Re: Can't diff against the 00000000 revision

From: Jeff King <peff@peff.net>
Date: Jan 12, 2016, 15:32 UTC
Message-ID: <20160112153239.GA8041@sigill.intra.peff.net>
In-Reply-To: <jwv4meiygrc.fsf-monnier+gmane.comp.version-control.git@gnu.org>

On Tue, Jan 12, 2016 at 10:17:24AM -0500, Stefan Monnier wrote:

Show 28 quoted lines

> If I look at the initial commit on a branch, I see something like:
> 
>     % git show d59cfff346c3e210adc26501f8cebf8da5ab2e7d
>     commit d59cfff346c3e210adc26501f8cebf8da5ab2e7d
>     Author: Stefan Monnier <monnier@iro.umontreal.ca>
>     Date:   Wed Dec 2 20:46:51 2015 -0500
>     
>         Initial release
>     
>     diff --git a/bugit b/bugit
>     new file mode 100755
>     index 0000000..681bd38
>     --- /dev/null
>     +++ b/bugit
>     @@ -0,0 +1,512 @@
>     ...
> 
> which is great.  But I can't get the same result with
> 
>     git diff 0000000..681bd38
> 
> because it complains:
> 
>     % git diff 0000000..681bd38
>     fatal: ambiguous argument '0000000..681bd38': unknown revision or path not in the working tree.
>     Use '--' to separate paths from revisions, like this:
>     'git <command> [<revision>...] -- [<file>...]'
>     %

Right. There is no "000000" blob; it's just a syntactic placeholder.

If you want to diff against the empty blob, you can. Its name is:

  $ git hash-object -t blob /dev/null
  e69de29bb2d1d6434b8b29ae775ad8c2e48c5391

> I bumped into this problem in a post-receive hook where I need to pay
> attention to all newly added files, and where this problem means that
> I can't use the same code for a newly added branch as for a push on
> a pre-existing branch.

Keep in mind that the "000000" in your example is showing the change in the _blob_, not the change in revisions. Even if it were not 0's, you could not run "git diff A..B" on it, because that syntax only works with commits.

So the corner case you need to deal with is not about a newly added branch; it is about a newly added file (or in the opposite direction, a deleted file).

Or from your description, maybe you are also running into the all-zero sha1 in the argument to the post-receive hook (where it is standing in for a commit sha1 on a newly-pushed branch).

Show 5 quoted lines

> I currently work around the problem by adding a dummy empty branch, but
> being able to use the revision 00000000 as a known reference to an empty
> tree would come in really handy, and since it's already used at various
> places in Git (post-receive hook and "git show" output, at least), it
> would seem like a natural extension.

The empty tree also has a name:

  $ git hash-object -t tree /dev/null
  4b825dc642cb6eb9a060e54bf8d69288fbee4904

and you can diff against that.

And hopefully that explains why "000000" does not necessarily make a good placeholder for "the empty thing". There are multiple empty things, and it is not clear what:

  git diff 0000000 1234abcd

means. Is 0000000 a tree? A blob?

-Peff