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

From: brian m. carlson <sandals@crustytoothpaste.net>
Date: Apr 26, 2023, 20:40 UTC
Message-ID: <ZEmMUFR7AJn+v7jV@tapette.crustytoothpaste.net>
In-Reply-To: <20230426105134.GA130148@coredump.intra.peff.net>

On 2023-04-26 at 10:51:34, Jeff King wrote:

Show 5 quoted lines

> 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.

> 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.

Show 7 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.

-- 
brian m. carlson (he/him or they/them)
Toronto, Ontario, CA