usbip: inline thin wrappers and drop pass-through darwin session

Apply the <10-LoC / <3-non-test-caller heuristic across the package:
inline writeControlFrame, removeLeaseWaiter, vhciAttach, vhciHubForSpeed,
applyControlSnapshot, applyRemoteEntries, shouldRetryBusID, entrySerial,
and the trivial darwin/linux import-host constructors at their call
sites.

Delete the darwinClientSession wrapper -- darwinVirtualController already
satisfies DataSession, so attach the Description() method to it directly
and return the controller from darwinImportHost.Attach.

Fold the 11-line data_session.go into host.go beside AttachedSession,
which embeds DataSession.

service/usbip/client.go

@@ -139,7 +139,16 @@ func (c *ClientService) runBusIDLoop(ctx context.Context, busid, description str
 		if ctx.Err() != nil {
 			return
 		}
-		if !c.shouldRetryBusID(ctx, busid) {
+		retry := true
+		if !c.assignment.Matched() {
+			err = c.syncRemoteStateContext(ctx)
+			if err != nil {
+				c.logger.Warn("refresh remote exports after releasing ", busid, ": ", err)
+			} else {
+				retry = c.assignment.IsRetryDesired(busid)
+			}
+		}
+		if !retry {
 			c.logger.Info("remote export ", busid, " disappeared; stopping import worker")
 			return
 		}

service/usbip/client_control.go

@@ -72,7 +72,9 @@ func (s *clientControlSession) requestLease(ctx context.Context, busid string) (
 		Version: controlProtocolVersion,
 	}, request)
 	if err != nil {
-		s.removeLeaseWaiter(nonce)
+		s.access.Lock()
+		delete(s.pending, nonce)
+		s.access.Unlock()
 		return controlLeaseResponse{}, err
 	}
 
@@ -80,7 +82,9 @@ func (s *clientControlSession) requestLease(ctx context.Context, busid string) (
 	case result := <-waiter:
 		return result.response, result.err
 	case <-ctx.Done():
-		s.removeLeaseWaiter(nonce)
+		s.access.Lock()
+		delete(s.pending, nonce)
+		s.access.Unlock()
 		return controlLeaseResponse{}, ctx.Err()
 	}
 }
@@ -100,12 +104,6 @@ func (s *clientControlSession) deliverLeaseResponse(response controlLeaseRespons
 	return true
 }
 
-func (s *clientControlSession) removeLeaseWaiter(nonce uint64) {
-	s.access.Lock()
-	delete(s.pending, nonce)
-	s.access.Unlock()
-}
-
 func (s *clientControlSession) closeWithError(err error) {
 	s.access.Lock()
 	if s.closed {
@@ -144,15 +142,6 @@ func (c *ClientService) requestImportLease(ctx context.Context, busid string) (c
 	}, nil
 }
 
-func (c *ClientService) applyControlSnapshot(snapshot controlDeviceSnapshot) {
-	devices := deviceInfoV2Map(snapshot.Devices)
-	values := sortedDeviceInfoV2Values(devices)
-	c.remoteAccess.Lock()
-	c.remoteDevicesV2 = devices
-	c.remoteAccess.Unlock()
-	c.applyRemoteDeviceState(values)
-}
-
 func (c *ClientService) applyControlDelta(delta controlDeviceDelta) {
 	c.remoteAccess.Lock()
 	if c.remoteDevicesV2 == nil {
@@ -194,6 +183,10 @@ func (c *ClientService) syncRemoteStateAndResetControlState(ctx context.Context)
 	c.remoteAccess.Lock()
 	c.remoteDevicesV2 = devices
 	c.remoteAccess.Unlock()
-	c.applyRemoteEntries(entries)
+	if !c.assignment.Matched() {
+		c.applyRemoteExports(entries)
+		return nil
+	}
+	c.applyMatchedExportsWithRetained(entries, nil)
 	return nil
 }

service/usbip/client_darwin.go

@@ -34,7 +34,7 @@ type darwinPendingSubmit struct {
 	reply     chan SubmitResponse
 }
 
-var _ DataSession = (*darwinVirtualController)(nil)
+var _ AttachedSession = (*darwinVirtualController)(nil)
 
 type darwinVirtualController struct {
 	ctx       context.Context
@@ -124,6 +124,10 @@ func (c *darwinVirtualController) Err() error {
 	return c.runErr
 }
 
+func (c *darwinVirtualController) Description() string {
+	return "IOUSBHostControllerInterface"
+}
+
 func (c *darwinVirtualController) enqueueEvent(event darwinControllerEvent) {
 	select {
 	case c.events <- event:

service/usbip/client_shared.go

@@ -99,11 +99,11 @@ func (c *ClientService) runControlSession() error {
 	if err != nil {
 		return E.Cause(errControlUnsupported, "write control preface: ", err)
 	}
-	err = writeControlFrame(conn, controlFrame{
+	err = writeControlMessage(conn, controlFrame{
 		Type:         controlFrameHello,
 		Version:      controlProtocolVersion,
 		Capabilities: controlCapabilities,
-	})
+	}, nil)
 	if err != nil {
 		return E.Cause(errControlUnsupported, "write control hello: ", err)
 	}
@@ -190,7 +190,12 @@ func (c *ClientService) runControlSession() error {
 				return E.Cause(errImmediateReconnect, "read device snapshot: ", err)
 			}
 			lastSeq = frame.Sequence
-			c.applyControlSnapshot(snapshot)
+			devices := deviceInfoV2Map(snapshot.Devices)
+			values := sortedDeviceInfoV2Values(devices)
+			c.remoteAccess.Lock()
+			c.remoteDevicesV2 = devices
+			c.remoteAccess.Unlock()
+			c.applyRemoteDeviceState(values)
 		case controlFrameDeviceDelta:
 			if !extended {
 				return E.Cause(errImmediateReconnect, "unexpected control frame ", frame.Type)
@@ -254,16 +259,12 @@ func (c *ClientService) syncRemoteStateContext(ctx context.Context) error {
 	if err != nil {
 		return err
 	}
-	c.applyRemoteEntries(entries)
-	return nil
-}
-
-func (c *ClientService) applyRemoteEntries(entries []DeviceEntry) {
 	if !c.assignment.Matched() {
 		c.applyRemoteExports(entries)
-		return
+		return nil
 	}
 	c.applyMatchedExportsWithRetained(entries, nil)
+	return nil
 }
 
 func (c *ClientService) applyRemoteDeviceState(devices []DeviceInfoV2) {
@@ -448,15 +449,3 @@ func (c *ClientService) fetchDevList(ctx context.Context) ([]DeviceEntry, error)
 	}
 	return ReadOpRepDevListBody(conn)
 }
-
-func (c *ClientService) shouldRetryBusID(ctx context.Context, busid string) bool {
-	if c.assignment.Matched() {
-		return true
-	}
-	err := c.syncRemoteStateContext(ctx)
-	if err != nil {
-		c.logger.Warn("refresh remote exports after releasing ", busid, ": ", err)
-		return true
-	}
-	return c.assignment.IsRetryDesired(busid)
-}

service/usbip/control_protocol.go

@@ -161,10 +161,6 @@ func (cr *controlReader) read(r io.Reader) (controlMessage, error) {
 	return controlMessage{Frame: frame, Payload: payload}, nil
 }
 
-func writeControlFrame(w io.Writer, frame controlFrame) error {
-	return writeControlMessage(w, frame, nil)
-}
-
 func writeControlMessage(w io.Writer, frame controlFrame, payload any) error {
 	rawPayload, err := marshalControlPayload(payload)
 	if err != nil {
@@ -224,12 +220,16 @@ func deviceInfoV2FromEntry(entry DeviceEntry, backend string, stableID string, s
 	if state == "" {
 		state = deviceStateAvailable
 	}
+	serial := entry.Serial
+	if serial == "" {
+		serial = entry.Info.SerialString()
+	}
 	return DeviceInfoV2{
 		BusID:              entry.Info.BusIDString(),
 		StableID:           stableID,
 		Backend:            backend,
 		Path:               cstring(entry.Info.Path[:]),
-		Serial:             entrySerial(entry),
+		Serial:             serial,
 		VendorID:           entry.Info.IDVendor,
 		ProductID:          entry.Info.IDProduct,
 		BCDDevice:          entry.Info.BCDDevice,

service/usbip/darwin_integration_test.go

@@ -144,7 +144,7 @@ func TestDarwinClientSessionClosesOnContextCancel(t *testing.T) {
 	defer serverConn.Close()
 	controller := newDarwinVirtualController(context.Background(), newTestLogger(t), clientConn, DeviceInfoTruncated{})
 	go controller.readLoop()
-	session := &darwinClientSession{controller: controller}
+	session := controller
 
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
@@ -304,11 +304,11 @@ func (s *darwinFakeUSBIPServer) handleControlConn(conn net.Conn) {
 		return
 	}
 	capabilities := hello.Capabilities & controlCapabilities
-	err = writeControlFrame(conn, controlFrame{
+	err = writeControlMessage(conn, controlFrame{
 		Type:         controlFrameAck,
 		Version:      controlProtocolVersion,
 		Capabilities: capabilities,
-	})
+	}, nil)
 	if err != nil {
 		return
 	}
@@ -345,7 +345,7 @@ func (s *darwinFakeUSBIPServer) handleControlConn(conn net.Conn) {
 			}
 			return
 		}
-		err = writeControlFrame(conn, controlFrame{Type: controlFramePong, Version: controlProtocolVersion})
+		err = writeControlMessage(conn, controlFrame{Type: controlFramePong, Version: controlProtocolVersion}, nil)
 		if err != nil {
 			return
 		}

service/usbip/data_session.go

@@ -1,11 +0,0 @@
-package usbip
-
-// DataSession implementations MUST close the channel returned by Done
-// when the session terminates for any reason. Err is only valid after
-// Done is closed; it returns nil for a clean detach. Close is idempotent
-// and safe to call from any goroutine.
-type DataSession interface {
-	Done() <-chan struct{}
-	Err() error
-	Close() error
-}

service/usbip/host.go

@@ -48,6 +48,16 @@ type ExportSnapshot struct {
 	RawStatus    int
 }
 
+// DataSession implementations MUST close the channel returned by Done
+// when the session terminates for any reason. Err is only valid after
+// Done is closed; it returns nil for a clean detach. Close is idempotent
+// and safe to call from any goroutine.
+type DataSession interface {
+	Done() <-chan struct{}
+	Err() error
+	Close() error
+}
+
 type AttachedSession interface {
 	DataSession
 	Description() string

service/usbip/host_darwin.go

@@ -24,7 +24,7 @@ func newPlatformExportHost(logger log.ContextLogger, matches []option.USBIPDevic
 }
 
 func newPlatformImportHost(logger log.ContextLogger) (ImportHost, error) {
-	return newDarwinImportHost(logger), nil
+	return &darwinImportHost{logger: logger}, nil
 }
 
 // darwinExportHost retains stale captures: devices that reconcile
@@ -285,10 +285,6 @@ type darwinImportHost struct {
 	logger log.ContextLogger
 }
 
-func newDarwinImportHost(logger log.ContextLogger) *darwinImportHost {
-	return &darwinImportHost{logger: logger}
-}
-
 func (h *darwinImportHost) Start(ctx context.Context) error {
 	return nil
 }
@@ -304,27 +300,7 @@ func (h *darwinImportHost) Attach(ctx context.Context, info DeviceInfoTruncated,
 		_ = controller.Close()
 		return nil, err
 	}
-	return &darwinClientSession{controller: controller}, nil
-}
-
-type darwinClientSession struct {
-	controller *darwinVirtualController
-}
-
-func (s *darwinClientSession) Done() <-chan struct{} {
-	return s.controller.Done()
-}
-
-func (s *darwinClientSession) Err() error {
-	return s.controller.Err()
-}
-
-func (s *darwinClientSession) Close() error {
-	return s.controller.Close()
-}
-
-func (s *darwinClientSession) Description() string {
-	return "IOUSBHostControllerInterface"
+	return controller, nil
 }
 
 var _ DataSession = (*darwinServerDataSession)(nil)

service/usbip/host_linux.go

@@ -26,7 +26,10 @@ func newPlatformExportHost(logger log.ContextLogger, matches []option.USBIPDevic
 }
 
 func newPlatformImportHost(logger log.ContextLogger) (ImportHost, error) {
-	return newLinuxImportHost(logger), nil
+	return &linuxImportHost{
+		logger: logger,
+		ports:  make(map[int]struct{}),
+	}, nil
 }
 
 func isMissingUSBDeviceError(err error) bool {
@@ -463,13 +466,6 @@ type linuxImportHost struct {
 	ports       map[int]struct{}
 }
 
-func newLinuxImportHost(logger log.ContextLogger) *linuxImportHost {
-	return &linuxImportHost{
-		logger: logger,
-		ports:  make(map[int]struct{}),
-	}
-}
-
 func (h *linuxImportHost) Start(ctx context.Context) error {
 	return ensureKernelPath(sysVHCIControllerV0, "vhci-hcd", "vhci_hcd.0")
 }
@@ -512,7 +508,8 @@ func (h *linuxImportHost) attachOnce(ctx context.Context, info DeviceInfoTruncat
 			triedPorts[port] = struct{}{}
 			continue
 		}
-		err = vhciAttach(port, handoff.file.Fd(), info.DevID(), info.Speed)
+		attachLine := fmt.Sprintf("%d %d %d %d", port, int(handoff.file.Fd()), info.DevID(), info.Speed)
+		err = writeSysfs(filepath.Join(sysVHCIControllerV0, "attach"), attachLine)
 		if err != nil {
 			h.releasePort(port)
 			if errors.Is(err, unix.EBUSY) {

service/usbip/protocol.go

@@ -287,18 +287,15 @@ func trailingCString(b []byte) string {
 	return ""
 }
 
-func entrySerial(entry DeviceEntry) string {
-	if entry.Serial != "" {
-		return entry.Serial
-	}
-	return entry.Info.SerialString()
-}
-
 func entryDeviceKey(entry DeviceEntry) DeviceKey {
+	serial := entry.Serial
+	if serial == "" {
+		serial = entry.Info.SerialString()
+	}
 	return DeviceKey{
 		BusID:     entry.Info.BusIDString(),
 		VendorID:  entry.Info.IDVendor,
 		ProductID: entry.Info.IDProduct,
-		Serial:    entrySerial(entry),
+		Serial:    serial,
 	}
 }

service/usbip/server.go

@@ -198,12 +198,12 @@ func (s *ServerService) handleControlConn(conn net.Conn) {
 	capabilities := hello.Capabilities & controlCapabilities
 	sub, seq := s.ledger.Subscribe(s.ctx, conn, capabilities)
 	defer s.ledger.Unsubscribe(sub)
-	err = writeControlFrame(conn, controlFrame{
+	err = writeControlMessage(conn, controlFrame{
 		Type:         controlFrameAck,
 		Version:      controlProtocolVersion,
 		Capabilities: capabilities,
 		Sequence:     seq,
-	})
+	}, nil)
 	if err != nil {
 		s.logger.Debug("write control ack: ", err)
 		return

service/usbip/sysfs_linux.go

@@ -177,7 +177,11 @@ func vhciPickFreePort(speed uint32, skip map[int]struct{}) (int, error) {
 	if err != nil {
 		return -1, err
 	}
-	targetHub := vhciHubForSpeed(speed)
+	targetHub := "hs"
+	switch speed {
+	case SpeedSuper, SpeedSuperPlus:
+		targetHub = "ss"
+	}
 	for _, record := range records {
 		if record.hub != targetHub || record.state != 4 {
 			continue
@@ -190,11 +194,6 @@ func vhciPickFreePort(speed uint32, skip map[int]struct{}) (int, error) {
 	return -1, E.New("no free ", targetHub, " vhci port")
 }
 
-func vhciAttach(port int, fd uintptr, devid uint32, speed uint32) error {
-	line := fmt.Sprintf("%d %d %d %d", port, int(fd), devid, speed)
-	return writeSysfs(filepath.Join(sysVHCIControllerV0, "attach"), line)
-}
-
 func readVHCIStatus() ([]vhciStatusRecord, error) {
 	raw, err := os.ReadFile(filepath.Join(sysVHCIControllerV0, "status"))
 	if err != nil {
@@ -237,15 +236,6 @@ func parseVHCIStatus(raw string) []vhciStatusRecord {
 	return records
 }
 
-func vhciHubForSpeed(speed uint32) string {
-	switch speed {
-	case SpeedSuper, SpeedSuperPlus:
-		return "ss"
-	default:
-		return "hs"
-	}
-}
-
 func ensureKernelPath(path string, module string, description string) error {
 	_, err := os.Stat(path)
 	if err == nil {