Bug#19960 Inconsistent results when joining InnoDB tables using partial UTF8 indexes

  Adding a multibyte-aware VARCHAR copying function, to put correct column prefix,
  taking in account number of characters (instead just limiting on number of bytes).
  For example, for a KEY(col(3)) on a UTF8 column when copying the string 'foo bar foo',
  we should put only 3 leftmost characters: 'foo'.
  9 characters were incorrectly put before this fix.

mysql-test/r/ctype_utf8.result

@@ -1462,3 +1462,21 @@ set @a:=null;
 execute my_stmt using @a;
 a	b
 drop table if exists t1;
+CREATE TABLE t1 (
+colA int(11) NOT NULL,
+colB varchar(255) character set utf8 NOT NULL,
+PRIMARY KEY  (colA)
+) ENGINE=InnoDB DEFAULT CHARSET=latin1;
+INSERT INTO t1 (colA, colB) VALUES (1, 'foo'), (2, 'foo bar');
+CREATE TABLE t2 (
+colA int(11) NOT NULL,
+colB varchar(255) character set utf8 NOT NULL,
+KEY bad  (colA,colB(3))
+) ENGINE=InnoDB DEFAULT CHARSET=latin1;
+INSERT INTO t2 (colA, colB) VALUES (1, 'foo'),(2, 'foo bar');
+SELECT * FROM t1 JOIN t2 ON t1.colA=t2.colA AND t1.colB=t2.colB
+WHERE t1.colA < 3;
+colA	colB	colA	colB
+1	foo	1	foo
+2	foo bar	2	foo bar
+DROP TABLE t1, t2;

mysql-test/t/ctype_utf8.test

@@ -1164,3 +1164,23 @@ execute my_stmt using @a;
 set @a:=null;
 execute my_stmt using @a;
 drop table if exists t1;
+
+#
+# Bug#19960: Inconsistent results when joining
+# InnoDB tables using partial UTF8 indexes
+#
+CREATE TABLE t1 (
+  colA int(11) NOT NULL,
+  colB varchar(255) character set utf8 NOT NULL,
+   PRIMARY KEY  (colA)
+) ENGINE=InnoDB DEFAULT CHARSET=latin1;
+INSERT INTO t1 (colA, colB) VALUES (1, 'foo'), (2, 'foo bar');
+CREATE TABLE t2 (
+  colA int(11) NOT NULL,
+  colB varchar(255) character set utf8 NOT NULL,
+   KEY bad  (colA,colB(3))
+) ENGINE=InnoDB DEFAULT CHARSET=latin1;
+INSERT INTO t2 (colA, colB) VALUES (1, 'foo'),(2, 'foo bar');
+SELECT * FROM t1 JOIN t2 ON t1.colA=t2.colA AND t1.colB=t2.colB
+WHERE t1.colA < 3;
+DROP TABLE t1, t2;

sql/field_conv.cc

@@ -428,6 +428,21 @@ static void do_varstring2(Copy_field *copy)
          length);
 }
 
+
+static void do_varstring2_mb(Copy_field *copy)
+{
+  int well_formed_error;
+  CHARSET_INFO *cs= copy->from_field->charset();
+  uint char_length= (copy->to_length - HA_KEY_BLOB_LENGTH) / cs->mbmaxlen;
+  uint from_length= uint2korr(copy->from_ptr);
+  const char *from_beg= copy->from_ptr + HA_KEY_BLOB_LENGTH;
+  uint length= cs->cset->well_formed_len(cs, from_beg, from_beg + from_length,
+                                         char_length, &well_formed_error);
+  int2store(copy->to_ptr, length);
+  memcpy(copy->to_ptr+HA_KEY_BLOB_LENGTH, from_beg, length);
+}
+ 
+
 /***************************************************************************
 ** The different functions that fills in a Copy_field class
 ***************************************************************************/
@@ -587,7 +602,8 @@ void (*Copy_field::get_copy_func(Field *to,Field *from))(Copy_field*)
           return do_field_string;
         if (to_length != from_length)
           return (((Field_varstring*) to)->length_bytes == 1 ?
-                  do_varstring1 : do_varstring2);
+                  do_varstring1 : (from->charset()->mbmaxlen == 1 ?
+                                   do_varstring2 : do_varstring2_mb));
       }
       else if (to_length < from_length)
 	return (from->charset()->mbmaxlen == 1 ?