Subject: Re: [PATCH] diff: don't read index when --no-index is given

From: Jonathan Nieder <[email-masked]>
Date: Mon, 9 Dec 2013 07:16:09 -0800

Thomas Gummerer wrote:

> git diff --no-index ... currently reads the index, during setup, when
> calling gitmodules_config().  In the usual case this gives us some
> performance drawbacks,

Makes sense.

>                        but it's especially annoying if there is a broken
> index file.

Is this really a normal case?  It makes sense that as a side-effect it
is easier to use "git diff --no-index" as a general-purpose tool while
investigating a broken repo, but I would have thought that quickly
learning a repo is broken is a good thing in any case.

A little more information about the context where this came up would
be helpful, I guess.

[...]
> --- a/builtin/diff.c
> +++ b/builtin/diff.c
[...]
> @@ -282,9 +282,18 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
>  	 *
>  	 * Other cases are errors.
>  	 */
> +	for (i = 1; i < argc; i++) {
> +		if (!strcmp(argv[i], "--"))
> +			break;
> +		if (!strcmp(argv[i], "--no-index")) {
> +			no_index = 1;
> +			break;
> +		}

setup_revisions() uses the same logic that doesn't handle options that
take arguments in their "unstuck" form (e.g., "--word-diff-regex --"),
so this is probably not a regression, though I haven't checked. :)

[...]
>  	prefix = setup_git_directory_gently(&nongit);
> -	gitmodules_config();
> +	if (!no_index)
> +		gitmodules_config();

Perhaps we can improve performance and behavior by skipping the
setup_git_directory_gently() call, too?

That would help with the repairing-broken-repository case by
working even if .git/config or .git/HEAD is broken, and I think
it is more intuitive that the repository-local configuration is
ignored by "git diff --no-index".  It would also help with
performance by avoiding some filesystem access.

[...]
> +test_expect_success 'git diff --no-index with broken index' '
> +	cd repo &&
> +	echo broken >.git/index &&
> +	test_expect_code 0 git diff --no-index a ../non/git/a

Clever.  I wouldn't use "test_expect_code 0", since that's
already implied by including the "git diff --no-index" call
in the && chain.

Thanks and hope that helps,
Jonathan