Subject: Re: [PATCH 1/2] upload-pack: send shallow-info before wanted-refs in protocol v2

From: Patrick Steinhardt <[email-masked]>
Date: Mon, 5 Jan 2026 14:00:44 +0100

On Wed, Dec 24, 2025 at 01:35:03AM +0100, Matthew Dodd wrote:
> From: Mats-Dodd <[email-masked]>
> 
> The protocol v2 specification (Documentation/gitprotocol-v2.adoc) defines
> the ordering of optional sections in the fetch response as:
> 
>     [acknowledgments delim-pkt] [shallow-info delim-pkt]
>     [wanted-refs delim-pkt] [packfile-uris delim-pkt]
>     packfile flush-pkt
> 
> However, since the ref-in-want feature was introduced in 516e2b76bdc
> (upload-pack: implement ref-in-want, 2018-06-27), the server sends
> wanted-refs before shallow-info. This violates the specification and
> breaks the client (fetch-pack.c), which expects shallow-info first.
> 
> When a client performs a shallow clone/fetch against a server with
> uploadpack.allowRefInWant=true, the client receives sections in the
> wrong order and fails with:
> 
>     fatal: expected 'packfile', received 'shallow-info'
> 
> Fix by swapping the order of send_shallow_info() and

Nit: is there a word missing here? E.g. "Fix this by..."

> diff --git a/upload-pack.c b/upload-pack.c
> index 1e87ae9559..029ca93e69 100644
> --- a/upload-pack.c
> +++ b/upload-pack.c
> @@ -1830,8 +1830,8 @@ int upload_pack_v2(struct repository *r, struct packet_reader *request)
>  				state = UPLOAD_DONE;
>  			break;
>  		case UPLOAD_SEND_PACK:
> -			send_wanted_ref_info(&data);
>  			send_shallow_info(&data);
> +			send_wanted_ref_info(&data);

Indeed. The accompanying code in "fetch-pack.c" expects information the
other way round:

	if (process_section_header(&reader, "shallow-info", 1))
		receive_shallow_info(args, &reader, shallows, si);

	if (process_section_header(&reader, "wanted-refs", 1))
		receive_wanted_refs(&reader, sought, nr_sought);

The bug seems to exist since the inception of this feature. 516e2b76bd
(upload-pack: implement ref-in-want, 2018-06-27) implements the server
side in the current-broken way, and 733020517a (fetch-pack: implement
ref-in-want, 2018-06-27) implements the client side in the correct way.
So this combination has always been broken, and the fix looks obviously
correct to me indeed.

One nit though: I don't really think it's necessary to split up this
series into two patches. The new test can simply be added to this commit
here.

Thanks!

Patrick