Bug #55424: convert_tz crashes when fed invalid data

The CONVERT_TZ function crashes the server when the
timezone argument is an empty SET field value.

1) The CONVERT_TZ may find a timezone string in the
   tz_names hash.
2) A string representation of the empty SET is a
   String of zero length with the NULL pointer.
3) If the key argument length is zero, hash functions
   do comparison using the length of the record being
   compared against.

I.e. a zero-length String buffer is an invalid
argument for hash search functions, and if String
points to NULL buffer, hashcmp() fails with SEGV
accessing that memory.

The my_tz_find function has been modified to
treat empty Strings as invalid timezone values
to skip unnecessary hash search.

mysql-test/r/timezone2.result

@@ -296,4 +296,16 @@ CONVERT_TZ(NOW(), 'UTC', 'Europe/Moscow') IS NULL
 UPDATE t1 SET t = CONVERT_TZ(t, 'UTC', 'Europe/Moscow');
 UNLOCK TABLES;
 DROP TABLE t1;
+#
+# Bug #55424: convert_tz crashes when fed invalid data
+#
+CREATE TABLE t1 (a SET('x') NOT NULL);
+INSERT INTO t1 VALUES ('');
+SELECT CONVERT_TZ(1, a, 1) FROM t1;
+CONVERT_TZ(1, a, 1)
+NULL
+SELECT CONVERT_TZ(1, 1, a) FROM t1;
+CONVERT_TZ(1, 1, a)
+NULL
+DROP TABLE t1;
 End of 5.1 tests

mysql-test/t/timezone2.test

@@ -273,5 +273,14 @@ UNLOCK TABLES;
 
 DROP TABLE t1;
 
+--echo #
+--echo # Bug #55424: convert_tz crashes when fed invalid data
+--echo #
+
+CREATE TABLE t1 (a SET('x') NOT NULL);
+INSERT INTO t1 VALUES ('');
+SELECT CONVERT_TZ(1, a, 1) FROM t1;
+SELECT CONVERT_TZ(1, 1, a) FROM t1;
+DROP TABLE t1;
 
 --echo End of 5.1 tests

sql/sql_string.h

@@ -97,7 +97,7 @@ public:
   inline uint32 alloced_length() const { return Alloced_length;}
   inline char& operator [] (uint32 i) const { return Ptr[i]; }
   inline void length(uint32 len) { str_length=len ; }
-  inline bool is_empty() { return (str_length == 0); }
+  inline bool is_empty() const { return (str_length == 0); }
   inline void mark_as_const() { Alloced_length= 0;}
   inline const char *ptr() const { return Ptr; }
   inline char *c_ptr()

sql/tztime.cc

@@ -2259,7 +2259,7 @@ my_tz_find(THD *thd, const String *name)
   DBUG_PRINT("enter", ("time zone name='%s'",
                        name ? ((String *)name)->c_ptr_safe() : "NULL"));
 
-  if (!name)
+  if (!name || name->is_empty())
     DBUG_RETURN(0);
 
   VOID(pthread_mutex_lock(&tz_LOCK));