Re: [PATCH 5/5] serve: add support for a git_config() callback

From: Jeff King <peff@peff.net>
Date: Jun 16, 2021, 16:22 UTC
Message-ID: <YMolS8iJAMgbbg9D@coredump.intra.peff.net>
In-Reply-To: <patch-5.5-c8625025125-20210616T141332Z-avarab@gmail.com>

On Wed, Jun 16, 2021 at 04:16:21PM +0200, Ævar Arnfjörð Bjarmason wrote:

Show 16 quoted lines

> Since the introduction of serve.c we've added git_config() callbacks
> and other config reading for capabilities in the following commits:
> 
> - e20b4192a37 (upload-pack: support hidden refs with protocol v2, 2018-12-18)
> - 59e1205d167 (ls-refs: report unborn targets of symrefs, 2021-02-05)
> - 6b5b6e422ee (serve: advertise session ID in v2 capabilities, 2020-11-11)
> 
> This code can be simplified by simply adding support to serve.c itself
> for reading config at the right time before the capability is
> called. In particular the ensure_config_read() function being gone
> makes this whole thing much simpler.
> 
> A behavior change here is that we're eagerly doing the configuration
> for all our capabilities regardless of which one we end up serving,
> whereas before we'd defer it until the point where we were processing
> the specific command.

Hmm. I like simplifying things, but what's the plan for when we have a v2 receive-pack? We'll need different config based on whether we're serving a fetch or a push.

While working on e20b4192a37, I had originally proposed patches that would pass that context into the serve mechanism (based on whether we were called by upload-pack or receive-pack). But the review there indicated that v2 receive-pack would probably involve an extra capability argument passed by the client to ls-refs.

So we'd eventually need to parse config _after_ we've received the actual ls-refs command. To be clear, this patch isn't breaking anything now. But I think it's painting us into a corner that we'll regret later.

Of course there are ways around this. E.g., we could read config for both operations ahead of time, and then decide which to use later. But that definitely isn't how it works now, and we'd have to refactor the whole ref_is_hidden() system to fix that. Definitely do-able, but given the relatively limited upside of this patch, I'm not sure if it's worth it.

I'd also note that this patch doesn't touch upload_pack_v2() at all, which also reads config. That one is extra-weird because it reads the config fresh for every round of interaction with the client, and then cleans it up. Which led to fixes like the one in 08450ef791 (upload-pack: clear filter_options for each v2 fetch command, 2020-05-08).

Doing a single config read would probably make that simpler. But the config setup is shared between the v0 and v2 functions, so extracting it into serve() would potentially be awkward.

> In particular the ensure_config_read() function being gone
> makes this whole thing much simpler.

I do agree this is awkward. The two calls for that function seem to come from the "should we advertise ls-refs" and "actually do ls-refs" functions. Should we be able to assume that the former ran if we are in the latter? I'm not sure if that's true or not (i.e., because it's rare but possible for a client to make two separate HTTP requests, one to discover our v2 capabilities and another to actually issue ls-refs).

-Peff