Re: [RFC 00/14] Allow fetch-pack to send ref names (globs allowed)

From: Jeff King <peff@peff.net>
Date: Jan 26, 2017, 23:00 UTC
Message-ID: <20170126230046.aknesybfyzxhx3ia@sigill.intra.peff.net>
In-Reply-To: <cover.1485381677.git.jonathantanmy@google.com>

On Wed, Jan 25, 2017 at 02:02:53PM -0800, Jonathan Tan wrote:

Show 7 quoted lines

> Negotiation currently happens by upload-pack initially sending a list of
> refs with names and SHA-1 hashes, and then several request/response
> pairs in which the request from fetch-pack consists of SHA-1 hashes
> (selected from the initial list). Allowing the request to consist of
> names instead of SHA-1 hashes increases tolerance to refs changing
> (due to time, and due to having load-balanced servers without strong
> consistency),

Interesting. My big question is: what happens when a ref _does_ change? How does the client handle this?

The existing uploadpack.allowReachableSHA1InWant is there to work around the problem that an http client may get a ref advertisement in one step, and then come back later to do the want/have negotiation, at which point the server has moved on (or maybe it's even a different server). There the client says "I want sha1 X", and the server needs to say "well, X isn't my tip now, but it's still acceptable for you to fetch".

But this seems to go in the opposite direction. After the advertisement, the client decides "OK, I want to fetch refs/heads/master which is at SHA1 X". It connects to the server and says "I want refs/heads/master". Let's say the server has moved its version of the ref to SHA1 Y.

What happens? I think the server will say "wanted-ref master Y". Does the client just decide to use "Y" then? How does that interact with any decisions the client might have made about X? I guess things like fast-forwards have to be decided after we fetch the objects anyway (since we cannot compute them until we get the pack), so maybe there aren't any such decisions. I haven't checked.

> and is a step towards eliminating the need for the server
> to send the list of refs first (possibly improving performance).

I'm not sure it is all that useful towards that end. You still have to break compatibility so that the client tells the server to suppress the ref advertisement. After that, it is just a question of asking for the refs. And you have two options:

  1. Ask the server to tell you the values of some subset of the refs,
     pick what you want, and then do the want/have as normal.

  2. Go straight to the want/have, but tell the server the refs you want
     instead of their sha1s.

I think your approach here would lead to (2).

But (1), besides being closer to how the protocol works now, seems like it's more flexible. I can ask about the ref state without necessarily having to retrieve the objects. How would you write git-ls-remote with such a system?

Show 10 quoted lines

> [1] There has been some discussion about whether the server should
> accept partial ref names, e.g. [2]. In this patch set, I have made the
> server only accept full names, and it is the responsibility of the
> client to send the multiple patterns which it wants to match. Quoting
> from the commit message of the second patch:
> 
>   For example, a client could reasonably expand an abbreviated
>   name "foo" to "want-ref foo", "want-ref refs/heads/foo", "want-ref
>   refs/tags/foo", among others, and ensure that at least one such ref has
>   been fetched.

That has a cost that scales linearly with the number of refs, because you have to ask for each name 6 times. After the discussion you linked, I think my preference is more like:

  1. Teach servers to accept a list of patterns from the client
     which will be resolved in order. Unlike your system, the client
     only needs to specify the list once per session, rather than once
     per ref.

  2. (Optional) Give a shorthand for the stock patterns that git has had
     in place for years. That saves some bytes over specifying the
     patterns completely (though it's really not _that_ many bytes, so
     perhaps the complication isn't a big deal).

-Peff