How to make Emacs lock-buffer fail when it cannot lock a file?

Question

I want to some Emacs lisp to manipulate same file from different Emacs processes. So I wrote the following script to check how lock-buffer works. However, it stops when trying to lock the file by the second Emacs process (find-and-lock-file $es2 /tmp/dummy). I need to go to another terminal and send emacsclient --socket-name server-2 --eval '(kill-emacs)' to stop the Emacs process. Emacs prompts what to do for the file if I open an UI by emacsclient -t --socket-name server-2, but I want to make it all done in the background and avoid using Emacs prompt to continue the process. How can I do this? Is it possible to make Emacs raise some error when it fails to lock a file?

EDIT: @event_jr proposed an answer using file-locked-p. I think it works most of the time. However, I think other Emacs process can lock the file between the execution of file-locked-p and lock-buffer. ~~So, I will keep this question open.~~ Solved. Thanks, @event_jr!

#!/bin/bash
es1="server-1"
es2="server-2"

start-server () {
    emacs -q --daemon --eval "(progn (setq server-name \"$1\") (server-start) (require 'cl))"
}

emacs-eval () {
    echo "@$1 >>> $2"
    emacsclient --socket-name "$1" --eval "$2"
}

kill-emacs () {
    emacs-eval "$1" '(kill-emacs)'
}

find-and-lock-file () {
    emacs-eval "$1" "(progn (find-file \"$2\") (set-buffer-modified-p t) (lock-buffer))"
}

start-server $es1
start-server $es2

find-and-lock-file $es1 /tmp/dummy
find-and-lock-file $es2 /tmp/dummy

kill-emacs $es1
kill-emacs $es2

event_jr · Accepted Answer · 2012-03-11T13:41:50.310

4

There doesn't seem to be a way to make emacsclient --eval return error code. But You can make it print what you need to know:

#!/usr/bin/env bash

es1="server-1"
es2="server-2"

emacs=/Applications/Emacs.app/Contents/MacOS/Emacs
[ -e $emacs ] || emacs=emacs

start-server () {
  read -r -d '' script <<EOF
(progn
  (setq server-name "$1")
  (server-start)
  (require 'cl)

  (defun my-set-buffer-modified-p (flag)
    (flet ((ask-user-about-lock
              (&rest args)
;;              (signal 'file-locked args)
              (apply 'error "%s was locked by %s" args)))
      (set-buffer-modified-p flag))))
EOF

  $emacs -q --daemon --eval "$script"
}

emacs-eval () {
    echo "@$1 >>> $2"
  emacsclient --socket-name "$1" --eval "$2"
}

kill-emacs () {
  emacs-eval "$1" '(kill-emacs)'
}

find-and-lock-file () {
  read -r -d '' script <<EOF
(with-current-buffer (find-file-noselect "$2")
  (my-set-buffer-modified-p t))
EOF

  emacs-eval "$1" "$script"

}

start-server $es1
start-server $es2

find-and-lock-file $es1 /tmp/dummy
find-and-lock-file $es2 /tmp/dummy

kill-emacs $es1
kill-emacs $es2

EDIT: I've dug around the source a little bit and found a reference to ask-user-about-lock, which solves it nicely.

edited Mar 11 '12 at 13:41

answered Mar 08 '12 at 15:57

event_jr

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Thank you! However, I am not convinced. Other Emacs process may lock the file between `(res (file-locked-p (buffer-file-name)))` and `(lock-buffer)`, right? And it will stop the execution like the first example. – tkf Mar 09 '12 at 00:11
You are correct. I don't have a solution that covers all corner cases. This question is getting old, maybe you should consider offering a bounty so it gets more attention from other people. – event_jr Mar 10 '12 at 06:53
Thank you very much! I did find `ask-user-about-lock` but I could not make it work. I grepped through el files to find out how it is used but there was no caller. I guess it is called from C. Anyway, you get the bounty! – tkf Mar 11 '12 at 21:12
Yes, it was in C. I've also filed bug #10989 because `signal` doesn't work from inside the eval block, it has to be `error`. – event_jr Mar 11 '12 at 23:49
Is this the bug report? http://debbugs.gnu.org/cgi/bugreport.cgi?bug=10989 I don't understand because both emacsclient calls use `--eval "(error \"foo\")"` which does not hang, even if it is called multiple times. But `emacsclient --eval "(signal 'file-locked \"file is locked\")"` does hang. Probably this is what the bug report means to say. – tkf Mar 12 '12 at 16:03
Hi, I wrote another answer to check that `(signal 'file-locked ...)` works if it is not called from emacsclient --eval. – tkf Mar 12 '12 at 16:33
Thanks for checking the bug report. I added to it with the correction you pointed out. – event_jr Mar 12 '12 at 17:11

tkf · Answer 2 · 2012-03-12T16:36:56.870

I found another answer using run-with-timer, to bypass the emacsclient --eval bug, so that I can check that (signal 'file-locked ...) works in "normal" situation.

#!/usr/bin/env bash

es1="server-1"
es2="server-2"

emacs=/Applications/Emacs.app/Contents/MacOS/Emacs
[ -e $emacs ] || emacs=emacs

start-server () {
  read -r -d '' script <<EOF
(progn
  (setq server-name "$1")
  (server-start)
  (require 'cl)
  (defvar my-file-is-locked "undefined")

  (defun my-set-buffer-modified-p (flag)
    (flet ((ask-user-about-lock
              (&rest args)
              (setq my-file-is-locked "no")
              (signal 'file-locked args)))
      (set-buffer-modified-p flag)
      (setq my-file-is-locked "yes"))))
EOF

  $emacs -q --daemon --eval "$script"
}

emacs-eval () {
    echo "@$1 >>> $2"
  emacsclient --socket-name "$1" --eval "$2"
}

kill-emacs () {
  emacs-eval "$1" '(kill-emacs)'
}

find-and-lock-file () {
  read -r -d '' script <<EOF
(run-with-timer 0 nil (lambda ()
  (with-current-buffer (find-file-noselect "$2")
    (my-set-buffer-modified-p t))))
EOF

  emacs-eval "$1" "$script"
}

file-locked-p () {
  emacs-eval "$1" "(message \"my-file-is-locked = %s\" my-file-is-locked)"
}

start-server $es1
start-server $es2

find-and-lock-file $es1 /tmp/dummy
find-and-lock-file $es2 /tmp/dummy

file-locked-p $es1
file-locked-p $es2

kill-emacs $es1
kill-emacs $es2

How to make Emacs lock-buffer fail when it cannot lock a file?

2 Answers2

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