Diftool problems

Diftool problems

[Marcin Zalewski]

Hi,

When git-difftool calls a diff tool, it uses file names given to it by
git-diff. This is a problem because often one of the files to be
compared is the same as the file to be merged into. What I mean is
that, in the following fragment of the git-difftool--helper file, $1
and $2 (I think) may end up being the same:

launch_merge_tool () {
	# Merged is the filename as it appears in the work tree
	# Local is the contents of a/filename
	# Remote is the contents of b/filename
	# Custom merge tool commands might use $BASE so we provide it
	MERGED="$1"
	LOCAL="$2"
	REMOTE="$3"
	BASE="$1"

Git-mergetool creates a temporary file for merging, but git-difftool
does not. Since git-diff tools is not meant for merging anything, it
may seem that there is no problem. However, some merge tools (such as
ediff) do not like when the merge target is the same as one of the
files to be compared. I use the following emacs snippet by Theodore
Tso:

http://kerneltrap.org/mailarchive/git/2007/7/2/250505

With that emacs code, ediff refuses to do a diff with the way that
difftool is done now. I do not have a patch, but it seems that a
simple fix would be to copy the code that creates temporary files from
mergetool.

Cheers,
Marcin Zalewski

Re: Diftool problems

[Markus Heidelberg]

Marcin Zalewski, 29.04.2009:
> Hi,
> 
> When git-difftool calls a diff tool, it uses file names given to it by
> git-diff. This is a problem because often one of the files to be
> compared is the same as the file to be merged into. What I mean is
> that, in the following fragment of the git-difftool--helper file, $1
> and $2 (I think) may end up being the same:
> 
> launch_merge_tool () {
> 	# Merged is the filename as it appears in the work tree
> 	# Local is the contents of a/filename
> 	# Remote is the contents of b/filename
> 	# Custom merge tool commands might use $BASE so we provide it
> 	MERGED="$1"
> 	LOCAL="$2"
> 	REMOTE="$3"
> 	BASE="$1"
> 
> Git-mergetool creates a temporary file for merging, but git-difftool
> does not. Since git-diff tools is not meant for merging anything, it
> may seem that there is no problem. However, some merge tools (such as
> ediff) do not like when the merge target is the same as one of the
> files to be compared. I use the following emacs snippet by Theodore
> Tso:
> 
> http://kerneltrap.org/mailarchive/git/2007/7/2/250505
> 
> With that emacs code, ediff refuses to do a diff with the way that
> difftool is done now. I do not have a patch, but it seems that a
> simple fix would be to copy the code that creates temporary files from
> mergetool.

The real fix would be to adjust the ediff snippet for difftool support.

As you said yourself, git-difftool is not meant for merging files, so
there is no reason to open more than 2 files at all.

The built-in difftools 'emerge' and 'ecmerge' still seem to open LOCAL,
REMOTE and MERGED. This should be fixed, so that they don't open MERGED
any more, but I don't have emacs installed, so I shouldn't try it
myself.

Oh, and LOCAL shouldn't be copied to a temporary file in the first
place, because people don't use git-difftool in read-only mode only.

Markus

Re: Diftool problems

[Marcin Zalewski]

> The real fix would be to adjust the ediff snippet for difftool support.

The emas snippet was meant to work with mergetool and it does (I
think). Changing the emacs code could indeed help with difftool but it
would break mergetool.

> As you said yourself, git-difftool is not meant for merging files, so
> there is no reason to open more than 2 files at all.

I agree, but the current implementation of difftool uses mergetool
library. That may be the reason why difftool is trying to come up with
the third file. Here is the snippet of code from mergetool library
that executes emerge in case of non-merge-mode:

"$merge_tool_path" -f emerge-files-command \
				"$LOCAL" "$REMOTE" "$(basename "$MERGED")"

> The built-in difftools 'emerge' and 'ecmerge' still seem to open LOCAL,
> REMOTE and MERGED. This should be fixed, so that they don't open MERGED
> any more, but I don't have emacs installed, so I shouldn't try it
> myself.

Again, I agree. This could be one of the possible solutions, but it
would require that mergetool library is changed or rewriting pieces of
mergetool in difftool. Correct me if I am wrong.

> Oh, and LOCAL shouldn't be copied to a temporary file in the first
> place, because people don't use git-difftool in read-only mode only.

I think that merge result could be a temporary file, like in
mergetool. In a situation where I use git to track an SVN repository,
difftool can be actually used to merge my uncommitted changes with a
commit from someone else after doing svn rebase. On the other hand, I
am no git expert so there may be a better way to handle this case.

-m

Re: Diftool problems

[Markus Heidelberg]

Marcin Zalewski, 29.04.2009:
> > The real fix would be to adjust the ediff snippet for difftool support.
> 
> The emas snippet was meant to work with mergetool and it does (I
> think). Changing the emacs code could indeed help with difftool but it
> would break mergetool.

I'm sure the emacs snippet can be adjusted to work with both. If it is
called with 2 files, then it's for difftool, else for mergetool.

> > As you said yourself, git-difftool is not meant for merging files, so
> > there is no reason to open more than 2 files at all.
> 
> I agree, but the current implementation of difftool uses mergetool
> library.

The file is called git-mergetool--lib, but more exactly it should be
called git-mergetool-difftool--lib, but who wants this? Difftool was
originally based on mergetool, but the recent refactoring introduced the
lib, which is shared by both, without belonging to one of them more than
to the other.

> That may be the reason why difftool is trying to come up with
> the third file.

Difftool isn't forcing a third file on you.

> Here is the snippet of code from mergetool library
> that executes emerge in case of non-merge-mode:
> 
> "$merge_tool_path" -f emerge-files-command \
> 				"$LOCAL" "$REMOTE" "$(basename "$MERGED")"

As said, this command shouldn't open the 3rd MERGED file, this should be
fixed. This part of the lib is not responsible for difftool.

> > The built-in difftools 'emerge' and 'ecmerge' still seem to open LOCAL,
> > REMOTE and MERGED. This should be fixed, so that they don't open MERGED
> > any more, but I don't have emacs installed, so I shouldn't try it
> > myself.
> 
> Again, I agree. This could be one of the possible solutions,

No, that doesn't solve your problem with ediff.
Or do you set merge.tool=emerge and ediff is called due to the snippet
in your ~/.emacs? If yes, then I got it now... Sure, I just read the
post from Ted Tso again, use-ediff-instead.el, aha.

I guess it would be cleaner to do this in ~/.gitconfig:

[diff]
	tool = ediff
[difftool "ediff"]
	cmd = emacs --options "$LOCAL" "$REMOTE"

Which is annoying, of course.

But now that I got it (see above) I think you should leave
merge.tool=emerge, since the snippet somehow seems to rely on it.

> but it
> would require that mergetool library is changed

What's wrong with that?
If this solves the 'ediff' problem and also makes 'emerge' to work right
as difftool, then everyone would benefit from.

> or rewriting pieces of
> mergetool in difftool. Correct me if I am wrong.
> 
> > Oh, and LOCAL shouldn't be copied to a temporary file in the first
> > place, because people don't use git-difftool in read-only mode only.
> 
> I think that merge result could be a temporary file, like in
> mergetool. In a situation where I use git to track an SVN repository,
> difftool can be actually used to merge my uncommitted changes with a
> commit from someone else after doing svn rebase.

Above you want your ediff mergetool snippet to work with difftool. And
here you want to use difftool as a mergetool. I'm confused :)

BTW, difftool doesn't work with files in unmerged state.

Markus

Re: Diftool problems

[Marcin Zalewski]

Markus,

I think I mostly agree with everything that you have written. I hope
that someone will fix something now. :)

Cheers,
-m

Re: Diftool problems

[David Aguilar]

On  0, Marcin Zalewski <marcin.zalewski@gmail.com> wrote:
> > The real fix would be to adjust the ediff snippet for difftool support.
> 
> "$merge_tool_path" -f emerge-files-command \
> 				"$LOCAL" "$REMOTE" "$(basename "$MERGED")"

Hi

Would the right command be:
	"$merge_tool_path" -f emerge-files-command \
		"$LOCAL" "$REMOTE"

(without $MERGED) ?


> I think that merge result could be a temporary file, like in
> mergetool. In a situation where I use git to track an SVN repository,
> difftool can be actually used to merge my uncommitted changes with a
> commit from someone else after doing svn rebase. On the other hand, I
> am no git expert so there may be a better way to handle this case.
> 
> -m

Are you able to save the result to a file with the above
snippet?  I'd prefer to not touch the way difftool deals
with files this late in the RC cycle, and if just
saving within emacs works without changing difftool
to use temporary files then that's even better.

I tried it and didn't notice any difference w/ or w/out
the patch, but if not specifying $MERGED is more
correct then we should do that.

You should have just seen a patch from me.
Let me know if you have any other suggestions.

-- 
		David