Re: [PATCH v5 1/3] ls-refs: report unborn targets of symrefs

Jeff King [off-list ref] writes:

See the
discussion in the v4 thread (sorry, seems not to have hit the archive
yet, but hopefully this link will work soon):

  https://lore.kernel.org/git/YBCitNb75rpnuW2L@coredump.intra.peff.net/ (local)

I guess vger having some sort of constipation, we (not you-and-me
but list participants as a whole) would be doing this kind of
back-and-force while untable to read what others have said. 

https://lore.kernel.org/git/xmqqmtwvz4g9.fsf@gitster.c.googlers.com/ (local)

will have the following.

It feels somewhat brittle that we have to read the same variable and
apply the same "default to true" logic in two places and have to
keep them in sync.  Is this because the decision to advertize or not
has to be made way before the code that is specific to the
implementation of ls-refs is run?

If ls_refs_advertise() is always called first before ls_refs(), I
wonder if it makes sense to reuse what we found out about the
configured (or left unconfigured) state here and use it when
ls_refs() gets called?  I know that the way serve.c infrastructure
calls "do we advertise?" helper from each protocol-element handler
is too narrow and does not allow us to pass such a necessary piece
of information but I view it as a misdesign that can be corrected
(and until that happens, we could use file-local static limited to
ls-refs.c).

After giving the above a bit more thought, here are a few random
thoughts around the area.

 * As "struct protocol_capability" indicates, we have <name of
   service, the logic to advertise, the logic to serve> as a
   three-tuple for services.  The serving logic should know what
   advertising logic advertised (or more precisely, what information
   advertising logic used to make that decision) so that they can
   work consistently.

   For that, there should be a mechanism that advertising logic can
   use to leave a note to serving logic, perhaps by adding a "void
   *" to both of these functions.  The advertising function would
   allocate a piece of memory it wants to use and returns the
   pointer to it to the caller in serve.c, and that pointer is given
   to the corresponding ls_refs() when it is called by serve.c.
   Then ls_refs_advertise can say "I found this configuration
   setting and decided to advertise" to later ls_refs() and the
   latter can say "ah, as you have advertised, I have to respond to
   such a request".

 * I am not sure if "lsrefs.allowunborn = yes/no" is a good way to
   configure this feature.  Wouldn't it be more natural to make this
   three-way, i.e. "lsrefs.unborn = advertise/serve/ignore", where
   the server operator can choose among (1) advertise the presence
   of the capability and respond to requests, (2) do not advertise
   the capability but if a request comes, respond to it, and (3) do
   not advertise and do not respond.  We could throw in 'deny' that
   causes the request to result in a failure but I do not care too
   deeply about that fourth option.

   Using such a configuration mechanism, ls_refs_advertise may leave
   the value of "lsrefs.unborn" (or lack thereof) it found and used
   to base its decision to advertise, for use by ls_refs.  ls_refs
   in turn can use the value found there to decide if it ignores or
   responds to the "unborn" request.