Hi.

I am observing that processes running over TRAMP overwrite all the
buffer contents after (point). This is irrelevant in the usual case
where the process output is going to the end of the buffer (there's
nothing after the point in that case). This is also something that works
fine with local (i.e. non-TRAMP) processes.

To be clear, an example emacs invocation appears below. This populates
the *scratch* buffer with

a b c
1 2 3

Then moves the point to the begging of the "1 2 3" line, and runs a
TRAMP process to insert the text "inserted". I would expect the
*scratch* buffer to end up with

a b c
inserted
1 2 3

If the process runs without TRAMP, this is indeed what happens. With
TRAMP, however, I get

a b c
inserted

Note that with TRAMP the "1 2 3" line got deleted. The example invocation:


 $ emacs -Q \
   --eval \
 '(defun test-filter(process output)
         (with-current-buffer (process-buffer process) (insert output)))' \
   --eval \
 '(defun test-sentinel(process event)
         )' \
   --eval \
 '(progn
    (insert "a b c\n1 2 3\n")
    (forward-line -1)
    (cd "/sudo::/tmp")
    (let ((process (start-file-process "echo" (get-buffer "*scratch*") "echo" "inserted")))
      (set-process-sentinel process `test-sentinel)
      (set-process-filter   process `test-filter)))'


This creates a process filter that simply inserts the process output,
and a sentinel that does nothing. The (cd "/sudo::/tmp") form is there
to force the process to run with TRAMP. Every TRAMP-based path I've
tried tickles this bug for me. Removing that form makes the process run
without TRAMP, and I do not see the buggy behavior then.

I'm observing this issue with the latest emacs24 release and with the
latest emacs built from the sources as of 2013/09/30. Emacs 23.4.1
appears to NOT have this bug. This is on a machine running Debian.

I'm attaching a patch that fixes this issue for me. The cause appears to
be a bit of code that deletes the system prompt from the TRAMP output.
There's a bug in that code that deletes more than just the prompt in the
case described above.