[PATCH] fetch-pack: demonstrate --all failure when remote is empty

[Kirill Smelkov <kirr@nexedi.com>]

( Junio, please pick up the patch provided in the end )

On Tue, Jun 12, 2018 at 06:54:17PM +0000, Kirill Smelkov wrote:
> On Tue, Jun 12, 2018 at 05:48:49AM -0400, Jeff King wrote:
> > On Mon, Jun 11, 2018 at 09:43:02AM +0000, Kirill Smelkov wrote:
[...]

> > > I'm not sure, but I would say that `fetch-pack --all` from an empty
> > > repository should not fail and should just give empty output as fetch
> > > does.
> > 
> > Yeah, that seems reasonable to me. The die() that catches this dates
> > back to 2005-era, and we later taught the "fetch" porcelain to handle
> > this. I don't _think_ anybody would be upset that the plumbing learned
> > to treat this as a noop. It's probably a one-liner change in
> > fetch_pack() to return early instead of dying.
> 
> Ok, I will try to send related testcase, and it is indeed easy to find
> - the fix itself.

I started doing it in full with the following

--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -1581,6 +1581,8 @@ struct ref *fetch_pack(struct fetch_pack_args *args,

        if (!ref) {
                packet_flush(fd[1]);
+               if (nr_sought == 0)	// XXX or better args->fetch_all
+                       return NULL; /* nothing to fetch */
                die(_("no matching remote head"));
        }
        prepare_shallow_info(&si, shallow);


but then came to the fact that !ref fetch_pack() return is analyzed in 2
places:

- in builtin/fetch-pack.c itself:

        ref = fetch_pack(&args, fd, conn, ref, dest, sought, nr_sought,
                 &shallow, pack_lockfile_ptr, protocol_v0);

	...

        ret = !ref;

- and in transport.c in fetch_refs_via_pack():

        case protocol_v1:
        case protocol_v0:
                refs = fetch_pack(&args, data->fd, data->conn,
                                  refs_tmp ? refs_tmp : transport->remote_refs,
                                  dest, to_fetch, nr_heads, &data->shallow,
                                  &transport->pack_lockfile, data->version);
                break;

	...

        if (refs == NULL)
                ret = -1;


As I don't know git codebase well-enough I don't see offhand how to
distinguish empty result from a real error when something was requested
and not fetched. If it would be only builtin/fetch-pack I could start to
play ugly games with analyzing too in the calling site args.fetch_all
and nr_though and if at that level we also know we requested nothing,
don't treat !refs as an error.

However with transport.c being there too, since I'm no longer using
`fetch-pack --all`, now it is best for me to not delve into this story
and just stop with attached patch.

Thanks,
Kirill

---- 8< ----
From 76d80ffcfd4574715545c62413d64d40af063d09 Mon Sep 17 00:00:00 2001
From: Kirill Smelkov <kirr@nexedi.com>
Date: Wed, 13 Jun 2018 15:46:00 +0300
Subject: [PATCH] fetch-pack: demonstrate --all failure when remote is empty

Currently `fetch-pack --all` from an empty repository gives:

	fatal: no matching remote head

However it would be logical for this fetch operation to succeed with
empty result. Add test showing the failure.

Signed-off-by: Kirill Smelkov <kirr@nexedi.com>

---
 t/t5500-fetch-pack.sh | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/t/t5500-fetch-pack.sh b/t/t5500-fetch-pack.sh
index 82aee1c2d8..2234bad411 100755
--- a/t/t5500-fetch-pack.sh
+++ b/t/t5500-fetch-pack.sh
@@ -528,6 +528,14 @@ test_expect_success 'test --all with tag to non-tip' '
 	)
 '
 
+test_expect_failure 'test --all wrt empty.git' '
+	git init --bare empty.git &&
+	(
+		cd client &&
+		git fetch-pack --all ../empty.git
+	)
+'
+
 test_expect_success 'shallow fetch with tags does not break the repository' '
 	mkdir repo1 &&
 	(
-- 
2.18.0.rc1.253.gf85a566b11.dirty