Subject: Re: [PATCH v2 2/3] send-pack: fix push nego. when remote has refs

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Tue, 27 Jul 2021 10:09:35 +0200

On Thu, Jul 15 2021, Jonathan Tan wrote:

> Commit 477673d6f3 ("send-pack: support push negotiation", 2021-05-05)
> did not test the case in which a remote advertises at least one ref. In
> such a case, "remote_refs" in get_commons_through_negotiation() in
> send-pack.c would also contain those refs with a zero ref->new_oid (in
> addition to the refs being pushed with a nonzero ref->new_oid). Passing
> them as negotiation tips to "git fetch" causes an error, so filter them
> out.
>
> (The exact error that would happen in "git fetch" in this case is a
> segmentation fault, which is unwanted. This will be fixed in the
> subsequent commit.)

Let's add the test from the subsequent here as a test_expect_failure and
flip it to "success".

> @@ -425,8 +425,10 @@ static void get_commons_through_negotiation(const char *url,
>  	child.no_stdin = 1;
>  	child.out = -1;
>  	strvec_pushl(&child.args, "fetch", "--negotiate-only", NULL);
> -	for (ref = remote_refs; ref; ref = ref->next)
> -		strvec_pushf(&child.args, "--negotiation-tip=%s", oid_to_hex(&ref->new_oid));
> +	for (ref = remote_refs; ref; ref = ref->next) {
> +		if (!is_null_oid(&ref->new_oid))
> +			strvec_pushf(&child.args, "--negotiation-tip=%s", oid_to_hex(&ref->new_oid));
> +	}
>  	strvec_push(&child.args, url);

This will run into my eff40457a4 (fetch: fix segfault in
--negotiate-only without --negotiation-tip=*, 2021-07-08) if we supply a
--negotiate-only without --negotiation-tip=, but trying it it looks like
even when you push to an empty repo and your repo is itself empty we'll
always add the tip you're pushing as the negotiation tip.

Let's add a test for that, i.e. I instrumented your test to check what
happens whe I do the push without any remote/local refs, both for
one/both cases (and both combinations), it seems to work...

For code that's doing a loop over "refs" testing that seems to be
worthwhile, i.e. we don't actually depend on "refs" in the sense that
they exist, but the refs we've constructed in-memory to be created on
the remote, correct?

I.e. this on top would be OK (not saying you need this, but I for one
would find it easier to follow with this):
	
	diff --git a/send-pack.c b/send-pack.c
	index b3a495b7b1..d1e231076c 100644
	--- a/send-pack.c
	+++ b/send-pack.c
	@@ -420,15 +420,20 @@ static void get_commons_through_negotiation(const char *url,
	 	struct child_process child = CHILD_PROCESS_INIT;
	 	const struct ref *ref;
	 	int len = the_hash_algo->hexsz + 1; /* hash + NL */
	+	int got_tip = 0;
	 
	 	child.git_cmd = 1;
	 	child.no_stdin = 1;
	 	child.out = -1;
	 	strvec_pushl(&child.args, "fetch", "--negotiate-only", NULL);
	 	for (ref = remote_refs; ref; ref = ref->next) {
	-		if (!is_null_oid(&ref->new_oid))
	-			strvec_pushf(&child.args, "--negotiation-tip=%s", oid_to_hex(&ref->new_oid));
	+		if (is_null_oid(&ref->new_oid))
	+			continue;
	+		strvec_pushf(&child.args, "--negotiation-tip=%s", oid_to_hex(&ref->new_oid));
	+		got_tip = 1;
	 	}
	+	if (!got_tip)
	+		BUG("should get at least one ref tip, even with no remote/local refs");
	 	strvec_push(&child.args, url);
	 
	 	if (start_command(&child))

But also: looking at the trace output we already have the ref
advertisement at this point, so in the case of an empty repo we'll see
it has no refs, but then we're going to provide a --negotiation-tip=*
pointing to our local OID anyway.

That seems like a fairly non-obvious edge case that should be called out
/ tested.

I.e. aren't we at least just going to engage in redundant work there in
trying to negotiate with empty repos, or is it going to noop anyway.

Or maybe we'll get lucky and they have the OID already, they just
recently deleted their reference(s), then we won't need to send as much
over? Is that what this is trying to do?

But hrm, won't that sort of thing increase the odds of repository
corruption?

I.e. now we make the implicit assumption that an OID we see in the
advertisement is one the server isn't going to aggressively prune while
our push is underday (Jeff King has a good E-Mail summarizing that
somewhere, not digging it up now, but I could...).

So such a remote will negotiate with us using that OID, but unlike with
advertised OIDs we can't safely assume that the OID won't be racily
deleted during our negotiation.

Or maybe I'm entirely wrong here....