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From:   Junio C Hamano <gitster@pobox.com>
To:     ZheNing Hu <adlternative@gmail.com>
Cc:     Git List <git@vger.kernel.org>
Subject: Re: [QUESTION] how to diff one blob with nothing
References: <CAOLTT8TVGna+C9nYy9nj3h5bT7AdAT0SKtUM3YVz6h=KZhGHHg@mail.gmail.com>
Date:   Wed, 19 Jul 2023 09:15:03 -0700
In-Reply-To: <CAOLTT8TVGna+C9nYy9nj3h5bT7AdAT0SKtUM3YVz6h=KZhGHHg@mail.gmail.com>
        (ZheNing Hu's message of "Wed, 19 Jul 2023 17:59:15 +0800")
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ZheNing Hu <adlternative@gmail.com> writes:

> I want to diff two blobs right now, and one of them
> may be empty, so I tried using
> 0000000000000000000000000000000000000000 or
> e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 (empty blobID)
> to test its effect, and the result I found was:
>
> git diff 00750edc07d6415dcc07ae0351e9397b0222b7ba
> 0000000000000000000000000000000000000000
> fatal: bad object 0000000000000000000000000000000000000000

As the object name for an empty blob is not all-0, this is expected.

> git diff 00750edc07d6415dcc07ae0351e9397b0222b7ba
> e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
> fatal: bad object e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
>
> Since the "empty object" has not been created, the
> git diff operation fails.

If you haven't created one, of course it would fail.  It should help
to do

    $ git hash-object -w --stdin </dev/null

before running

    $ git diff 00750edc e69de29bb

Long time ago, with 346245a1 (hard-code the empty tree object,
2008-02-13) we taught git what an empty-tree looks like, but it
seems that we never did the same for an empty blob, perhaps?

Interesting.  I am not sure if it is a good idea to teach empty_blob
to find_cached_object() and leaning negative but I haven't thought
things through on this.