Fixing the p4merge launch shell script

Hi,

I've noticed that the p4merge shell script could do with some improvement when it comes to merging. Because p4merge throws up an error when one of the files it's given to diff is "/dev/null", git needs to create a temporary empty file and pass that to p4merge when diffing a file that has been created/deleted (eg. create file, git add ., git diff --cached).

At the moment, the hack I'm using, which works, is to create a c:/blank.txt file which is an empty file, and modify "libexec/git-core/mergetools/p4merge" to start with this:

diff_cmd () {
	if [ ! -f "$LOCAL" ]
	then
		LOCAL="C:/blank.txt"
	fi
	if [ ! -f "$REMOTE" ]
	then
		REMOTE="C:/blank.txt"
	fi
	"$merge_tool_path" "$LOCAL" "$REMOTE"
}

Obviously, this isn't good enough because c:/blank.txt probably won't exist on the system. It would be nice for the temporary blank file to have the same extension as the one that's been added, so we could diff (say) c:/users/jez/Temp/abc123_newFile.foo c:/development/bar/newfile.foo. However, I can't see a way to do this purely from within the shell script (even `mktemp` doesn't exist on all systems so that isn't good enough). I believe git needs to create this temporary blank file itself, much like it creates a temporary file for the previous version of a file that's been modified when it's being diff'd. It then needs to expose the location of this temporary file as a variable; say $LOCALBLANK. So, we would be able to modify the shell script to this:

diff_cmd () {
	if [ ! -f "$LOCAL" ]
	then
		LOCAL=$LOCALBLANK
	fi
	if [ ! -f "$REMOTE" ]
	then
		REMOTE=$REMOTEBLANK
	fi
	"$merge_tool_path" "$LOCAL" "$REMOTE"
}

Thoughts?  Is there an easier way to do this?

Jeremy Morton <admin@game-point.net> writes:

Which version of git? Perhaps you do not have ec245ba (mergetool: Provide an empty file when needed, 2012-01-19) yet?

Junio C Hamano <gitster <at> pobox.com> writes:

That patch fixes the mergetool part, but the part I was referring to was the difftool part, which still has this problem.

Best regards, Jeremy Morton (Jez)

On Wed, Oct 3, 2012 at 1:34 AM, Jeremy Morton <admin@game-point.net> wrote:

Thanks for the heads-up, Jez.

I should have some time to take a look this weekend. If someone beats me to it I'll be happy to review the patches before then (Jeremy? ;-)

Is `git diff` giving us /dev/null as the path here, and p4merge doesn't like that, or is something else going on?

If that's indeed happening, then I would imagine a sensible thing for git-difftool--helper to do would be to check for /dev/null and create some temporary empty file.

Does that sound like the right way to go? Another option would be for git-diff's external diff code to handle it at a much earlier point, perhaps guarded behind an option that git-difftool knows to pass (changing existing behavior is off the table).

These both sound like pretty big changes given that we're getting ready for 1.8.0.

If it's just p4merge that doesn't like this then perhaps the right decision is to paper over it in p4merge's mergetools/ scriptlet.

It's certainly the least-impactful option.  Thoughts?