MDEV-23499 Assertion c.same_type(*o) failed

dict_col_t::same_encoding(), dict_col_t::same_format(): Allow an instantaneous change of a column to a compatible encoding, just like ha_innobase::can_convert_string() and similar functions do.

Author: dr-m

mysql-test/suite/innodb/r/instant_alter_bugs.result

@@ -407,4 +407,10 @@ CREATE TABLE t (i INT PRIMARY KEY) ENGINE=InnoDB;
 INSERT INTO t SET i=1;
 ALTER TABLE t ADD e CHAR(255) CHARACTER SET UTF32 FIRST, ALGORITHM=INSTANT;
 DROP TABLE t;
+#
+# MDEV-23499 Assertion c.same_type(*o) failed
+#
+CREATE TABLE t (pk SERIAL, b TEXT CHARACTER SET utf8) ENGINE=InnoDB;
+ALTER TABLE t MODIFY b TEXT CHARACTER SET utf8mb4 FIRST;
+DROP TABLE t;
 SET GLOBAL innodb_purge_rseg_truncate_frequency=@save_frequency;

mysql-test/suite/innodb/t/instant_alter_bugs.test

@@ -429,4 +429,11 @@ INSERT INTO t SET i=1;
 ALTER TABLE t ADD e CHAR(255) CHARACTER SET UTF32 FIRST, ALGORITHM=INSTANT;
 DROP TABLE t;
 
+--echo #
+--echo # MDEV-23499 Assertion c.same_type(*o) failed
+--echo #
+CREATE TABLE t (pk SERIAL, b TEXT CHARACTER SET utf8) ENGINE=InnoDB;
+ALTER TABLE t MODIFY b TEXT CHARACTER SET utf8mb4 FIRST;
+DROP TABLE t;
+
 SET GLOBAL innodb_purge_rseg_truncate_frequency=@save_frequency;

storage/innobase/dict/dict0mem.cc

@@ -120,7 +120,7 @@ bool dict_col_t::same_encoding(uint16_t a, uint16_t b)
 {
  if (const CHARSET_INFO *acs= get_charset(a, MYF(MY_WME)))
  if (const CHARSET_INFO *bcs= get_charset(b, MYF(MY_WME)))
- return Charset(acs).same_encoding(bcs);
+ return Charset(bcs).encoding_allows_reinterpret_as(acs);
  return false;
 }
 

storage/innobase/include/dict0mem.h

@@ -728,7 +728,7 @@ struct dict_col_t{
  bool same_format(const dict_col_t &other) const
  {
  return same_type(other) && len >= other.len &&
- mbminlen == other.mbminlen && mbmaxlen == other.mbmaxlen &&
+ mbminlen == other.mbminlen && mbmaxlen >= other.mbmaxlen &&
  !((prtype ^ other.prtype) & ~(DATA_NOT_NULL | DATA_VERSIONED |
  CHAR_COLL_MASK << 16 |
  DATA_LONG_TRUE_VARCHAR));