MDEV-16729 VARCHAR COMPRESSED is created with a wrong length for multi-byte character sets

Field_varstring::sql_type() did not calculate character length correctly.
Using char_length() instead of the bad code.

mysql-test/main/column_compression.result

@@ -1472,3 +1472,17 @@ SELECT LEFT(a, 10), LENGTH(a) FROM t1;
 LEFT(a, 10)	LENGTH(a)
 aaaaaaaaaa	255
 DROP TABLE t1;
+#
+# MDEV-16729 VARCHAR COMPRESSED is created with a wrong length for multi-byte character sets
+#
+CREATE OR REPLACE TABLE t1 (a VARCHAR(1000) CHARACTER SET utf8 COMPRESSED);
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` varchar(1000) /*!100301 COMPRESSED*/ CHARACTER SET utf8 DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+SELECT COLUMN_TYPE FROM INFORMATION_SCHEMA.COLUMNS
+WHERE TABLE_SCHEMA='test' AND TABLE_NAME='t1';
+COLUMN_TYPE
+varchar(1000) /*!100301 COMPRESSED*/
+DROP TABLE t1;

mysql-test/main/column_compression.test

@@ -170,3 +170,14 @@ CREATE TABLE t1(a TINYTEXT COMPRESSED);
 INSERT INTO t1 VALUES(REPEAT('a', 255));
 SELECT LEFT(a, 10), LENGTH(a) FROM t1;
 DROP TABLE t1;
+
+
+--echo #
+--echo # MDEV-16729 VARCHAR COMPRESSED is created with a wrong length for multi-byte character sets
+--echo #
+
+CREATE OR REPLACE TABLE t1 (a VARCHAR(1000) CHARACTER SET utf8 COMPRESSED);
+SHOW CREATE TABLE t1;
+SELECT COLUMN_TYPE FROM INFORMATION_SCHEMA.COLUMNS
+WHERE TABLE_SCHEMA='test' AND TABLE_NAME='t1';
+DROP TABLE t1;

sql/field.cc

@@ -7715,10 +7715,9 @@ void Field_varstring::sql_type(String &res) const
   size_t length;
 
   length= cs->cset->snprintf(cs,(char*) res.ptr(),
-                             res.alloced_length(), "%s(%d)",
+                             res.alloced_length(), "%s(%u)",
                               (has_charset() ? "varchar" : "varbinary"),
-                             (int) field_length / charset()->mbmaxlen -
-                             MY_TEST(compression_method()));
+                              (uint) char_length());
   res.length(length);
   if ((thd->variables.sql_mode & (MODE_MYSQL323 | MODE_MYSQL40)) &&
       has_charset() && (charset()->state & MY_CS_BINSORT))