Re: [PATCH 7/8] refs: add 'update-symref' command to 'update-ref'

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> writes:

> Chris Torek <chris.torek@gmail.com> writes:
>
>> For these reasons, I'd suggest that the all-zero hash be officially
>> deprecated in favor of create/delete and of course create-symref
>> and delete-symref. Of course, compatibility requires some sort
>> of support for the old system for some time.  As to whether that
>> means something like the suggestion of ".missing" etc, I have no
>> particular opinion -- but since the symref options are new, they
>> would not *need* symmetric options, if we just say that "update-symref"
>> cannot create or delete a symref.
>
> I love that simplicity.

Having said that, the loose "update that can create or delete" may
actually be used by applications that do not care about overwriting
competing operation, so I am not sure if we can easily deprecate
that mode of operation.  Saying "update refs/heads/next to point at
this object" and have it created if it does not exist may be handy
for some loosely written applications.

So perhaps we can say "update with a concrete <old-oid> will ensure
that the <ref> poitns at <old-oid> before proceeding, but update
with 0{40} as <old-oid> to ensure creation is deprecated.  update
with 0{40} as <new-oid> as deletion is also deprecated.  Use create
and delete for these two deprecated operation modes".

This assumes that create and delete currently ensures that what is
asked to be created does not exist, and what is asked to be deleted
does exist, before the operation.  If we are loosely doing these two
operations, then we cannot easily deprecate the checking-update,
without breaking existing users.

As {create,update,delete,verify}-symref do not exist yet, we can
start with the right semantics from day one.  "update-symref" will
accept a <old-ref> only to ensure that the symref is pointing to
that ref and there is no "zero" value based creation/deletion
validation offered via "update-symref".  "create-symref" will error
out if the ref asked to be created already exists, "delete-symref"
will error out if the ref asked to be deleted does not exist.