Re: [RFC/PATCH] extend meaning of "--root" option to index comparisons

> The "--root" option generally means "treat any commits
> without parents as a big creation event". This extends the
> meaning to make an index comparison against a non-existant
> HEAD into a big creation event. In other words, "if this
> index _were_ to become a commit, this is how we would show
> it with --root."
>
> Specifically, we cover the case of
>
>  git diff --cached --root
>
> to show either the diff between the index and HEAD, or if
> there is no HEAD, show the diff against the empty tree.
> This can simplify calling scripts which must otherwise
> special-case the initial commit when showing the index
> status.
>
> We intentionally don't cover:
>
>  - git diff --cached --root HEAD
>
>    The user has specifically asked for HEAD, which doesn't
>    exist.
>
>  - git diff-index
>
>    The user is required to specify a tree-ish to
>    diff-index; if that tree-ish doesn't exist, we should
>    report an error.
>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
> On Tue, Sep 16, 2008 at 02:21:05AM -0400, Jeff King wrote:
>
>> Right, that was what I meant by "incomplete". I think there are several
>> other cases where giving "--root" would have expected behavior but is
>> currently ignored. I'll take a closer look, but I probably won't have
>> time for a few days.
>
> Actually, I wasn't able to find any more cases. I don't think it makes
> sense to override the behavior when an explicit tree-ish is given, so
> that cuts out the two places mentioned above. Though I think
> that scripts using this might prefer the plumbing
> diff-index, so maybe there is a better way to support this
> (e.g., if --root is specified without a tree-ish, assume
> HEAD or empty tree).
>
> diff-tree already handles --root itself. And there is no way to my
> knowledge to provoke the same kind of "show the diff against its parent"
> behavior via git-diff, since a single tree-ish there means "diff against
> the working tree".
>
> And of course for diff-files, such an option makes no sense.
>
> Can you think of any other cases?

>
>  builtin-diff.c       |    7 +++++--
>  revision.c           |   17 ++++++++++++++---
>  revision.h           |    1 +
>  t/t4030-diff-root.sh |   21 +++++++++++++++++++++

>  4 files changed, 41 insertions(+), 5 deletions(-)
>  create mode 100755 t/t4030-diff-root.sh
>
> diff --git a/builtin-diff.c b/builtin-diff.c
> index 037c303..0a1efb5 100644
> --- a/builtin-diff.c
> +++ b/builtin-diff.c
> @@ -315,8 +315,11 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
>                                break;
>                        else if (!strcmp(arg, "--cached")) {
>                                add_head_to_pending(&rev);
> -                               if (!rev.pending.nr)
> -                                       die("No HEAD commit to compare with (yet)");
> +                               if (!rev.pending.nr) {
> +                                       if (!rev.show_root_diff)
> +                                               die("No HEAD commit to compare with (yet)");
> +                                       add_empty_to_pending(&rev);
> +                               }
>                                break;
>                        }
>                }
> diff --git a/revision.c b/revision.c
> index 499f0e0..de0fd89 100644
> --- a/revision.c
> +++ b/revision.c
> @@ -145,16 +145,27 @@ void add_pending_object(struct rev_info *revs, struct object *obj, const char *n
>        add_pending_object_with_mode(revs, obj, name, S_IFINVALID);
>  }
>
> -void add_head_to_pending(struct rev_info *revs)
> +static void add_to_pending_by_name(struct rev_info *revs, const char *name)
>  {
>        unsigned char sha1[20];
>        struct object *obj;
> -       if (get_sha1("HEAD", sha1))
> +       if (get_sha1(name, sha1))
>                return;
>        obj = parse_object(sha1);
>        if (!obj)
>                return;
> -       add_pending_object(revs, obj, "HEAD");
> +       add_pending_object(revs, obj, name);
> +}
> +
> +void add_head_to_pending(struct rev_info *revs)
> +{
> +       add_to_pending_by_name(revs, "HEAD");
> +}
> +
> +void add_empty_to_pending(struct rev_info *revs)
> +{
> +       add_to_pending_by_name(revs,
> +                       "4b825dc642cb6eb9a060e54bf8d69288fbee4904");
>  }
>
>  static struct object *get_reference(struct rev_info *revs, const char *name, const unsigned char *sha1, unsigned int flags)
> diff --git a/revision.h b/revision.h
> index fc23522..108f43d 100644
> --- a/revision.h
> +++ b/revision.h
> @@ -151,6 +151,7 @@ extern void add_object(struct object *obj,
>  extern void add_pending_object(struct rev_info *revs, struct object *obj, const char *name);
>
>  extern void add_head_to_pending(struct rev_info *);
> +extern void add_empty_to_pending(struct rev_info *);
>
>  enum commit_action {
>        commit_ignore,
> diff --git a/t/t4030-diff-root.sh b/t/t4030-diff-root.sh
> new file mode 100755
> index 0000000..e5174b7
> --- /dev/null
> +++ b/t/t4030-diff-root.sh
> @@ -0,0 +1,21 @@
> +#!/bin/sh
> +
> +test_description='diff --root allows comparison between index and root'
> +. ./test-lib.sh
> +
> +test_expect_success 'setup' '
> +       echo content >file &&
> +       git add file
> +'
> +
> +test_expect_success 'diff --cached (without --root)' '
> +       test_must_fail git diff --cached --name-only
> +'
> +
> +test_expect_success 'diff --cached (with --root)' '
> +       echo file >expect &&
> +       git diff --cached --name-only --root >actual &&
> +       test_cmp expect actual
> +'
> +
> +test_done
> --
> 1.6.0.2.249.g97d7f.dirty