plumbing: http, Fix empty repos on Git v2.41+

Git v2.41.0 comes with [changes](https://github.com/git/git/commit/933e3a4ee205353d8f093d5dfcd226fa432c4e58)
that breaks go-git's assumptions for when detecting empty repositories.

Go-git expects a flush instead of the first hash line. Instead, a dummy capabilities^{}
with zero-id is returned. The change aims to allow for identifying
the object format even when cloning empty
repositories.

Signed-off-by: Paulo Gomes <pjbgf@linux.com>

4 files changed, 27 insertions, 4 deletions

diff --git a/plumbing/protocol/packp/advrefs_decode.go b/plumbing/protocol/packp/advrefs_decode.go
index 63bbe5a..f8d26a2 100644
--- a/plumbing/protocol/packp/advrefs_decode.go
+++ b/plumbing/protocol/packp/advrefs_decode.go
@@ -133,6 +133,7 @@ func decodeFirstHash(p *advRefsDecoder) decoderStateFn {
 		return nil
 	}
 
+	// TODO: Use object-format (when available) for hash size. Git 2.41+
 	if len(p.line) < hashSize {
 		p.error("cannot read hash, pkt-line too short")
 		return nil
diff --git a/plumbing/protocol/packp/advrefs_decode_test.go b/plumbing/protocol/packp/advrefs_decode_test.go
index 83b0b01..d127145 100644
--- a/plumbing/protocol/packp/advrefs_decode_test.go
+++ b/plumbing/protocol/packp/advrefs_decode_test.go
@@ -218,6 +218,16 @@ func (s *AdvRefsDecodeSuite) TestCaps(c *C) {
 			{Name: capability.SymRef, Values: []string{"HEAD:refs/heads/master"}},
 			{Name: capability.Agent, Values: []string{"foo=bar"}},
 		},
+	}, {
+		input: []string{
+			"0000000000000000000000000000000000000000 capabilities^{}\x00report-status report-status-v2 delete-refs side-band-64k quiet atomic ofs-delta object-format=sha1 agent=git/2.41.0\n",
+			pktline.FlushString,
+		},
+		capabilities: []entry{
+			{Name: capability.ReportStatus, Values: []string(nil)},
+			{Name: capability.ObjectFormat, Values: []string{"sha1"}},
+			{Name: capability.Agent, Values: []string{"git/2.41.0"}},
+		},
 	}} {
 		ar := s.testDecodeOK(c, test.input)
 		for _, fixCap := range test.capabilities {
diff --git a/plumbing/transport/http/common.go b/plumbing/transport/http/common.go
index f9b7a0e..a7cdc1e 100644
--- a/plumbing/transport/http/common.go
+++ b/plumbing/transport/http/common.go
@@ -73,6 +73,17 @@ func advertisedReferences(ctx context.Context, s *session, serviceName string) (
 		return nil, err
 	}
 
+	// Git 2.41+ returns a zero-id plus capabilities when an empty
+	// repository is being cloned. This skips the existing logic within
+	// advrefs_decode.decodeFirstHash, which expects a flush-pkt instead.
+	//
+	// This logic aligns with plumbing/transport/internal/common/common.go.
+	if ar.IsEmpty() &&
+		// Empty repositories are valid for git-receive-pack.
+		transport.ReceivePackServiceName != serviceName {
+		return nil, transport.ErrEmptyRemoteRepository
+	}
+
 	transport.FilterUnsupportedCapabilities(ar.Capabilities)
 	s.advRefs = ar
 
diff --git a/remote.go b/remote.go
index 25ec093..679e0af 100644
--- a/remote.go
+++ b/remote.go
@@ -224,11 +224,13 @@ func (r *Remote) PushContext(ctx context.Context, o *PushOptions) (err error) {
 		return err
 	}
 
-	if err = rs.Error(); err != nil {
-		return err
+	if rs != nil {
+		if err = rs.Error(); err != nil {
+			return err
+		}
 	}
 
-	return r.updateRemoteReferenceStorage(req, rs)
+	return r.updateRemoteReferenceStorage(req)
 }
 
 func (r *Remote) useRefDeltas(ar *packp.AdvRefs) bool {
@@ -347,7 +349,6 @@ func (r *Remote) newReferenceUpdateRequest(
 
 func (r *Remote) updateRemoteReferenceStorage(
 	req *packp.ReferenceUpdateRequest,
-	result *packp.ReportStatus,
 ) error {
 
 	for _, spec := range r.c.Fetch {