MDEV-21831: Assertion `length == pack_length()' failed in...

MDEV-21831: Assertion `length == pack_length()' failed in Field_inet6::sort_string upon INSERT into RocksDB table

For INET6 columns the values are stored as BINARY columns and returned to the client in TEXT format.
For rocksdb the indexes store mem-comparable images for columns, so use the pack_length() to store
the mem-comparable form for INET6 columns. This would also remain consistent with CHAR columns.

storage/rocksdb/mysql-test/rocksdb/r/rocksdb.result

@@ -2633,4 +2633,11 @@ DELETE t2, t1 FROM t2 LEFT JOIN t1 ON t2.id2 = t1.id2 AND t2.id1 = t1.id1 WHERE
 SET rocksdb_bulk_load_size= @save_rocksdb_bulk_load_size;
 SET rocksdb_commit_in_the_middle=0;
 DROP TABLE t1, t2;
+#
+# MDEV-21831: Assertion `length == pack_length()' failed in Field_inet6::sort_string upon
+# INSERT into RocksDB table
+#
+CREATE TABLE t1 (a INET6 NOT NULL, KEY (a)) ENGINE=RocksDB;
+INSERT INTO t1 VALUES ('41::1'),('61::1');
+DROP TABLE t1;
 SET GLOBAL ROCKSDB_PAUSE_BACKGROUND_WORK = @ORIG_PAUSE_BACKGROUND_WORK;

storage/rocksdb/mysql-test/rocksdb/t/rocksdb.test

@@ -1962,5 +1962,13 @@ SET rocksdb_bulk_load_size= @save_rocksdb_bulk_load_size;
 SET rocksdb_commit_in_the_middle=0;
 DROP TABLE t1, t2;
 
+--echo #
+--echo # MDEV-21831: Assertion `length == pack_length()' failed in Field_inet6::sort_string upon
+--echo # INSERT into RocksDB table
+--echo #
+
+CREATE TABLE t1 (a INET6 NOT NULL, KEY (a)) ENGINE=RocksDB;
+INSERT INTO t1 VALUES ('41::1'),('61::1');
+DROP TABLE t1;
 
 SET GLOBAL ROCKSDB_PAUSE_BACKGROUND_WORK = @ORIG_PAUSE_BACKGROUND_WORK;

storage/rocksdb/rdb_datadic.cc

@@ -3317,7 +3317,9 @@ bool Rdb_field_packing::setup(const Rdb_key_def *const key_descr,
       field->field_length = field->char_length() * cs->mbmaxlen.
     */
     const CHARSET_INFO *cs = field->charset();
-    m_max_image_len = cs->strnxfrmlen(field->field_length);
+    m_max_image_len = cs->strnxfrmlen(type == MYSQL_TYPE_STRING ?
+                                      field->pack_length() :
+                                      field->field_length);
   }
   const bool is_varchar = (type == MYSQL_TYPE_VARCHAR);
   const CHARSET_INFO *cs = field->charset();