Process output truncation when using UTF-8

[Milan Zamazal]

In GNU Emacs 21.3.2 (i386-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2003-04-24 on raven, modified by Debian
configured using `configure  i386-linux --prefix=/usr --sharedstatedir=/var/lib 
--libexecdir=/usr/lib --localstatedir=/var/lib --infodir=/usr/share/info 
--mandir=/usr/share/man --with-pop=yes --with-x=yes --with-x-toolkit=athena 
--without-gif'
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: cs_CZ.ISO8859-2
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: C
  locale-coding-system: iso-8859-2
  default-enable-multibyte-characters: t

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

When I start a process, set its input and output encoding to utf-8 and
send a long string to it through process-send-string, usually some final
part of the sent string is missing on the recipient's side.

You can try to reproduce the problem by evaluating the following Elisp
file:

utf-8-bug.el
Description: application/emacs-lisp

It should send Czech Emacs tutorial to a process, that stores it into
the /tmp/foo file.  On my system, some end part of the tutorial is
missing in /tmp/foo -- about the last 900 characters are missing.  It
doesn't happen when the process encoding is set to iso-8859-2 instead.

I can reproduce the problem on my Debian GNU/Linux i386 system with both
Emacs 21.3 as packaged by Debian and current Emacs CVS snapshot.