t2 values
   (11, 1+(select max(a+1) from t1));
 
 drop table t1,t2;
+
+--echo #
+--echo # MDEV-5122: "Commands out of sync", "Malformed packet" or client hang up on unique key violation
+--echo #
+drop table if exists t1;
+
+--disable_warnings
+drop function if exists f1;
+--enable_warnings
+
+create table t1 (a int, unique(a));
+
+delimiter |;
+create function f1(x int)
+  returns int
+begin
+  insert into t1 values(x),(x);
+  return 10;
+end|
+delimiter ;|
+
+--error ER_DUP_ENTRY
+select f1(100);
+select 'OK';
+
+drop function f1;
+drop table t1;
+
+

sql/sql_insert.cc

@@ -819,8 +819,8 @@ bool mysql_insert(THD *thd,TABLE_LIST *table_list,
   save_insert_query_plan(thd, table_list);
   if (thd->lex->describe)
   {
-    retval= 0;
-    goto exit_without_my_ok;
+    retval= thd->lex->explain->send_explain(thd);
+    goto free_and_exit;
   }
 
   /*
@@ -1170,15 +1170,14 @@ bool mysql_insert(THD *thd,TABLE_LIST *table_list,
   DBUG_RETURN(FALSE);
 
 abort:
-exit_without_my_ok:
 #ifndef EMBEDDED_LIBRARY
   if (lock_type == TL_WRITE_DELAYED)
     end_delayed_insert(thd);
 #endif
   if (table != NULL)
     table->file->ha_release_auto_increment();
-  retval= thd->lex->explain->send_explain(thd);
 
+free_and_exit:
   if (!joins_freed)
     free_underlaid_joins(thd, &thd->lex->select_lex);
   thd->abort_on_warning= 0;