[client] Stage incoming file drop payloads through a platform sink

Android cannot address the user's shared storage by path, so a received
file had to land in app-private storage and be copied out afterwards,
needing twice the space of the transfer.

Put the staging area behind a Sink interface the receiver writes every
payload through. The filesystem spool implements it unchanged and stays
the default; a platform that cannot be addressed by path implements the
gomobile-bound half instead and stages payloads wherever it can reach.

The writer reports its own total rather than returning a written count:
gomobile copies a []byte argument into a fresh Java array and carries no
count back out.

A failed delivery now drops the staged payloads. The filesystem spool
swept them up on its next pass, but a sink holding entries the engine
cannot address by path has no such fallback.

client/android/client.go

@@ -113,8 +113,9 @@ type Client struct {
 
 	// The file drop handle survives engine restarts so the UI keeps one listener
 	// registration and one history view across reconnects. See fileDropFor.
-	fileDropMu sync.Mutex
-	fileDrop   *FileDrop
+	fileDropMu   sync.Mutex
+	fileDrop     *FileDrop
+	fileDropSink FileDropSink
 }
 
 func (c *Client) setState(cfg *profilemanager.Config, cacheDir string, cfgPath string, cc *internal.ConnectClient) {

client/android/client_filedrop.go

@@ -10,6 +10,15 @@ import (
 	"github.com/netbirdio/netbird/client/internal"
 )
 
+// SetFileDropSink installs the platform sink incoming payloads are staged
+// through. It takes effect on the next handle the client opens, so the platform
+// sets it before asking for one.
+func (c *Client) SetFileDropSink(sink FileDropSink) {
+	c.fileDropMu.Lock()
+	defer c.fileDropMu.Unlock()
+	c.fileDropSink = sink
+}
+
 // FileDrop returns the handle of the active profile, creating it on first use.
 // The UI calls this to list transfers and change settings while disconnected.
 func (c *Client) FileDrop(configDir string) (*FileDrop, error) {
@@ -32,7 +41,7 @@ func (c *Client) fileDropFor(configDir, profileID string) (*FileDrop, error) {
 		return fd, nil
 	}
 
-	fd, err := NewFileDrop(configDir, profileID)
+	fd, err := NewFileDrop(configDir, profileID, c.fileDropSink)
 	if err != nil {
 		c.fileDropMu.Unlock()
 		return nil, err

client/android/filedrop.go

@@ -28,8 +28,10 @@ type FileDrop struct {
 	listener  FileDropListener
 }
 
-// NewFileDrop opens the file drop state of the given profile.
-func NewFileDrop(configDir, profileID string) (*FileDrop, error) {
+// NewFileDrop opens the file drop state of the given profile. A nil sink leaves
+// payloads staged and delivered on the filesystem, under the destination
+// directory the policy names.
+func NewFileDrop(configDir, profileID string, sink FileDropSink) (*FileDrop, error) {
 	if configDir == "" || profileID == "" {
 		return nil, errors.New("file drop requires a config dir and profile ID")
 	}
@@ -39,12 +41,21 @@ func NewFileDrop(configDir, profileID string) (*FileDrop, error) {
 		return nil, err
 	}
 
+	var platformSink filedrop.Sink
+	if sink != nil {
+		platformSink, err = filedrop.NewPlatformSink(newPlatformSinkAdapter(sink))
+		if err != nil {
+			return nil, fmt.Errorf("wrap file drop sink: %w", err)
+		}
+	}
+
 	fd := &FileDrop{configDir: configDir, profileID: profileID}
 	manager, err := filedrop.NewManager(filedrop.ManagerConfig{
 		Profile: profilemanager.ID(profileID),
 		DataDir: filepath.Join(configDir, filedropDataSubdir, profileID),
 		Store:   filedrop.NewProfileStore(prefs.prefs),
 		Events:  fd.publish,
+		Sink:    platformSink,
 	})
 	if err != nil {
 		return nil, fmt.Errorf("create file drop manager: %w", err)

client/android/filedrop_sink.go

@@ -0,0 +1,83 @@
+//go:build android
+
+package android
+
+import (
+	"fmt"
+
+	"github.com/netbirdio/netbird/client/internal/filedrop"
+)
+
+// FileDropSink stages incoming payloads on the platform's behalf. Android
+// cannot address the user's shared storage by path, so the writing itself
+// happens in Java and the engine only drives it.
+//
+// The methods mirror filedrop.PlatformSink; see that interface for the contract.
+type FileDropSink interface {
+	// DestinationLabel names where payloads land, for the UI to show in place of
+	// a path the platform does not have.
+	DestinationLabel() string
+	Prepare(offerID string) error
+	Received(offerID string, index int) (int64, error)
+	OpenWriter(offerID string, index int, name string, offset int64, size int64) (FileDropWriter, error)
+	Deliver(offerID string) (string, error)
+	Remove(offerID string)
+	Cleanup(maxAgeSeconds int64)
+}
+
+// FileDropWriter is one payload's destination, opened by a FileDropSink.
+//
+// WriteChunk takes the bytes rather than filling a caller-supplied buffer, and
+// the total is read back through Written: gomobile copies a []byte argument
+// into a fresh Java array and never carries a written count back out.
+type FileDropWriter interface {
+	WriteChunk(p []byte) error
+	Close() error
+	Written() int64
+}
+
+// platformSinkAdapter bridges the gomobile-bound FileDropSink onto the
+// interface the engine consumes. The two differ only in the writer type, which
+// gomobile cannot share between packages.
+type platformSinkAdapter struct {
+	sink FileDropSink
+}
+
+func newPlatformSinkAdapter(sink FileDropSink) filedrop.PlatformSink {
+	return &platformSinkAdapter{sink: sink}
+}
+
+func (a *platformSinkAdapter) DestinationLabel() string {
+	return a.sink.DestinationLabel()
+}
+
+func (a *platformSinkAdapter) Prepare(offerID string) error {
+	return a.sink.Prepare(offerID)
+}
+
+func (a *platformSinkAdapter) Received(offerID string, index int) (int64, error) {
+	return a.sink.Received(offerID, index)
+}
+
+func (a *platformSinkAdapter) OpenWriter(offerID string, index int, name string, offset int64, size int64) (filedrop.PlatformWriter, error) {
+	w, err := a.sink.OpenWriter(offerID, index, name, offset, size)
+	if err != nil {
+		return nil, err
+	}
+	if w == nil {
+		return nil, fmt.Errorf("no destination for %s item %d", offerID, index)
+	}
+	return w, nil
+}
+
+func (a *platformSinkAdapter) Deliver(offerID string) (string, error) {
+	return a.sink.Deliver(offerID)
+}
+
+func (a *platformSinkAdapter) Remove(offerID string) {
+	a.sink.Remove(offerID)
+}
+
+func (a *platformSinkAdapter) Cleanup(maxAgeSeconds int64) {
+	a.sink.Cleanup(maxAgeSeconds)
+}

client/internal/filedrop/filedrop_test.go

@@ -166,7 +166,7 @@ func TestAutoAcceptTransfersPayload(t *testing.T) {
 
 	assert.Equal(t, int64(len(content)), lastSent, "progress must reach the full payload size")
 
-	staged, err := os.ReadFile(srv.Spool().Path(id, 0))
+	staged, err := os.ReadFile(srv.FileSpool().Path(id, 0))
 	require.NoError(t, err)
 	assert.Equal(t, content, staged, "staged bytes should match what was sent")
 
@@ -200,7 +200,7 @@ func TestAskModeAcceptReleasesUpload(t *testing.T) {
 	id, err := client.Send(context.Background(), testAddr, []Payload{payload}, nil)
 	require.NoError(t, err)
 
-	staged, err := os.ReadFile(srv.Spool().Path(id, 0))
+	staged, err := os.ReadFile(srv.FileSpool().Path(id, 0))
 	require.NoError(t, err)
 	assert.Equal(t, content, staged, "payload should arrive after acceptance")
 
@@ -226,7 +226,7 @@ func TestAskModeDeclineKeepsPayloadOut(t *testing.T) {
 	id, err := client.Send(context.Background(), testAddr, []Payload{filePayload(t, "x.bin", []byte("data"))}, nil)
 	require.ErrorIs(t, err, ErrDeclined, "sender must see the decline")
 
-	_, statErr := os.Stat(srv.Spool().Path(id, 0))
+	_, statErr := os.Stat(srv.FileSpool().Path(id, 0))
 	assert.True(t, os.IsNotExist(statErr), "declined payload must never be staged")
 }
 
@@ -257,7 +257,7 @@ func TestUploadResumesFromConfirmedOffset(t *testing.T) {
 	require.NoError(t, srv.Spool().Prepare(offer.ID))
 
 	half := int64(len(content) / 2)
-	_, err := srv.Spool().Write(offer.ID, 0, 0, strings.NewReader(string(content[:half])), half)
+	_, err := srv.Spool().Write(offer.ID, 0, "", 0, strings.NewReader(string(content[:half])), half)
 	require.NoError(t, err)
 
 	srv.mu.RLock()
@@ -270,7 +270,7 @@ func TestUploadResumesFromConfirmedOffset(t *testing.T) {
 
 	require.NoError(t, client.putFile(context.Background(), base, offer.ID, 0, payload, confirmed, nil))
 
-	staged, err := os.ReadFile(srv.Spool().Path(offer.ID, 0))
+	staged, err := os.ReadFile(srv.FileSpool().Path(offer.ID, 0))
 	require.NoError(t, err)
 	assert.Equal(t, content, staged, "resumed upload must reconstruct the full payload")
 }
@@ -302,7 +302,7 @@ func TestUploadIsBoundedByAnnouncedSize(t *testing.T) {
 	_, err = io.ReadAll(conn)
 	require.NoError(t, err)
 
-	staged, err := os.ReadFile(srv.Spool().Path(offer.ID, 0))
+	staged, err := os.ReadFile(srv.FileSpool().Path(offer.ID, 0))
 	require.NoError(t, err)
 	assert.Len(t, staged, int(announced), "staged size must be capped at the announced size")
 }
@@ -349,7 +349,7 @@ func TestTextPayloadStaysInline(t *testing.T) {
 	assert.Equal(t, "hello peer", offer.Files[0].Text, "text must arrive in the offer itself")
 	assert.Equal(t, StateCompleted, offer.State, "a text-only offer completes without an upload")
 
-	_, statErr := os.Stat(srv.Spool().Path(id, 0))
+	_, statErr := os.Stat(srv.FileSpool().Path(id, 0))
 	assert.True(t, os.IsNotExist(statErr), "text payloads must not be written to the spool")
 }
 
@@ -583,10 +583,10 @@ func TestSpoolWriteTruncatesStaleTail(t *testing.T) {
 	id := OfferID("offer")
 	require.NoError(t, spool.Prepare(id))
 
-	_, err = spool.Write(id, 0, 0, strings.NewReader("AAAAAAAAAA"), 10)
+	_, err = spool.Write(id, 0, "", 0, strings.NewReader("AAAAAAAAAA"), 10)
 	require.NoError(t, err)
 
-	total, err := spool.Write(id, 0, 2, strings.NewReader("BB"), 10)
+	total, err := spool.Write(id, 0, "", 2, strings.NewReader("BB"), 10)
 	require.NoError(t, err)
 	assert.Equal(t, int64(4), total, "staged size follows the resumed write")
 

client/internal/filedrop/manager.go

@@ -50,6 +50,7 @@ type ManagerConfig struct {
 	Store    Store
 	Events   EventSink
 	OfferTTL time.Duration
+	Sink     Sink
 }
 
 type sendHandle struct {
@@ -69,6 +70,7 @@ type Manager struct {
 	history  *History
 	events   EventSink
 	offerTTL time.Duration
+	sink     Sink
 
 	server     *Server
 	ports      *PortRegistry
@@ -95,6 +97,7 @@ func NewManager(cfg ManagerConfig) (*Manager, error) {
 		history:  LoadHistory(cfg.Store),
 		events:   cfg.Events,
 		offerTTL: cfg.OfferTTL,
+		sink:     cfg.Sink,
 		ports:    NewPortRegistry(),
 		sends:    make(map[OfferID]*sendHandle),
 	}
@@ -141,8 +144,13 @@ func (m *Manager) DeleteTransfer(id OfferID) {
 	m.history.Delete(id)
 }
 
-// DestinationDir returns the directory received files are delivered to.
+// DestinationDir returns where received files are delivered: the platform
+// sink's own name for it when one stages payloads, otherwise the configured
+// directory.
 func (m *Manager) DestinationDir() string {
+	if labeller, ok := m.sink.(interface{ DestinationLabel() string }); ok {
+		return labeller.DestinationLabel()
+	}
 	return m.policy.DestinationDir()
 }
 
@@ -162,6 +170,7 @@ func (m *Manager) StartReceiver(ctx context.Context, addr netip.AddrPort, netsta
 
 	server, err := NewServer(ServerConfig{
 		SpoolDir: filepath.Join(m.dataDir, "spool"),
+		Sink:     m.sink,
 		Policy:   m.policy,
 		Resolver: resolver,
 		Notifier: m,
@@ -577,9 +586,12 @@ func (m *Manager) OnCompleted(offer Offer) {
 		return
 	}
 
-	delivered, err := deliver(server.Spool(), offer, m.policy.DestinationDir())
+	delivered, err := server.Spool().Deliver(offer, m.policy.DestinationDir())
 	if err != nil {
 		log.Errorf("failed to deliver file drop payloads: %v", err)
+		// Nothing will ask for these payloads again, and a sink that cannot
+		// address them by path has no sweep of its own to fall back on.
+		server.Spool().Remove(offer.ID)
 		m.finishTransfer(offer.ID, StateFailed, err.Error())
 		m.emit(EventFailed, m.transferOf(offer.ID))
 		return

client/internal/filedrop/receiver.go

@@ -27,11 +27,11 @@ type receiver struct {
 	resolver    PeerResolver
 	notifier    Notifier
 	offers      *OfferStore
-	spool       *Spool
+	spool       Sink
 	spoolMaxAge time.Duration
 }
 
-func newReceiver(cfg ServerConfig, spool *Spool, maxAge time.Duration) *receiver {
+func newReceiver(cfg ServerConfig, spool Sink, maxAge time.Duration) *receiver {
 	return &receiver{
 		policy:      cfg.Policy,
 		resolver:    cfg.Resolver,
@@ -162,7 +162,7 @@ func (r *receiver) upload(sender senderIdentity, id OfferID, index int, offset i
 		},
 	}
 
-	received, err := r.spool.Write(id, index, offset, staged, size)
+	received, err := r.spool.Write(id, index, offer.Files[index].Name, offset, staged, size)
 
 	// A withdrawn offer is an answer, not a failure: the state it moved to is
 	// the one the user chose, and the spool is already gone.

client/internal/filedrop/server.go

@@ -35,9 +35,12 @@ type Notifier interface {
 	OnWithdrawn(offer Offer)
 }
 
-// ServerConfig configures the receiving side.
+// ServerConfig configures the receiving side. Sink overrides the filesystem
+// spool, for a platform that stages payloads somewhere the engine cannot
+// address; SpoolDir is then unused.
 type ServerConfig struct {
 	SpoolDir    string
+	Sink        Sink
 	Policy      *PolicyStore
 	Resolver    PeerResolver
 	Notifier    Notifier
@@ -70,9 +73,13 @@ func NewServer(cfg ServerConfig) (*Server, error) {
 		return nil, errors.New("receiving policy is required")
 	}
 
-	spool, err := NewSpool(cfg.SpoolDir)
-	if err != nil {
-		return nil, fmt.Errorf("create spool: %w", err)
+	spool := cfg.Sink
+	if spool == nil {
+		fsSpool, err := NewSpool(cfg.SpoolDir)
+		if err != nil {
+			return nil, fmt.Errorf("create spool: %w", err)
+		}
+		spool = fsSpool
 	}
 
 	maxAge := cfg.SpoolMaxAge
@@ -96,10 +103,17 @@ func (s *Server) Offers() *OfferStore {
 }
 
 // Spool returns the staging area, so the platform layer can deliver completed payloads.
-func (s *Server) Spool() *Spool {
+func (s *Server) Spool() Sink {
 	return s.recv.spool
 }
 
+// FileSpool returns the staging area as a filesystem spool, nil when the
+// platform staged payloads elsewhere.
+func (s *Server) FileSpool() *Spool {
+	spool, _ := s.recv.spool.(*Spool)
+	return spool
+}
+
 // Policy returns the active profile's receiving policy store.
 func (s *Server) Policy() *PolicyStore {
 	return s.recv.policy

client/internal/filedrop/sink.go

@@ -0,0 +1,135 @@
+package filedrop
+
+import (
+	"fmt"
+	"io"
+	"strings"
+	"time"
+)
+
+// Sink stages incoming payloads until an offer completes. The receiver writes
+// every payload through it and never touches the filesystem directly, so a
+// platform whose destination is not a filesystem path can implement its own.
+type Sink interface {
+	Prepare(id OfferID) error
+	Received(id OfferID, index int) (int64, error)
+	Write(id OfferID, index int, name string, offset int64, r io.Reader, limit int64) (int64, error)
+	Deliver(offer Offer, destDir string) ([]string, error)
+	Remove(id OfferID)
+	Cleanup(maxAge time.Duration, now time.Time)
+}
+
+// PlatformSink is the platform-facing half of a Sink, bound over gomobile. It
+// carries the same calls with only types gomobile can bind, so a platform can
+// stage payloads somewhere the engine cannot address by path, such as an
+// Android MediaStore entry.
+type PlatformSink interface {
+	// DestinationLabel names where payloads land, for the UI to show in place of
+	// a path the platform does not have.
+	DestinationLabel() string
+	Prepare(offerID string) error
+	Received(offerID string, index int) (int64, error)
+	OpenWriter(offerID string, index int, name string, offset int64, size int64) (PlatformWriter, error)
+	// Deliver publishes an offer's payloads and returns where each one landed,
+	// newline-separated, in the order the offer announced them, text payloads
+	// skipped.
+	Deliver(offerID string) (string, error)
+	Remove(offerID string)
+	Cleanup(maxAgeSeconds int64)
+}
+
+// PlatformWriter is one payload's destination, opened by a PlatformSink.
+//
+// WriteChunk takes the bytes by value rather than filling a caller-supplied
+// buffer: gomobile copies a []byte argument into a fresh Java array, which is
+// the right direction here, but the written count never crosses back, so the
+// writer reports its own total through Written instead, read after Close.
+type PlatformWriter interface {
+	WriteChunk(p []byte) error
+	Close() error
+	Written() int64
+}
+
+// platformSink adapts a PlatformSink to the Sink the receiver uses.
+type platformSink struct {
+	platform PlatformSink
+}
+
+// platformWriter adapts a PlatformWriter to io.Writer.
+type platformWriter struct {
+	w PlatformWriter
+}
+
+// NewPlatformSink wraps a platform-provided sink for the receiver to stage into.
+func NewPlatformSink(platform PlatformSink) (Sink, error) {
+	if platform == nil {
+		return nil, fmt.Errorf("platform sink is required")
+	}
+	return &platformSink{platform: platform}, nil
+}
+
+// DestinationLabel reports where the platform delivers payloads.
+func (s *platformSink) DestinationLabel() string {
+	return s.platform.DestinationLabel()
+}
+
+func (s *platformSink) Prepare(id OfferID) error {
+	return s.platform.Prepare(string(id))
+}
+
+func (s *platformSink) Received(id OfferID, index int) (int64, error) {
+	return s.platform.Received(string(id), index)
+}
+
+func (s *platformSink) Write(id OfferID, index int, name string, offset int64, r io.Reader, limit int64) (int64, error) {
+	if offset < 0 {
+		return 0, fmt.Errorf("negative offset %d", offset)
+	}
+
+	w, err := s.platform.OpenWriter(string(id), index, name, offset, limit)
+	if err != nil {
+		return offset, fmt.Errorf("open destination: %w", err)
+	}
+
+	_, copyErr := io.Copy(&platformWriter{w: w}, io.LimitReader(r, limit-offset))
+	closeErr := w.Close()
+
+	written := w.Written()
+	if copyErr != nil {
+		return written, fmt.Errorf("write destination: %w", copyErr)
+	}
+	if closeErr != nil {
+		return written, fmt.Errorf("close destination: %w", closeErr)
+	}
+	return written, nil
+}
+
+func (s *platformSink) Deliver(offer Offer, _ string) ([]string, error) {
+	delivered, err := s.platform.Deliver(string(offer.ID))
+	if err != nil {
+		return nil, err
+	}
+	return splitDelivered(delivered), nil
+}
+
+func (s *platformSink) Remove(id OfferID) {
+	s.platform.Remove(string(id))
+}
+
+func (s *platformSink) Cleanup(maxAge time.Duration, _ time.Time) {
+	s.platform.Cleanup(int64(maxAge / time.Second))
+}
+
+func (p *platformWriter) Write(b []byte) (int, error) {
+	if err := p.w.WriteChunk(b); err != nil {
+		return 0, err
+	}
+	return len(b), nil
+}
+
+func splitDelivered(delivered string) []string {
+	if delivered == "" {
+		return nil
+	}
+	return strings.Split(delivered, "\n")
+}

client/internal/filedrop/sink_test.go

@@ -0,0 +1,219 @@
+package filedrop
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+type fakePlatformWriter struct {
+	written  []byte
+	offset   int64
+	closed   bool
+	writeErr error
+	closeErr error
+}
+
+type fakePlatformSink struct {
+	writers   map[string]*fakePlatformWriter
+	prepared  []string
+	removed   []string
+	cleanups  []int64
+	openErr   error
+	closeErr  error
+	openCalls []string
+	delivered string
+	label     string
+}
+
+func newFakePlatformSink() *fakePlatformSink {
+	return &fakePlatformSink{writers: map[string]*fakePlatformWriter{}}
+}
+
+func (w *fakePlatformWriter) WriteChunk(p []byte) error {
+	if w.writeErr != nil {
+		return w.writeErr
+	}
+	w.written = append(w.written, p...)
+	return nil
+}
+
+func (w *fakePlatformWriter) Close() error {
+	w.closed = true
+	return w.closeErr
+}
+
+func (w *fakePlatformWriter) Written() int64 {
+	return w.offset + int64(len(w.written))
+}
+
+func (s *fakePlatformSink) DestinationLabel() string {
+	return s.label
+}
+
+func (s *fakePlatformSink) Prepare(offerID string) error {
+	s.prepared = append(s.prepared, offerID)
+	return nil
+}
+
+func (s *fakePlatformSink) Received(offerID string, index int) (int64, error) {
+	w, ok := s.writers[key(offerID, index)]
+	if !ok {
+		return 0, nil
+	}
+	return w.Written(), nil
+}
+
+func (s *fakePlatformSink) OpenWriter(offerID string, index int, name string, offset int64, _ int64) (PlatformWriter, error) {
+	s.openCalls = append(s.openCalls, name)
+	if s.openErr != nil {
+		return nil, s.openErr
+	}
+	w := &fakePlatformWriter{offset: offset, closeErr: s.closeErr}
+	s.writers[key(offerID, index)] = w
+	return w, nil
+}
+
+func (s *fakePlatformSink) Deliver(string) (string, error) {
+	return s.delivered, nil
+}
+
+func (s *fakePlatformSink) Remove(offerID string) {
+	s.removed = append(s.removed, offerID)
+}
+
+func (s *fakePlatformSink) Cleanup(maxAgeSeconds int64) {
+	s.cleanups = append(s.cleanups, maxAgeSeconds)
+}
+
+func key(offerID string, index int) string {
+	return fmt.Sprintf("%s:%d", offerID, index)
+}
+
+func TestPlatformSinkWritesThroughToTheDestination(t *testing.T) {
+	platform := newFakePlatformSink()
+	sink, err := NewPlatformSink(platform)
+	require.NoError(t, err)
+
+	written, err := sink.Write("offer", 0, "report.bin", 0, strings.NewReader("netbird"), 7)
+	require.NoError(t, err)
+	assert.Equal(t, int64(7), written)
+
+	w := platform.writers[key("offer", 0)]
+	assert.Equal(t, "netbird", string(w.written))
+	assert.True(t, w.closed, "the destination must be closed after the write")
+	assert.Equal(t, []string{"report.bin"}, platform.openCalls,
+		"the announced name must reach the platform, which owns the final naming")
+}
+
+func TestPlatformSinkStopsAtTheAnnouncedSize(t *testing.T) {
+	platform := newFakePlatformSink()
+	sink, err := NewPlatformSink(platform)
+	require.NoError(t, err)
+
+	written, err := sink.Write("offer", 0, "x.bin", 0, strings.NewReader("more than announced"), 4)
+	require.NoError(t, err)
+	assert.Equal(t, int64(4), written, "a sender overrunning its own offer must be cut off")
+	assert.Equal(t, "more", string(platform.writers[key("offer", 0)].written))
+}
+
+func TestPlatformSinkResumeReportsTheTotal(t *testing.T) {
+	platform := newFakePlatformSink()
+	sink, err := NewPlatformSink(platform)
+	require.NoError(t, err)
+
+	written, err := sink.Write("offer", 0, "x.bin", 3, strings.NewReader("def"), 6)
+	require.NoError(t, err)
+	assert.Equal(t, int64(6), written,
+		"the platform counts from the offset it resumed at, not from this call alone")
+}
+
+func TestPlatformSinkFailsWhenTheDestinationWillNotOpen(t *testing.T) {
+	platform := newFakePlatformSink()
+	platform.openErr = errors.New("no space")
+	sink, err := NewPlatformSink(platform)
+	require.NoError(t, err)
+
+	written, err := sink.Write("offer", 0, "x.bin", 5, strings.NewReader("data"), 9)
+	require.Error(t, err)
+	assert.Equal(t, int64(5), written, "the offset already staged survives a failed reopen")
+}
+
+func TestPlatformSinkReportsWhatLandedWhenTheCloseFails(t *testing.T) {
+	platform := newFakePlatformSink()
+	sink, err := NewPlatformSink(platform)
+	require.NoError(t, err)
+
+	// A destination that accepts every chunk but fails to flush has to report the
+	// bytes that did land, or a resume would restart from the wrong offset.
+	platform.closeErr = errors.New("flush failed")
+
+	written, err := sink.Write("offer", 0, "x.bin", 0, strings.NewReader("netbird"), 7)
+	require.Error(t, err)
+	assert.Equal(t, int64(7), written)
+}
+
+func TestPlatformSinkRejectsNegativeOffset(t *testing.T) {
+	sink, err := NewPlatformSink(newFakePlatformSink())
+	require.NoError(t, err)
+
+	_, err = sink.Write("offer", 0, "x.bin", -1, strings.NewReader("data"), 4)
+	require.Error(t, err)
+}
+
+func TestPlatformSinkDeliverSplitsTheReportedDestinations(t *testing.T) {
+	platform := newFakePlatformSink()
+	platform.delivered = "content://media/1\ncontent://media/2"
+	sink, err := NewPlatformSink(platform)
+	require.NoError(t, err)
+
+	delivered, err := sink.Deliver(Offer{ID: "offer"}, "")
+	require.NoError(t, err)
+	assert.Equal(t, []string{"content://media/1", "content://media/2"}, delivered)
+}
+
+func TestPlatformSinkDeliverWithoutDestinationsIsEmpty(t *testing.T) {
+	sink, err := NewPlatformSink(newFakePlatformSink())
+	require.NoError(t, err)
+
+	delivered, err := sink.Deliver(Offer{ID: "offer"}, "")
+	require.NoError(t, err)
+	assert.Empty(t, delivered, "a text-only offer delivers nothing, rather than one empty path")
+}
+
+func TestPlatformSinkCleanupPassesWholeSeconds(t *testing.T) {
+	platform := newFakePlatformSink()
+	sink, err := NewPlatformSink(platform)
+	require.NoError(t, err)
+
+	sink.Cleanup(90*time.Minute, time.Now())
+	assert.Equal(t, []int64{5400}, platform.cleanups)
+}
+
+func TestNewPlatformSinkRequiresAPlatform(t *testing.T) {
+	_, err := NewPlatformSink(nil)
+	require.Error(t, err)
+}
+
+func TestManagerReportsThePlatformDestinationLabel(t *testing.T) {
+	platform := newFakePlatformSink()
+	platform.label = "Download/NetBird"
+	sink, err := NewPlatformSink(platform)
+	require.NoError(t, err)
+
+	mgr, err := NewManager(ManagerConfig{
+		Profile: testProfile,
+		DataDir: t.TempDir(),
+		Sink:    sink,
+	})
+	require.NoError(t, err)
+
+	require.NoError(t, mgr.SetDestinationDir("/data/data/io.netbird.client/files"))
+	assert.Equal(t, "Download/NetBird", mgr.DestinationDir(),
+		"the platform's own name for the destination wins over the stored path")
+}

client/internal/filedrop/spool.go

@@ -61,8 +61,10 @@ func (s *Spool) Received(id OfferID, index int) (int64, error) {
 	return info.Size(), nil
 }
 
-// Write appends the payload at offset, truncating any bytes past it first.
-func (s *Spool) Write(id OfferID, index int, offset int64, r io.Reader, limit int64) (int64, error) {
+// Write appends the payload at offset, truncating any bytes past it first. The
+// announced name plays no part: payloads are staged under their index and only
+// take their name in Deliver.
+func (s *Spool) Write(id OfferID, index int, _ string, offset int64, r io.Reader, limit int64) (int64, error) {
 	if offset < 0 {
 		return 0, fmt.Errorf("negative offset %d", offset)
 	}
@@ -92,6 +94,12 @@ func (s *Spool) Write(id OfferID, index int, offset int64, r io.Reader, limit in
 	return offset + written, nil
 }
 
+// Deliver moves an offer's staged payloads into destDir under their announced
+// names and returns where each one landed.
+func (s *Spool) Deliver(offer Offer, destDir string) ([]string, error) {
+	return deliver(s, offer, destDir)
+}
+
 // Path returns the staged path of one item for the platform layer to deliver from.
 func (s *Spool) Path(id OfferID, index int) string {
 	return s.filePath(id, index)