Bug#45168: assertion with convert() and empty set value

The assertion in String::copy was added in order to avoid
valgrind errors when the destination was the same as the source.
Eased restriction to allow for the case when str == NULL.

mysql-test/r/func_set.result

@@ -146,3 +146,16 @@ NULL
 0
 0
 drop table t1;
+CREATE TABLE t1( a SET('a', 'b', 'c') );
+CREATE TABLE t2( a SET('a', 'b', 'c') );
+INSERT INTO t1 VALUES ('d');
+Warnings:
+Warning	1265	Data truncated for column 'a' at row 1
+INSERT INTO t2 VALUES ('');
+SELECT CONVERT( a USING latin1 ) FROM t1;
+CONVERT( a USING latin1 )
+
+SELECT CONVERT( a USING latin1 ) FROM t2;
+CONVERT( a USING latin1 )
+
+DROP TABLE t1, t2;

mysql-test/t/func_set.test

@@ -84,3 +84,16 @@ engine=myisam default charset=latin1;
 insert into t1 values (''),(null),(null),(''),(''),('');
 select find_in_set(f1,f1) as a from t1,(select find_in_set(f1,f1) as b from t1) a;
 drop table t1;
+#
+# Bug#45168: assertion with convert() and empty set value
+#
+CREATE TABLE t1( a SET('a', 'b', 'c') );
+CREATE TABLE t2( a SET('a', 'b', 'c') );
+
+INSERT INTO t1 VALUES ('d');
+INSERT INTO t2 VALUES ('');
+
+SELECT CONVERT( a USING latin1 ) FROM t1;
+SELECT CONVERT( a USING latin1 ) FROM t2;
+
+DROP TABLE t1, t2;

sql/item_strfunc.cc

@@ -2710,13 +2710,12 @@ String *Item_func_conv_charset::val_str(String *str)
     return null_value ? 0 : &str_value;
   /* 
     Here we don't pass 'str' as a parameter to args[0]->val_str()
-    as 'str' may points to 'str_value' (e.g. see Item::save_in_field()),
+    as 'str' may point to 'str_value' (e.g. see Item::save_in_field()),
     which we use below to convert string. 
     Use argument's 'str_value' instead.
   */
-  String *arg= args[0]->val_str(&args[0]->str_value);;
+  String *arg= args[0]->val_str(&args[0]->str_value);
   uint dummy_errors;
-  arg= args[0]->val_str(&args[0]->str_value);  
   if (!arg)
   {
     null_value=1;

sql/sql_string.cc

@@ -322,14 +322,23 @@ bool String::set_or_copy_aligned(const char *str,uint32 arg_length,
   return copy_aligned(str, arg_length, offset, cs);
 }
 
-	/* Copy with charset conversion */
+
+/**
+   Copies the character data into this String, with optional character set
+   conversion.
+
+   @return
+   FALSE ok
+   TRUE  Could not allocate result buffer
+
+*/
 
 bool String::copy(const char *str, uint32 arg_length,
 		  CHARSET_INFO *from_cs, CHARSET_INFO *to_cs, uint *errors)
 {
   uint32 offset;
-  
-  DBUG_ASSERT(str != Ptr);
+
+  DBUG_ASSERT(!str || str != Ptr);
   
   if (!needs_conversion(arg_length, from_cs, to_cs, &offset))
   {