MergingWithEmacs

Using Emacs as a merge program

Emacs is bundled with an elisp program called Ediff whose purpose is to help developers to visually apply patches. One of the ediff commands is well-suited to three-way merging and can be used as a merge program with Mercurial.

Mercurial 1.0 and later

Add the following to your ~/.hgrc file (see MergeToolConfiguration):

[ui]
merge = emacs

[merge-tools]
emacs.args = -q --eval "(ediff-merge-with-ancestor \"$local\" \"$other\" \"$base\" nil \"$output\")"

If you want to use the plain emerge tool in emacs, the following line works instead in the merge-tools section:

emacs.args = -q --eval "(emerge-files-with-ancestor nil \"$local\" \"$other\" \"$base\" \"$output\" nil 'kill-emacs)"

Mercurial 0.9.5 and earlier: Wrap Emacs+Ediff call in a script

Dump the following content into a file in your PATH (don't forget to turn on the execute bit):

   #!/bin/sh
   
   set -e # bail out quickly on failure
   
   LOCAL="$1"
   BASE="$2"
   OTHER="$3"

   BACKUP="$LOCAL.orig"
   
   Restore ()
   {
       cp "$BACKUP" "$LOCAL" 
   }

   ExitOK ()
   {
       exit $?
   }

   # Back up our file
   cp "$LOCAL" "$BACKUP"

   # Attempt to do a non-interactive merge
   if which merge > /dev/null 2>&1 ; then
       if merge "$LOCAL" "$BASE" "$OTHER" 2> /dev/null; then
           # success!
           ExitOK 
       fi
       Restore
   elif which diff3 > /dev/null 2>&1 ; then
       if diff3 -m "$BACKUP" "$BASE" "$OTHER" > "$LOCAL" ; then
           # success
           ExitOK
       fi
       Restore
   fi

   if emacs -q --no-site-file --eval "(ediff-merge-with-ancestor \"$BACKUP\" \"$OTHER\" \"$BASE\" nil \"$LOCAL\")" 
   then       
       ExitOK
   fi

   echo "emacs-merge: failed to merge files"
   exit 1

   # End of file

Considering Mercurial is now able to premerge before running the merge tool and even does it by default (see hgrc(5)), I feel that attempts of merge with merge and diff3 are not needed anymore.

0.1. How the script works

This script tries first to automatically merge the files using the RCS merge program or the diff3 program. If the automatic merger fails merging the files because of a conflict or, neither merge nor diff3 are available on the system, then emacs is launched to let the developer resolve the conflicts.

0.2. Enabling the script usage

Don't forget to add an entry in your hgrc file (either ~/.hgrc or the local working copy .hg/hgrc) to point Mercurial at your merge command (let's call it emacs-merge)

   [ui]
   merge = emacs-merge

All Mercurial versions: using emacsclient

You may want to use 'emacsclient' to reuse an already running Emacs session. However, emacsclient does not relay errors from the server process properly (it always exits with status 0), so hg has no way to know if an error occured, and will happily commit the merge, possibly without you ever noticing something is amiss.

As a workaround until Emacs Bug #6963 is resolved, you'll need a wrapper script like the one below to supply the error handling.

(save it as e. g. 'emacsclient-merge' and configure it like this)

   [ui]
   merge = emacsclient-merge

    #!/bin/bash

    if [ $# -lt 1 ]; then
      echo 1>&2 "Usage: $0 local other base output"
      exit 1
    fi

    local=$1
    other=$2
    base=$3
    output=$4

    OUTPUT=`emacsclient --no-wait --eval "(ediff-merge-with-ancestor \"$local\" \"$other\" \"$base\" nil \"$output\")" 2>&1`
    echo $OUTPUT | grep -v "Ediff Control Panel"

    if echo "$OUTPUT" | grep -q '^*ERROR*'; then
        exit 1
    fi