# Discover target HEAD via concurrent upload-pack info-refs

`03819d2`·

Soph·2mo ago·2 files·+44 added/-7 removed

receive-pack advertisements don't include HEAD (server-side go-git's addReferences passes addHead=false for forPush, matching real git servers). So the session now does a second info-refs against the target's upload-pack endpoint, runs it concurrently with the existing receive-pack info-refs, and parses HEAD's symref capability from the result.

Cost: one extra HTTP GET per session, parallelised with the existing target request → effectively zero added latency. Failures are non-fatal: push-only auth that 401s on upload-pack just leaves targetSession.headTarget empty.

`gitproto.DiscoverHEAD` wraps the v1 advertisement parse. The session goroutine swallows errors; future telemetry could surface them if useful.

Refs #45.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

## Sessions

bf6948e65b2dView transcript

[?\
do we have integration tests?Claude Code·Opus 4.7[1m]·1 step](/content/gh/entireio/git-sync/session/d0406407-d612-489d-b375-1372d062af82#timeline-bf6948e65b2d/index.html)

## Changes

2

- internal

- gitproto

- Mrefs.go+17

- syncer

- Msyncer.go+27/-7

```
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}
```

// DiscoverHEAD reads HEAD's symref target from an upload-pack info-refs
// advertisement against conn. Used to find the target's default branch
// since the receive-pack advertisement we already query doesn't include
// HEAD (server-side `addReferences` passes addHead=false for forPush).
//
// Returns empty when HEAD is detached, the target advertisement omits it
// (typical for empty bare repos where HEAD's underlying ref doesn't yet
// exist), or any error occurs. Errors are returned for telemetry but
// callers typically treat this as best-effort.
func DiscoverHEAD(ctx context.Context, conn *Conn) (plumbing.ReferenceName, error) {
	adv, err := AdvertisedRefsV1(ctx, conn, transport.UploadPackService)
	if err != nil {
		return "", fmt.Errorf("upload-pack info-refs: %w", err)
	}
	return headTargetFromAdv(adv), nil
}

// headTargetFromAdv extracts the branch HEAD points to from v1 advertised
// capabilities. Returns empty when HEAD is detached or no symref is advertised.
func headTargetFromAdv(adv *packp.AdvRefs) plumbing.ReferenceName {
```

Minternal/gitproto/refs.go+17

```

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}

type targetSession struct {
	conn     *gitproto.Conn
	adv      *packp.AdvRefs
	refMap   map[plumbing.ReferenceName]plumbing.Hash
	features gitproto.TargetFeatures
	policy   planner.RelayTargetPolicy
	pusher   *gitproto.Pusher
	conn       *gitproto.Conn
	adv        *packp.AdvRefs
	refMap     map[plumbing.ReferenceName]plumbing.Hash
	features   gitproto.TargetFeatures
	policy     planner.RelayTargetPolicy
	pusher     *gitproto.Pusher
	headTarget plumbing.ReferenceName
}

// newSession performs the shared setup: protocol validation, mapping validation,
55 unmodified lines

return nil, fmt.Errorf("create target transport: %w", err)
	}
		targetConn.ProgressOut = &sessionStderr{s: s}

// Run upload-pack discovery concurrently with the receive-pack
		// advertisement: receive-pack tells us capabilities and current
		// refs (for push), upload-pack tells us HEAD's symref target
		// (receive-pack omits HEAD by protocol design). Errors on the
		// upload-pack side are non-fatal — push-only auth, for example,
		// would 401 and we just leave headTarget empty.
		headCh := make(chan plumbing.ReferenceName, 1)
		go func() {
			head, err := gitproto.DiscoverHEAD(ctx, targetConn)
			if err != nil {
				head = ""
			}
			headCh <- head
		}()

targetAdv, err := gitproto.AdvertisedRefsV1(ctx, targetConn, transport.ReceivePackService)
		if err != nil {
			<-headCh
			return nil, fmt.Errorf("list target refs: %w", err)
		}
		targetRefSlice, err := gitproto.AdvRefsToSlice(targetAdv)
		if err != nil {
			<-headCh
			return nil, fmt.Errorf("decode target refs: %w", err)
		}
		targetRefMap := gitproto.RefHashMap(targetRefSlice)
7 unmodified lines

CapabilitiesKnown: targetFeatures.Known,
			NoThin:            targetFeatures.NoThin,
		},
		pusher: gitproto.NewPusher(targetConn, targetAdv, cfg.Verbose),
		pusher:     gitproto.NewPusher(targetConn, targetAdv, cfg.Verbose),
		headTarget: <-headCh,
	}
	if cfg.BestEffort {
		s.rejections = make(map[plumbing.ReferenceName]string)