Patrick Steinhardt <ps@pks.im> writes:

> On Tue, Apr 02, 2024 at 09:40:41AM -0700, Junio C Hamano wrote:
>> Patrick Steinhardt <ps@pks.im> writes:
>>
>> > because they have been supported by Git all along. It simply makes our
>> > lifes easier when we don't have to special-case creations and deletions
>> > in any way.
>> >
>> > So I'd really not want those to go away or become deprecated.
>>
>> That is a good input.
>>
>> Do you have anything to add as a counter-proposal?  The "I do not
>> care what was there before" update mode does make it necessary to
>> have a "zero" value for symrefs that can be distinguishable from
>> not having a value at all.
>>
>> Thanks.
>
> Sorry for taking this long to answer your question.
>
> I might have missed it while scanning through this thread, but why
> exactly is the zero OID not a good enough placeholder here to say that
> the ref must not exist? A symref cannot point to a ref named after the
> zero OID anyway.
>
> In my opinion, "update-symref" with an old-value must be able to accept
> both object IDs and symrefs as old value. Like this it would be possible
> to update a proper ref to a symref in a race-free way. So you can say:
>
>     git update-ref SYMREF refs/heads/main 19981daefd7c147444462739375462b49412ce33
>
> To update "SYRMEF" to "refs/heads/main", but only in case it currently
> is a proper ref that points to 19981daefd7c147444462739375462b49412ce33.
> Similarly...
>
>     git update-ref SYMREF refs/heads/main refs/heads/master
>
> would update "SYMREF" to "refs/heads/main", but only if it currently
> points to the symref "refs/heads/master". And by extension I think that
> the zero OID should retain its established meaning of "This ref must not
> exist":
>
>     git update-ref SYMREF refs/heads/main 0000000000000000000000000000000000000000
>
> This would only update "SYMREF" to "refs/heads/main" if it does not yet
> exist.
>

This is definitely nicer experience for the user. From looking at the
other commands, {verify, create, delete} I can only see this applying to
`symref-update`. Making the syntax for update something like:

    symref-update SP <ref> SP <new-ref> [SP (<old-oid> | <old-rev>)] LF

I think this makes sense to me, will incorporate this and send the next
version in the next few days.

On a side-note: This would also mean that we should somehow support
moving from symrefs to a regular ref via a transaction, so that means we
should allow

    update SP <ref> SP <new-oid> [SP (<old-oid> | <old-rev>)] LF

too, but I'm not going to tackle that in my patches.