MDEV-25900 Assertion `octets < 1024' failed in...

MDEV-25900 Assertion `octets < 1024' failed in Binlog_type_info_fixed_string::Binlog_type_info_fixed_string OR Assertion `field_length < 1024' failed in Field_string::save_field_metadata

A CHAR column cannot be longer than 1024, because
Binlog_type_info_fixed_string::Binlog_type_info_fixed_string
replies on this fact - it cannot store binlog metadata for longer columns.

In case of the filename character set mbmaxlen is equal to 5,
so only 1024/5=204 characters can fit into the 1024 limit.
- In strict mode:
  Disallowing creation of a CHAR column with octet length grater than 1024.
- In non-strict mode:
  Automatically convert CHAR with octet length>1024 into VARCHAR.

mysql-test/main/ctype_filename.result

@@ -132,4 +132,31 @@ a	c
 @002d1	@002d1
 DROP TABLE t1;
 SET NAMES utf8;
+#
+# MDEV-25900 Assertion `octets < 1024' failed in Binlog_type_info_fixed_string::Binlog_type_info_fixed_string OR Assertion `field_length < 1024' failed in Field_string::save_field_metadata
+#
+CREATE TABLE t1 (a CHAR(204)) CHARACTER SET filename;
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` char(204) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=filename COLLATE=filename
+DROP TABLE t1;
+CREATE TABLE t1 (a CHAR(205)) CHARACTER SET filename;
+ERROR 42000: Column length too big for column 'a' (max = 204); use BLOB or TEXT instead
+SET sql_mode='';
+CREATE TABLE t1 (a CHAR(205)) CHARACTER SET filename;
+Warnings:
+Note	1246	Converting column 'a' from CHAR to VARCHAR
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` varchar(205) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=filename COLLATE=filename
+DROP TABLE t1;
+SET sql_mode=DEFAULT;
+CREATE TABLE t1 (a CHAR(205) CHARACTER SET latin1);
+ALTER TABLE t1 CONVERT TO CHARACTER SET filename;
+ERROR 42000: Column length too big for column 'a' (max = 204); use BLOB or TEXT instead
+DROP TABLE t1;
 # End of 10.5 tests

mysql-test/main/ctype_filename.test

@@ -141,4 +141,26 @@ SET NAMES utf8;
 
 --enable_ps_protocol
 
+--echo #
+--echo # MDEV-25900 Assertion `octets < 1024' failed in Binlog_type_info_fixed_string::Binlog_type_info_fixed_string OR Assertion `field_length < 1024' failed in Field_string::save_field_metadata
+--echo #
+
+CREATE TABLE t1 (a CHAR(204)) CHARACTER SET filename;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;
+
+--error ER_TOO_BIG_FIELDLENGTH
+CREATE TABLE t1 (a CHAR(205)) CHARACTER SET filename;
+
+SET sql_mode='';
+CREATE TABLE t1 (a CHAR(205)) CHARACTER SET filename;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;
+SET sql_mode=DEFAULT;
+
+CREATE TABLE t1 (a CHAR(205) CHARACTER SET latin1);
+--error ER_TOO_BIG_FIELDLENGTH
+ALTER TABLE t1 CONVERT TO CHARACTER SET filename;
+DROP TABLE t1;
+
 --echo # End of 10.5 tests

sql/sql_table.cc

@@ -4734,6 +4734,33 @@ bool Column_definition::prepare_blob_field(THD *thd)
 {
   DBUG_ENTER("Column_definition::prepare_blob_field");
 
+  if (real_field_type() == FIELD_TYPE_STRING && length > 1024)
+  {
+    DBUG_ASSERT(charset->mbmaxlen > 4);
+    /*
+      Convert long CHAR columns to VARCHAR.
+      CHAR has an octet length limit of 1024 bytes.
+      The code in Binlog_type_info_fixed_string::Binlog_type_info_fixed_string
+      relies on this limit. If octet length of a CHAR column is greater
+      than 1024, then it cannot write its metadata to binlog properly.
+      In case of the filename character set with mbmaxlen=5,
+      the maximum possible character length is 1024/5=204 characters.
+      Upgrade to VARCHAR if octet length is greater than 1024.
+    */
+    char warn_buff[MYSQL_ERRMSG_SIZE];
+    if (thd->is_strict_mode())
+    {
+      my_error(ER_TOO_BIG_FIELDLENGTH, MYF(0), field_name.str,
+               static_cast<ulong>(1024 / charset->mbmaxlen));
+      DBUG_RETURN(1);
+    }
+    set_handler(&type_handler_varchar);
+    my_snprintf(warn_buff, sizeof(warn_buff), ER_THD(thd, ER_AUTO_CONVERT),
+                field_name.str, "CHAR", "VARCHAR");
+    push_warning(thd, Sql_condition::WARN_LEVEL_NOTE, ER_AUTO_CONVERT,
+                 warn_buff);
+  }
+
   if (length > MAX_FIELD_VARCHARLENGTH && !(flags & BLOB_FLAG))
   {
     /* Convert long VARCHAR columns to TEXT or BLOB */