Re: [2.22.0] difftool no longer passes through to git diff if diff.tool is unset

[Jeff King <peff@peff.net>]

On Thu, Jun 20, 2019 at 07:29:36PM +0000, Pugh, Logan wrote:

> Thanks for the explanation. It sounds like I was under the incorrect 
> assumption that I could use the difftool command the same way as the 
> diff command. Part of my confusion could be blamed on the git-difftool 
> documentation (https://git-scm.com/docs/git-difftool) which near the top 
> states:

Well, it _is_ true that you can use it the same way. It's just that you
need to configure it to use whatever 3rd-party tool you want (and if you
do not want to configure a tool, then you are better off just using
git-diff directly). It was only due to a bug/historical accident that it
behaved just like git-diff in the no-index case (but not in the regular
case -- AFAICT, that would have been broken for your script always).

> My use case is a CLI program I've written that processes and then 
> compares two arbitrary files using the git difftool apparatus as 
> configured by the end user, leaving the choice to them whether to use 
> the internal diff tool or an external tool.
> 
> Now, if I'm understanding correctly, I should not rely on the behavior 
> of git difftool --no-index passing through to git diff. I could add 
> another CLI switch and code path to my program that calls git diff 
> directly instead of git difftool but the passthrough behavior seemed 
> more elegant at the time.
> 
> Ideally, in my mind, git difftool should work as it says on the tin, as 
> a straight up passthrough to git diff *unless* explicitly configured to 
> use external tools (e.g. diff.tool and diff.guitool).

That does make some sense to me for your use case. But I'm worried it
would be a worse experience for people new to difftool (they run it and
scratch their heads why it does not do anything different, whereas now
they get walked through an interactive configuration).

I dunno. I do not use difftool myself, so I don't have strong opinions.

In the meantime, I think you can probably switch behavior in your script
by checking if the diff.tool config is set. It might be nice if difftool
had a better way to query that without you having to know if it's
configured. Or in your case I suppose even better would just be an
option like "--if-not-configured-just-use-regular-diff". Then it would
do what you want, without impacting users who do want the interactive
setup.

-Peff