MDEV-5506 safe_mutex: Trying to lock unitialized mutex at safemalloc.c

on server shutdown after SELECT with CONVERT_TZ

It's wrong to return my_empty_string from val_str().
Removing my_empty_string. Using make_empty_result() instead.

mysql-test/r/date_formats.result

@@ -24,6 +24,8 @@ SET date_format='%Y%m%d';
 SET date_format='%Y.%m.%d';
 SET date_format='%d.%m.%Y';
 SET date_format='%m-%d-%Y';
+SET datetime_format= NULL;
+ERROR 42000: Variable 'datetime_format' can't be set to the value of 'NULL'
 set datetime_format= '%Y%m%d%H%i%s';
 set datetime_format= '%Y-%m-%d %H:%i:%s';
 set datetime_format= '%m-%d-%y %H:%i:%s.%f';

mysql-test/r/timezone2.result

@@ -326,5 +326,11 @@ NULL
 Warnings:
 Warning	1292	Incorrect datetime value: '00:00:00'
 #
+# MDEV-5506 safe_mutex: Trying to lock unitialized mutex at safemalloc.c on server shutdown after SELECT with CONVERT_TZ
+#
+SELECT CONVERT_TZ('2001-10-08 00:00:00', MAKE_SET(0,'+01:00'), '+00:00' );
+CONVERT_TZ('2001-10-08 00:00:00', MAKE_SET(0,'+01:00'), '+00:00' )
+NULL
+#
 # End of 5.3 tests
 #

mysql-test/t/date_formats.test

@@ -32,6 +32,8 @@ SET date_format='%Y.%m.%d';
 SET date_format='%d.%m.%Y';
 SET date_format='%m-%d-%Y';
 
+--error ER_WRONG_VALUE_FOR_VAR
+SET datetime_format= NULL;
 set datetime_format= '%Y%m%d%H%i%s';
 set datetime_format= '%Y-%m-%d %H:%i:%s';
 set datetime_format= '%m-%d-%y %H:%i:%s.%f';

mysql-test/t/timezone2.test

@@ -297,6 +297,12 @@ DROP TABLE t1;
 SELECT CONVERT_TZ(TIME('00:00:00'),'+00:00','+7:5');
 SELECT CONVERT_TZ(TIME('2010-01-01 00:00:00'),'+00:00','+7:5');
 
+--echo #
+--echo # MDEV-5506 safe_mutex: Trying to lock unitialized mutex at safemalloc.c on server shutdown after SELECT with CONVERT_TZ
+--echo #
+SELECT CONVERT_TZ('2001-10-08 00:00:00', MAKE_SET(0,'+01:00'), '+00:00' ); 
+
+
 --echo #
 --echo # End of 5.3 tests
 --echo #

sql/item_strfunc.cc

@@ -40,11 +40,6 @@ C_MODE_START
 #include "../mysys/my_static.h"			// For soundex_map
 C_MODE_END
 
-/**
-   @todo Remove this. It is not safe to use a shared String object.
- */
-String my_empty_string("",default_charset_info);
-
 
 /*
   Convert an array of bytes to a hexadecimal representation.
@@ -2250,7 +2245,7 @@ String *Item_func_make_set::val_str(String *str)
   ulonglong bits;
   bool first_found=0;
   Item **ptr=args+1;
-  String *result=&my_empty_string;
+  String *result= make_empty_result();
 
   bits=args[0]->val_int();
   if ((null_value=args[0]->null_value))

sql/mysql_priv.h

@@ -2357,7 +2357,6 @@ extern pthread_attr_t connection_attrib;
 extern I_List<THD> threads;
 extern I_List<NAMED_LIST> key_caches;
 extern MY_BITMAP temp_pool;
-extern String my_empty_string;
 extern const String my_null_string;
 extern SHOW_VAR status_vars[];
 #endif /* MYSQL_SERVER */

sql/set_var.cc

@@ -2196,13 +2196,11 @@ bool sys_var_thd_date_time_format::check(THD *thd, set_var *var)
   String str(buff,sizeof(buff) - 1, system_charset_info), *res;
   DATE_TIME_FORMAT *format;
 
-  if (!(res=var->value->val_str(&str)))
-    res= &my_empty_string;
-
-  if (!(format= date_time_format_make(date_time_type,
-				      res->ptr(), res->length())))
+  if (!(res= var->value->val_str(&str)) ||
+      !(format= date_time_format_make(date_time_type,
+                                      res->ptr(), res->length())))
   {
-    my_error(ER_WRONG_VALUE_FOR_VAR, MYF(0), name, res->c_ptr());
+    my_error(ER_WRONG_VALUE_FOR_VAR, MYF(0), name, res ? res->c_ptr() : "NULL");
     return 1;
   }
   

sql/sql_class.cc

@@ -1923,12 +1923,13 @@ bool select_result::check_simple_select() const
 static String default_line_term("\n",default_charset_info);
 static String default_escaped("\\",default_charset_info);
 static String default_field_term("\t",default_charset_info);
+static String default_enclosed_and_line_start("", default_charset_info);
 
 sql_exchange::sql_exchange(char *name,bool flag)
   :file_name(name), opt_enclosed(0), dumpfile(flag), skip_lines(0)
 {
   field_term= &default_field_term;
-  enclosed=   line_start= &my_empty_string;
+  enclosed=   line_start= &default_enclosed_and_line_start;
   line_term=  &default_line_term;
   escaped=    &default_escaped;
   cs= NULL;