Re: [PATCH v2 2/2] fetch: send "refs/tags/" prefix upon CLI refspecs

From: Jonathan Tan <jonathantanmy@google.com>
Date: Jun 18, 2018, 23:16 UTC
Message-ID: <20180618231642.174650-1-jonathantanmy@google.com>
In-Reply-To: <xmqqwouvwzsz.fsf@gitster-ct.c.googlers.com>

Show 27 quoted lines

> Jonathan Tan <jonathantanmy@google.com> writes:
> 
> >> Wouldn't that allow us not having to advertise the whole tags
> >> namespace only to implement the tag following?
> >
> > Yes, it would, but as far as I can tell, it would add an extra burden on
> > the server to walk all refs requested in the ls-refs call (in order to
> > determine which tags to send back in the response). Also, this walk must
> > be done before any negotiation (since this is a ls-refs call),...
> 
> My comment was that I doubt the "must be done" part of the above.
> How would refs-in-want be responded where client-supplied "I want
> 'master' branch---I am not asking for the exact object the first
> server I contacted said where the 'master' is at" gets turned into
> "So the final value of 'master' among these servers that are not
> quite in sync is this" by the one that gives you the pack, not
> necessarily the one that responds to ls-refs upon initial contact?
> Can't we do something similar, i.e. let the client say "I want tags
> that refer to new objects you are going to send me, I do not know
> what they are offhand" and the server that actually gives you the
> pack to say "here are the tags I ended up including"?  The
> "include-tag" process to generate pack with extra objects (i.e. the
> tags that point at packed objects) has to involve walking for
> reachabliity anyway, so as long as the feature is supported,
> somebody has to do the work, and if you want to cut down the
> transfer cost of the refs/tags/* enumeration, it needs to happen on
> the server end, no?

Ah, I think I see. There are these possible worlds:
 (1) the current world
 (2) no ref-in-want, and upload-pack sends tag information as part of
     its response to ls-refs
 (3) no ref-in-want, but upload-pack can send ref information right
     before the packfile
 (4) ref-in-want, and upload-pack will send ref information right before
     the packfile

I was only thinking about (2) and (4), but I think you are talking about (3). Yes, that would work, although I don't think it's worth the protocol churn to do (3) then (4), especially since we already have ref-in-want patches sent to the mailing list - but I should have discussed this option in my previous e-mails too.

> Or perhaps v2 fetch should implement the automated tag following
> without using include-tag and instead as an extended feature of
> ref-in-want.  I think that is merely giving a different name to the
> same idea outlined above, though ;-)

Instead of not using include-tag, I would define include-tag in the presence of want-refs to also include the refs, but I agree with this solution.