Multibyte char encoding atttypmod weirdness

Version: PostgreSQL 7.2.1 (7.3 not tested)

Summary:

When locale is set to multibyte char encoding languages,
such as ja_JP.eucjp, and char encoding set to EUC_JP, for the char(20)
columns (attributes), the libpq ((PGresult *)res)->attDescs[0].atttypmod
returned by PQfmod(res, 0) is not correct. It's neither 20, nor 20+4 as
reported in the hackers' mail list [1], but something varying (which I
failed
to figure out). In my specific case, it's 25.

Is it a bug, or a feature that needs special care which is not
documented
in the postgresql documents? Is this extra byte overhead reflected by
VARHDRSZ? But a simple fgrep -r VARHDRSZ in the header files showed:

internal/c.h:#define VARHDRSZ ((int32) sizeof(int32))
internal/c.h: * always VARSIZE(ptr) - VARHDRSZ.
server/access/tuptoaster.h: VARHDRSZ))
server/utils/varbit.h:/* Header overhead *in addition to* VARHDRSZ */
server/utils/varbit.h:#define VARBITBYTES(PTR) (VARSIZE(PTR) -
VARHDRSZ - VARBITHDRSZ)
server/utils/varbit.h:
VARHDRSZ + VARBITHDRSZ)
server/c.h:#define VARHDRSZ ((int32) sizeof(int32))
server/c.h: * always VARSIZE(ptr) - VARHDRSZ.

which means VARHDRSZ should be sizeof(int32), which is always a constant
4
bytes. Is the VARBITHDRSZ relevant to this problem? But VARBITHDRSZ is
not
defined in any header files "make install-all-headers" installed.

BTW, if it's not a bug, this kind of implementation inconsistent with
common
sense is ugly and a potential of buggy code.

[1] http://archives.postgresql.org/pgsql-hackers/1998-03/msg00430.php