MDEV-6752 Trailing incomplete characters are not replaced to question marks on conversion

mysql-test/r/ctype_latin1.result

@@ -7660,5 +7660,60 @@ DROP FUNCTION iswellformed;
 DROP TABLE allbytes;
 # End of ctype_backslash.inc
 #
+# MDEV-6752 Trailing incomplete characters are not replaced to question marks on conversion
+#
+SET NAMES utf8, character_set_connection=latin1;
+SELECT '<27>';
+?
+?
+SELECT HEX('<27>');
+HEX('<27>')
+3F
+SELECT HEX(CAST('<27>' AS CHAR CHARACTER SET utf8));
+HEX(CAST('<27>' AS CHAR CHARACTER SET utf8))
+3F
+SELECT HEX(CAST('<27>' AS CHAR CHARACTER SET latin1));
+HEX(CAST('<27>' AS CHAR CHARACTER SET latin1))
+3F
+SELECT HEX(CONVERT('<27>' USING utf8));
+HEX(CONVERT('<27>' USING utf8))
+3F
+SELECT HEX(CONVERT('<27>' USING latin1));
+HEX(CONVERT('<27>' USING latin1))
+3F
+SELECT '<27>x';
+?x
+?x
+SELECT HEX('<27>x');
+HEX('<27>x')
+3F78
+SELECT HEX(CAST('<27>x' AS CHAR CHARACTER SET utf8));
+HEX(CAST('<27>x' AS CHAR CHARACTER SET utf8))
+3F78
+SELECT HEX(CAST('<27>x' AS CHAR CHARACTER SET latin1));
+HEX(CAST('<27>x' AS CHAR CHARACTER SET latin1))
+3F78
+SELECT HEX(CONVERT('<27>x' USING utf8));
+HEX(CONVERT('<27>x' USING utf8))
+3F78
+SELECT HEX(CONVERT('<27>x' USING latin1));
+HEX(CONVERT('<27>x' USING latin1))
+3F78
+SET NAMES utf8;
+CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET latin1);
+INSERT INTO t1 VALUES ('<27>'),('<27>#');
+Warnings:
+Warning	1366	Incorrect string value: '\xC2' for column 'a' at row 1
+Warning	1366	Incorrect string value: '\xC2#' for column 'a' at row 2
+SHOW WARNINGS;
+Level	Code	Message
+Warning	1366	Incorrect string value: '\xC2' for column 'a' at row 1
+Warning	1366	Incorrect string value: '\xC2#' for column 'a' at row 2
+SELECT HEX(a),a FROM t1;
+HEX(a)	a
+3F	?
+3F23	?#
+DROP TABLE t1;
+#
 # End of 10.0 tests
 #

mysql-test/suite/sys_vars/r/character_set_client_func.result

@@ -30,7 +30,7 @@ SET @@session.character_set_client = utf8;
 INSERT INTO t1 values('<03>');
 SELECT hex(a),CHAR_LENGTH(a) FROM t1;
 hex(a)	CHAR_LENGTH(a)
-03	1
+033F	2
 DELETE FROM t1;
 DROP TABLE IF EXISTS t1;
 SET @@global.character_set_client = @global_character_set_client;

mysql-test/t/ctype_latin1.test

@@ -210,6 +210,29 @@ set names latin1;
 let $ctype_unescape_combinations=selected;
 --source include/ctype_unescape.inc
 
+--echo #
+--echo # MDEV-6752 Trailing incomplete characters are not replaced to question marks on conversion
+--echo #
+SET NAMES utf8, character_set_connection=latin1;
+SELECT '<27>';
+SELECT HEX('<27>');
+SELECT HEX(CAST('<27>' AS CHAR CHARACTER SET utf8));
+SELECT HEX(CAST('<27>' AS CHAR CHARACTER SET latin1));
+SELECT HEX(CONVERT('<27>' USING utf8));
+SELECT HEX(CONVERT('<27>' USING latin1));
+SELECT '<27>x';
+SELECT HEX('<27>x');
+SELECT HEX(CAST('<27>x' AS CHAR CHARACTER SET utf8));
+SELECT HEX(CAST('<27>x' AS CHAR CHARACTER SET latin1));
+SELECT HEX(CONVERT('<27>x' USING utf8));
+SELECT HEX(CONVERT('<27>x' USING latin1));
+SET NAMES utf8;
+CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET latin1);
+INSERT INTO t1 VALUES ('<27>'),('<27>#');
+SHOW WARNINGS;
+SELECT HEX(a),a FROM t1;
+DROP TABLE t1;
+
 --echo #
 --echo # End of 10.0 tests
 --echo #

sql/sql_string.cc

@@ -1022,8 +1022,15 @@ well_formed_copy_nchars(CHARSET_INFO *to_cs,
         wc= '?';
       }
       else
-        break;  // Not enough characters
+      {
-
+        if ((uchar *) from >= from_end)
+          break; // End of line
+        // Incomplete byte sequence
+        if (!*well_formed_error_pos)
+          *well_formed_error_pos= from;
+        from++;
+        wc= '?';
+      }
 outp:
       if ((cnvres= (*wc_mb)(to_cs, wc, (uchar*) to, to_end)) > 0)
         to+= cnvres;

strings/ctype.c

@@ -1066,7 +1066,14 @@ my_convert_internal(char *to, uint32 to_length,
       wc= '?';
     }
     else
-      break;  // Not enough characters
+    {
+      if ((uchar *) from >= from_end)
+        break;  /* End of line */
+      /* Incomplete byte sequence */
+      error_count++;
+      from++;
+      wc= '?';
+    }
 
 outp:
     if ((cnvres= (*wc_mb)(to_cs, wc, (uchar*) to, to_end)) > 0)