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From:   Junio C Hamano <gitster@pobox.com>
To:     Han-Wen Nienhuys <hanwen@google.com>
Cc:     Han-Wen Nienhuys via GitGitGadget <gitgitgadget@gmail.com>,
        git <git@vger.kernel.org>, Han-Wen Nienhuys <hanwenn@gmail.com>
Subject: Re: [PATCH v3 0/3] Remove special casing for PSEUDOREF updates
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Date:   Wed, 05 Aug 2020 08:54:01 -0700
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        (Han-Wen Nienhuys's message of "Wed, 5 Aug 2020 12:48:21 +0200")
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Han-Wen Nienhuys <hanwen@google.com> writes:

>> All of which means FETCH_HEAD is special and we may not want to
>> burden the special casing of it to newer backends.
>
> Can you confirm that FETCH_HEAD is the only thing that can store more
> than just a symref / SHA1 ? Based on the name, and a comment in the
> JGit source, I thought that MERGE_HEAD might contain more than one
> SHA1 at a time.

No I cannot.  I do not think MERGE_HEAD is something I added with as
deep a thought as I did with FETCH_HEAD.  If it mimics FETCH_HEAD,
then perhaps it does, but I somehow doubt it.

As can be seen in builtin/merge.c::collect_parents(), we do special
case FETCH_HEAD when grabbing what commit*S* to merge, but all
others are read with get_merge_parent() that makes a single call to
get_oid(), i.e. anything other than FETCH_HEAD cannot have more than
one object recorded.