Paul Gortmaker <> writes:

> The '-D' or '--irreversible-delete' option of format-patch is
> great for sending out patches to mailing lists, where there
> is little value in seeing thousands of lines of deleted code.
> Attention can then be focused on the changes relating to
> the binding of the deleted code (Makefiles, etc).
> However the original intent of commit 467ddc14f ("git diff -D: omit
> the preimage of deletes") was as follows:
>     To prevent such a patch from being applied by mistake, the
>     output is designed not to be usable by "git apply" (or GNU "patch");
>     it is strictly for human consumption.
> The downside of this, is that patches to mailing lists which are
> then either managed with patchworks, or dealt with directly by
> maintainers, will need manual intervention if they are to be used.
> But with the index lines, there is no reason why we can't act
> intelligently and automatically on these with "git apply".
> If we can unambiguously map what was recorded as the deleted
> SHA prefix to the SHA of the matching blob filename in our tree,
> then we set the image len to zero which facilitates the delete.
> Signed-off-by: Paul Gortmaker <>
> ---
> For a recent use case example, see:
> Could be wrapped in an "am.applyirreversible" if for some reason
> global deployment was considered unwise?
>  Documentation/diff-options.txt |  5 +++--
>  builtin/apply.c                | 30 ++++++++++++++++++++++++++++++
>  2 files changed, 33 insertions(+), 2 deletions(-)
> diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
> index cf4b216..efaaf1c 100644
> --- a/Documentation/diff-options.txt
> +++ b/Documentation/diff-options.txt
> @@ -328,8 +328,9 @@ endif::git-log[]
>  --irreversible-delete::
>       Omit the preimage for deletes, i.e. print only the header but not
>       the diff between the preimage and `/dev/null`. The resulting patch
> -     is not meant to be applied with `patch` nor `git apply`; this is
> -     solely for people who want to just concentrate on reviewing the
> +     is not meant to be applied with `patch` (but can be with `git apply`).

... but only when you are applying to the exact version the patch
was created from, no?

> +     This is for people who want to avoid seeing/mailing all the deleted
> +     file content, and instead just concentrate on reviewing the
>       text after the change. In addition, the output obviously lack
>       enough information to apply such a patch in reverse, even manually,
>       hence the name of the option.
> diff --git a/builtin/apply.c b/builtin/apply.c
> index d453c83..363da63 100644
> --- a/builtin/apply.c
> +++ b/builtin/apply.c
> @@ -2933,6 +2933,36 @@ static int apply_fragments(struct image *img, struct 
> patch *patch)
>       if (patch->is_binary)
>               return apply_binary(img, patch);
> +     /* output from --irreversible-delete (looks like empty file delete) */
> +     if (patch->is_delete > 0 && !frag && img->len > 0) {

What is (img->len > 0) part trying to ensure?

If somebody gives you an irreversible deletion of an empty file,
shouldn't this codepath handle it the same way?

> +             unsigned char file_sha1[20], patch_sha1[20];
> +             struct object_context oc;
> +
> +             if (apply_in_reverse) {
> +                     error(_("can not reverse an irreversible-delete patch"
> +                           "on file '%s'."), name);
> +                     return -1;
> +             }

The return value of error() is already -1, so you can just return
it without { stmt; return -1; }.

> +
> +             strcpy(oc.path, name);
> +             if (get_sha1_with_context(patch->old_sha1_prefix,
> +                 GET_SHA1_BLOB | GET_SHA1_QUIETLY, patch_sha1, &oc)) {
> +                     error(_("the deleted SHA prefix of file '%s' (%s), does"
> +                           " not seem to exist in this repository."), name,
> +                           patch->old_sha1_prefix);
> +                     return -1;
> +             }

This is not sufficient to make sure patch_sha1 exists in your
repository and is indeed a blob object.  GET_SHA1_BLOB is a hint to
say "if there are more than one that shares this prefix, ignore ones
that are not blob---if there is only one remains, then even though
the prefix is ambiguous in this repository, we will take it".  If
you only have a commit or a tree that has the prefix but not the
blob object the patch wants to touch, you will get the object name
of that commit or tree you have in your repository.

Also oc is an output parameter; it is not about "I want to make sure
the found object is at this pathname"---that is an impossible request
to begin with.  I think something like this, without oc, would be
what you want:

        if (get_sha1_with_context(patch->old_sha1_prefix,
                        GET_SHA1_BLOB | GET_SHA1_QUIETLY,
                        patch_sha1, NULL) ||
            sha1_object_info(patch_sha1, NULL) != OBJ_BLOB)
                return error(...);

But I think the test itself (not the way you tested, but what you
are trying to test---the uniqueness of abbrevited object name) is
pointless.  The submitter of the patch may have far fewer objects
than you do, and it is perfectly normal if the old_sha1_prefix that
was sufficiently long to identify the blob for the submitter is not
unambiguous enough to identify the blob uniquely for you when you
try to apply the patch.  You may have other unrelated blobs that
happen to share the prefix in your repository.

Hashing img->buf and making sure it matches old_sha1_prefix is the
best you can do.  If the extra ambiguity coming from that approach
bothers you, then the entire "force apply an --irreversible-delete
patch" idea also should.

> +             hash_sha1_file(img->buf, img->len, blob_type, file_sha1);
> +             if (memcmp(file_sha1, patch_sha1, 20)) {
> +                     error(_("the delete requested of '%s' (%s), does not"
> +                           " match the current file contents."), name,
> +                           sha1_to_hex(patch_sha1));
> +                     return -1;

return error(...);
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