Re: How to use emacs as a "diff3 -m" compatible merge tool?

[Tassilo Horn <tsdh@gnu.org>]

Hi all,

> pacdiff calls the user-defined mergeprog like so:
>
> 	if $mergeprog "$file" "$base" "$pacfile" >"$merged"; then
> 		msg2 "Merged without conflicts."
> 	fi
>
>
> I'd like to use emacs as mergetool so need to support these calling
> convention (which are the ones of "diff3 -m"), i.e.,
>
>   1. args are MY-VERSION BASE THEIR-VERSION
>   2. a successful merge exits zero, an aborted merge exits non-zero
>   3. the merge result is printed to stdout

Here's a version which works for me (on GNU/Linux where /dev/stdout is a
thing) and fulfills all requirements except for 2 which turns out not to
be important at least in my pacdiff use-case.  It'll ask if I want to
use the merge result anyhow so exiting successfully even when I abort
the merge is no big deal.

--8<---------------cut here---------------start------------->8---
(defun th/ediff-spit-merge-result-and-kill-emacs ()
  (if-let ((merge-buf (ediff-get-buffer 'C)))
      (with-current-buffer merge-buf
        (append-to-file (point-min) (point-max)
                        "/dev/stdout")
        (kill-emacs))
    (error "There is no merge result buffer")))

(defun th/command-line-ediff-merge3 (_switch)
  "Do a 3-way merge using `ediff-merge-files-with-ancestor'.
Assumes the command line args were: my-version base-version their-version"
  (setq ediff-quit-merge-hook (list #'th/ediff-spit-merge-result-and-kill-emacs))
  (let ((make-backup-files nil)
        (my-version    (pop command-line-args-left))
        (base-version  (pop command-line-args-left))
        (their-version (pop command-line-args-left)))
    (ediff-merge-files-with-ancestor
     my-version their-version base-version)))

(add-to-list 'command-switch-alist
             '("--ediff-merge3" . th/command-line-ediff-merge3))
--8<---------------cut here---------------end--------------->8---

Bye,
Tassilo