MDEV-29019 Assertion `(length % 4) == 0' failed in my_lengthsp_utf32 on SELECT

Problem:

Item_func_conv::val_str() copied the ASCII string with the numeric base
conversion result directly to the function result string. In case of a
tricky character set (e.g. utf32) it produced an illformed string.

Fix:

Copy the base conversion result to the function result as is only if
the function character set is ASCII compatible, go through a
character set conversion otherwise.

mysql-test/main/ctype_utf32.result

@@ -2940,3 +2940,23 @@ DROP TABLE t1;
 #
 # End of 10.2 tests
 #
+#
+# Start of 10.4 tests
+#
+#
+# MDEV-29019 Assertion `(length % 4) == 0' failed in my_lengthsp_utf32 on SELECT
+#
+CREATE TABLE t (a INT);
+SET collation_connection=utf32_unicode_ci;
+INSERT INTO t VALUES (0);
+SELECT * FROM t ORDER BY (OCT(a));
+a
+0
+SELECT HEX(OCT(a)) FROM t;
+HEX(OCT(a))
+00000030
+DROP TABLE t;
+SET NAMES utf8;
+#
+# End of 10.4 tests
+#

mysql-test/main/ctype_utf32.test

@@ -1099,7 +1099,30 @@ CREATE TABLE t1 (
 SHOW CREATE TABLE t1;
 DROP TABLE t1;
 
---enable_service_connection
 --echo #
 --echo # End of 10.2 tests
 --echo #
+
+
+--echo #
+--echo # Start of 10.4 tests
+--echo #
+
+--echo #
+--echo # MDEV-29019 Assertion `(length % 4) == 0' failed in my_lengthsp_utf32 on SELECT
+--echo #
+
+CREATE TABLE t (a INT);
+SET collation_connection=utf32_unicode_ci;
+INSERT INTO t VALUES (0);
+SELECT * FROM t ORDER BY (OCT(a));
+SELECT HEX(OCT(a)) FROM t;
+DROP TABLE t;
+SET NAMES utf8;
+
+
+--echo #
+--echo # End of 10.4 tests
+--echo #
+
+--enable_service_connection

sql/item_strfunc.cc

@@ -3533,8 +3533,12 @@ String *Item_func_conv::val_str(String *str)
                                    from_base, &endptr, &err);
   }
 
+  uint dummy_errors;
   if (!(ptr= longlong2str(dec, ans, to_base)) ||
-      str->copy(ans, (uint32) (ptr - ans), default_charset()))
+      (collation.collation->state & MY_CS_NONASCII) ?
+       str->copy(ans, (uint32)  (ptr - ans), &my_charset_latin1,
+                 collation.collation, &dummy_errors) :
+       str->copy(ans, (uint32) (ptr - ans), collation.collation))
   {
     null_value= 1;
     return NULL;