Subject: Re: [PATCH] upload-pack: strip namespace from symref data

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Wed, 22 May 2019 12:33:56 +0200

On Wed, May 22 2019, Jeff King wrote:

> Since 7171d8c15f (upload-pack: send symbolic ref information as
> capability, 2013-09-17), we've sent cloning and fetching clients special
> information about which branch HEAD is pointing to, so that they don't
> have to guess based on matching up commit ids.

I'd add a 2nd paragraph here:

    The client will then use the corresponding code added in a45b5f0552
    ("connect: annotate refs with their symref information in
    get_remote_head()", 2013-09-17) to find the intended symref. See
    9907d1359c ("Merge branch 'jc/upload-pack-send-symref'", 2013-10-30)
    for the full set of changes related to this.

...because, see later...

> However, this feature has never worked properly with the GIT_NAMESPACE
> feature.  Because upload-pack uses head_ref_namespaced(find_symref), we
> do find and report on refs/namespaces/foo/HEAD instead of the actual
> HEAD of the repo. This makes sense, since the branch pointed to by the
> top-level HEAD may not be advertised at all. But we do two things wrong:
>
>   1. We report the full name refs/namespaces/foo/HEAD, instead of just
>      HEAD. Meaning no client is going to bother doing anything with that
>      symref, since we're not otherwise advertising it.
>
>   2. We report the symref destination using its full name (e.g.,
>      refs/namespaces/foo/refs/heads/master). That's similarly useless to
>      the client, who only saw "refs/heads/master" in the advertisement.
>
> We should be stripping the namespace prefix off of both places (which
> this patch fixes).
>
> Likely nobody noticed because bug (1) means that from the client's
> perspective, we did not report on HEAD at all. And thus it uses the
> pre-7171d8c15f fallback code to guess the correct HEAD, which is usually
> right. It only falls down in ambiguous cases (like the one laid out in
> the included test).

...because here you're talking about "the client's perspective" and "it
uses the pre-7171d8c15f [...] code", but this should say "the
pre-a45b5f0552 code", i.e. mention the commit that changed the *client*
logic.

Well, the client also "uses" the server-side code indirectly, but I
think it's easier to follow along if we note what both sides are (as I
dug up when reviewing this...).

> This also means that we don't have to worry about breaking anybody who
> was putting pre-stripped names into their namespace symrefs when we fix
> bug (2). Because of bug (1), nobody would have been using the symref we
> advertised in the first place (not to mention that those symrefs would
> have appeared broken for any non-namespaced access).
>
> Note that we have separate fixes here for the v0 and v2 protocols. The
> symref advertisement moved in v2 to be a part of the ls-refs command.
> This actually gets part (1) right, since the symref annotation
> piggy-backs on the existing ref advertisement, which is properly
> stripped. But it still needs a fix for part (2). The included tests
> cover both protocols.
>
> Reported-by: Bryan Turner <[email-masked]>
> Signed-off-by: Jeff King <[email-masked]>
> ---
> This is the same as my earlier fix, but with the v2 bit added, and of
> course tests and a commit message. Thanks (as usual) for a helpful bug
> report.
>
> I don't know if we have a general philosophy for testing v0 versus v2.
> Without specifying the protocol at all, we'd catch the former on a
> regular run and the latter under a GIT_TEST_PROTOCOL_VERSION=2 run. So
> we _could_ just rely on that, but since I had to do two separate fixes,
> it made sense to me to include explicit tests.

Yeah, we shouldn't rely on the GIT_TEST_* stuff for coverage, it should
just be used to find things that fall between the cracks.

>  ls-refs.c                        |  3 ++-
>  t/t5509-fetch-push-namespaces.sh | 28 ++++++++++++++++++++++++++++
>  upload-pack.c                    |  4 ++--
>  3 files changed, 32 insertions(+), 3 deletions(-)
>
> diff --git a/ls-refs.c b/ls-refs.c
> index 0a7dbc6442..818aef70a0 100644
> --- a/ls-refs.c
> +++ b/ls-refs.c
> @@ -57,7 +57,8 @@ static int send_ref(const char *refname, const struct object_id *oid,
>  		if (!symref_target)
>  			die("'%s' is a symref but it is not?", refname);
>
> -		strbuf_addf(&refline, " symref-target:%s", symref_target);
> +		strbuf_addf(&refline, " symref-target:%s",
> +			    strip_namespace(symref_target));
>  	}
>
>  	if (data->peel) {
> diff --git a/t/t5509-fetch-push-namespaces.sh b/t/t5509-fetch-push-namespaces.sh
> index c88df78c0b..75cbfcc392 100755
> --- a/t/t5509-fetch-push-namespaces.sh
> +++ b/t/t5509-fetch-push-namespaces.sh
> @@ -124,4 +124,32 @@ test_expect_success 'try to update a hidden full ref' '
>  	test_must_fail git -C original push pushee-namespaced master
>  '
>
> +test_expect_success 'set up ambiguous HEAD' '
> +	git init ambiguous &&
> +	(
> +		cd ambiguous &&
> +		git commit --allow-empty -m foo &&
> +		git update-ref refs/namespaces/ns/refs/heads/one HEAD &&
> +		git update-ref refs/namespaces/ns/refs/heads/two HEAD &&
> +		git symbolic-ref refs/namespaces/ns/HEAD \
> +			refs/namespaces/ns/refs/heads/two
> +	)
> +'
> +
> +test_expect_success 'clone chooses correct HEAD (v0)' '
> +	GIT_NAMESPACE=ns git -c protocol.version=0 \
> +		clone ambiguous ambiguous-v0 &&
> +	echo refs/heads/two >expect &&
> +	git -C ambiguous-v0 symbolic-ref HEAD >actual &&
> +	test_cmp expect actual
> +'
> +
> +test_expect_success 'clone chooses correct HEAD (v2)' '
> +	GIT_NAMESPACE=ns git -c protocol.version=2 \
> +		clone ambiguous ambiguous-v2 &&
> +	echo refs/heads/two >expect &&
> +	git -C ambiguous-v2 symbolic-ref HEAD >actual &&
> +	test_cmp expect actual
> +'
> +
>  test_done
> diff --git a/upload-pack.c b/upload-pack.c
> index 24298913c0..4d2129e7fc 100644
> --- a/upload-pack.c
> +++ b/upload-pack.c
> @@ -1037,8 +1037,8 @@ static int find_symref(const char *refname, const struct object_id *oid,
>  	symref_target = resolve_ref_unsafe(refname, 0, NULL, &flag);
>  	if (!symref_target || (flag & REF_ISSYMREF) == 0)
>  		die("'%s' is a symref but it is not?", refname);
> -	item = string_list_append(cb_data, refname);
> -	item->util = xstrdup(symref_target);
> +	item = string_list_append(cb_data, strip_namespace(refname));
> +	item->util = xstrdup(strip_namespace(symref_target));
>  	return 0;
>  }