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

Since some time my Gnus regularly errors when trying to upload a sent
message with attachments to my sent mail folder on my local Dovecot IMAP
server.  First I've thought it was dovecot's fault, but now I was able
to reproduce the error with stock emacs.

Here's the recipe:

1. emacs -Q
2. goto *scratch* and insert

--8<---------------cut here---------------start------------->8---
(progn
  (setq proctest
	(start-process "cat" (get-buffer-create "cat") "/bin/cat"))
  (with-temp-buffer
    (dotimes (dummy 999999)
      (insert
       (concat (number-to-string (random))
	       (number-to-string (random))
	       (number-to-string (random))
	       (number-to-string (random))
	       "\n")))
    (process-send-region proctest (point-min) (point-max)))
  (delete-process proctest))
--8<---------------cut here---------------end--------------->8---

3. eval it.

==> Eventually, after some seconds, I get this error:

--8<---------------cut here---------------start------------->8---
Debugger entered--Lisp error: (file-error "writing to process" "bad address" #<process cat>)
  process-send-region(#<process cat> 1 75145901)
  (progn (dotimes (dummy 999999) (insert ...)) (process-send-region proctest (point-min) (point-max)))
  (unwind-protect (progn (dotimes ... ...) (process-send-region proctest ... ...)) (and (buffer-name temp-buffer) (kill-buffer temp-buffer)))
  (save-current-buffer (set-buffer temp-buffer) (unwind-protect (progn ... ...) (and ... ...)))
  (with-current-buffer temp-buffer (unwind-protect (progn ... ...) (and ... ...)))
  (let ((temp-buffer ...)) (with-current-buffer temp-buffer (unwind-protect ... ...)))
  (with-temp-buffer (dotimes (dummy 999999) (insert ...)) (process-send-region proctest (point-min) (point-max)))
  (progn (setq proctest (start-process "cat" ... "/bin/cat")) (with-temp-buffer (dotimes ... ...) (process-send-region proctest ... ...)) (delete-process proctest))
  eval((progn (setq proctest (start-process "cat" ... "/bin/cat")) (with-temp-buffer (dotimes ... ...) (process-send-region proctest ... ...)) (delete-process proctest)))
  eval-last-sexp-1(nil)
  eval-last-sexp(nil)
  call-interactively(eval-last-sexp nil nil)
--8<---------------cut here---------------end--------------->8---

I can reproduce this bug reliable using a very recent CVS checkout

  GNU Emacs 23.0.90.1 (x86_64-pc-linux-gnu, GTK+ Version 2.14.7)
  of 2009-02-10 on thinkpad

and also with an older emacs 23 snapshot:

  GNU Emacs 23.0.60.1 (i486-pc-linux-gnu) of 2008-03-19 on vernadsky,
  modified by Debian

David (in the Cc) was also able to reproduce the bug with the recipe
above, although for him the error doesn't occur each time he evals the
code.  He uses emacs 23 on MacOS X.

I tried Emacs 22.3, but there the snippet from above works fine.

Maybe bug #471 is related.

  http://emacsbugs.donarmstrong.com/cgi-bin/bugreport.cgi?bug=471

The full backtrace using gdb is attached.