Re: git clone of empty repositories doesn't preserve hash

From: Jeff King <peff@peff.net>
Date: Apr 27, 2023, 04:56 UTC
Message-ID: <20230427045643.GB982277@coredump.intra.peff.net>
In-Reply-To: <ZEmMUFR7AJn+v7jV@tapette.crustytoothpaste.net>

On Wed, Apr 26, 2023 at 08:40:48PM +0000, brian m. carlson wrote:

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> On 2023-04-26 at 10:51:34, Jeff King wrote:
> > We could send a capabilities^{} line, which Git has supported on the
> > client side since eb398797cd (connect: advertized capability is not a
> > ref, 2016-09-09). So sending it should not break even old clients
> > (though we would have to check what alternate implementations like
> > libgit2 or dulwich do; we know JGit supports it).
> 
> I have a patch which does exactly this, which I will be sending shortly.
> I've confirmed that libgit2 and JGit support it, which is unsurprising,
> since all of the implementations, Git included, share the same code.  In
> addition, this is the behaviour we document as supporting, so all
> implementations should support it.

Yeah, I was worried about how accurate that "should" is. :)

Since you checked the others, I peeked at dulwich's code, and it appears to support it, too (I didn't actually run a test, but the code is pretty clear). It doesn't support sha256 at all, but that's OK. What I'd be concerned about is breaking clients when there is no useful capability to advertise (though of course we could decide to send capabilities^{} only when there is something useful to say).

Show 9 quoted lines

> > However, the object-format support here was broken until the very recent
> > 13e67aa39b (v0 protocol: fix sha1/sha256 confusion for capabilities^{},
> > 2023-04-14), so it would only be useful going forward (before then we'd
> > die(), but maybe that is preferable to having the wrong object format?).
> 
> I think it's better to die than to silently have the wrong object
> format, and it also prevents the problem if other clients using v0 or v1
> (which effectively have to be supported for compatibility, while v2 is
> optional) try to clone from a fixed server.

OK, good. So we can ignore that recently fixed bug (which only affects the case where older versions are cloning something whose hash format does not match theirs).

Show 12 quoted lines

> > I'm not sure it's worth the effort, though. If you want to use sha256
> > everywhere and tell the other side about it, you need a modern client
> > anyway, and that means the ability to speak v2. So this would only
> > matter if for some reason the v2 probe was being ignored (e.g., proxies
> > eating it, ssh refusing environment variable, etc), which itself are
> > things that ideally would be fixed (and can maybe one day even go away
> > if we optimistically default to v2).
> 
> Using v2 everywhere is difficult because many SSH servers still don't
> pass GIT_PROTOCOL by default, meaning that we're stuck with v0 and v1.
> In retrospect, sending an environment variable here was not a great
> decision, but we're stuck with it now.

True (I even got bit by this at one point, not realizing that I wasn't using v2 with one of my personal servers). I certainly don't have an objection if you're willing to do the work.

It would also allow us to fix the long-broken "unborn HEAD" problem for v0 in an empty repo (though I am also fine if we don't go that far).

-Peff