Bug#39591: Crash if table comment is longer than 62 characters

It was possible to crash a mysqld build with EXTRA_DEBUG using
CREATE TABLE ... COMMENT with a specially-crafted UTF-8 string.

This CS removes the check that caused it since it no longer
applies in current servers anyway, and adds comments instead
to avoid future confusion. 

mysql-test/r/strict.result:
  Try to crash mysqld with a "suitable" multi-byte (3-byte UTF-8) string
  for a table comment.
mysql-test/t/strict.test:
  Try to crash mysqld with a "suitable" multi-byte (3-byte UTF-8) string
  for a table comment.
sql/unireg.cc:
  Talk at length about limits in .frm form-info, characters vs bytes,
  inlined vs extra-segement TABLE-COMMENTS, and the differences in
  6.0+ vs <6.0 when it comes to UTF-8.
  
  Also, remove a check that no longer applies and that could lead to
  problems in pathological cases.

mysql-test/r/strict.result

@@ -1348,6 +1348,13 @@ t1	CREATE TABLE `t1` (
   `i` int(11) default NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1 COMMENT='123456789*123456789*123456789*123456789*123456789*123456789*'
 drop table t1;
+CREATE TABLE t3 (f1 INT) COMMENT 'כקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחן';
+SHOW CREATE TABLE t3;
+Table	Create Table
+t3	CREATE TABLE `t3` (
+  `f1` int(11) default NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1 COMMENT='כקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחן'
+DROP TABLE t3;
 set sql_mode= 'traditional';
 create table t1(col1 tinyint, col2 tinyint unsigned, 
 col3 smallint, col4 smallint unsigned,

mysql-test/t/strict.test

@@ -1199,6 +1199,15 @@ comment '123456789*123456789*123456789*123456789*123456789*123456789*';
 show create table t1;
 drop table t1;
 
+#
+# Bug #39591: Crash if table comment is longer than 62 characters
+#
+
+#60 chars, 120 (+1) bytes (UTF-8 with 2-byte chars)
+CREATE TABLE t3 (f1 INT) COMMENT 'כקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחןכקבהחן';
+SHOW CREATE TABLE t3;
+DROP TABLE t3;
+
 #
 # Bug #26359: Strings becoming truncated and converted to numbers under STRICT mode
 #

sql/unireg.cc

@@ -143,6 +143,24 @@ bool mysql_create_frm(THD *thd, my_string file_name,
 			     (create_info->min_rows == 1) && (keys == 0));
   int2store(fileinfo+28,key_info_length);
 
+  /*
+    This gives us the byte-position of the character at
+    (character-position, not byte-position) TABLE_COMMENT_MAXLEN.
+    The trick here is that character-positions start at 0, so the last
+    character in a maximum-allowed length string would be at char-pos
+    MAXLEN-1; charpos MAXLEN will be the position of the terminator.
+    Consequently, bytepos(charpos(MAXLEN)) should be equal to
+    comment[length] (which should also be the terminator, or at least
+    the first byte after the payload in the strict sense). If this is
+    not so (bytepos(charpos(MAXLEN)) comes /before/ the end of the
+    string), the string is too long.
+
+    For additional credit, realise that UTF-8 has 1-3 bytes before 6.0,
+    and 1-4 bytes in 6.0 (6.0 also has UTF-32). This means that the
+    inlined COMMENT supposedly does not exceed 60 character plus
+    terminator, vulgo, 181 bytes.
+  */
+
   tmp_len= system_charset_info->cset->charpos(system_charset_info,
                                               create_info->comment.str,
                                               create_info->comment.str +
@@ -165,14 +183,6 @@ bool mysql_create_frm(THD *thd, my_string file_name,
   strmake((char*) forminfo+47, create_info->comment.str ?
           create_info->comment.str : "", create_info->comment.length);
   forminfo[46]=(uchar) create_info->comment.length;
-#ifdef EXTRA_DEBUG
-  /*
-    EXTRA_DEBUG causes strmake() to initialize its buffer behind the
-    payload with a magic value to detect wrong buffer-sizes. We
-    explicitly zero that segment again.
-  */
-  memset((char*) forminfo+47 + forminfo[46], 0, 61 - forminfo[46]);
-#endif
   if (my_pwrite(file,(byte*) fileinfo,64,0L,MYF_RW) ||
       my_pwrite(file,(byte*) keybuff,key_info_length,
 		(ulong) uint2korr(fileinfo+6),MYF_RW))