ipn,ipnlocal,taildrop: use SAF for Android files (#15976)

Create FileOps for calling platform-specific file operations such as SAF APIs in Taildrop
Update taildrop.PutFile to support both traditional and SAF modes

Updates tailscale/tailscale#15263

Signed-off-by: kari-ts <kari@tailscale.com>

feature/taildrop/ext.go

@@ -73,6 +73,10 @@ type Extension struct {
 	// *.partial file to its final name on completion.
 	directFileRoot string
 
+	// FileOps abstracts platform-specific file operations needed for file transfers.
+	// This is currently being used for Android to use the Storage Access Framework.
+	FileOps FileOps
+
 	nodeBackendForTest ipnext.NodeBackend // if non-nil, pretend we're this node state for tests
 
 	mu             sync.Mutex // Lock order: lb.mu > e.mu
@@ -85,6 +89,30 @@ type Extension struct {
 	outgoingFiles map[string]*ipn.OutgoingFile
 }
 
+// safDirectoryPrefix is used to determine if the directory is managed via SAF.
+const SafDirectoryPrefix = "content://"
+
+// PutMode controls how Manager.PutFile writes files to storage.
+//
+//	PutModeDirect    – write files directly to a filesystem path (default).
+//	PutModeAndroidSAF – use Android’s Storage Access Framework (SAF), where
+//	                      the OS manages the underlying directory permissions.
+type PutMode int
+
+const (
+	PutModeDirect PutMode = iota
+	PutModeAndroidSAF
+)
+
+// FileOps defines platform-specific file operations.
+type FileOps interface {
+	OpenFileWriter(filename string) (io.WriteCloser, string, error)
+
+	// RenamePartialFile finalizes a partial file.
+	// It returns the new SAF URI as a string and an error.
+	RenamePartialFile(partialUri, targetDirUri, targetName string) (string, error)
+}
+
 func (e *Extension) Name() string {
 	return "taildrop"
 }
@@ -153,12 +181,18 @@ func (e *Extension) onChangeProfile(profile ipn.LoginProfileView, _ ipn.PrefsVie
 	if fileRoot == "" {
 		e.logf("no Taildrop directory configured")
 	}
+	mode := PutModeDirect
+	if e.directFileRoot != "" && strings.HasPrefix(e.directFileRoot, SafDirectoryPrefix) {
+		mode = PutModeAndroidSAF
+	}
 	e.setMgrLocked(managerOptions{
 		Logf:           e.logf,
 		Clock:          tstime.DefaultClock{Clock: e.sb.Clock()},
 		State:          e.stateStore,
 		Dir:            fileRoot,
 		DirectFileMode: isDirectFileMode,
+		FileOps:        e.FileOps,
+		Mode:           mode,
 		SendFileNotify: e.sendFileNotify,
 	}.New())
 }

feature/taildrop/paths.go

@@ -18,6 +18,12 @@ func (e *Extension) SetDirectFileRoot(root string) {
 	e.directFileRoot = root
 }
 
+// SetFileOps sets the platform specific file operations. This is used
+// to call Android's Storage Access Framework APIs.
+func (e *Extension) SetFileOps(fileOps FileOps) {
+	e.FileOps = fileOps
+}
+
 func (e *Extension) setPlatformDefaultDirectFileRoot() {
 	dg := distro.Get()
 

feature/taildrop/send.go

@@ -5,7 +5,7 @@ package taildrop
 
 import (
 	"crypto/sha256"
-	"errors"
+	"fmt"
 	"io"
 	"os"
 	"path/filepath"
@@ -82,126 +82,215 @@ func (m *manager) PutFile(id clientID, baseName string, r io.Reader, offset, len
 	case distro.Get() == distro.Unraid && !m.opts.DirectFileMode:
 		return 0, ErrNotAccessible
 	}
-	dstPath, err := joinDir(m.opts.Dir, baseName)
-	if err != nil {
-		return 0, err
-	}
 
-	redactAndLogError := func(action string, err error) error {
-		err = redactError(err)
-		m.opts.Logf("put %v error: %v", action, err)
-		return err
+	//Compute dstPath & avoid mid‑upload deletion
+	var dstPath string
+	if m.opts.Mode == PutModeDirect {
+		var err error
+		dstPath, err = joinDir(m.opts.Dir, baseName)
+		if err != nil {
+			return 0, err
+		}
+	} else {
+		// In SAF mode, we simply use the baseName as the destination "path"
+		// (the actual directory is managed by SAF).
+		dstPath = baseName
 	}
+	m.deleter.Remove(filepath.Base(dstPath)) // avoid deleting the partial file while receiving
 
 	// Check whether there is an in-progress transfer for the file.
-	partialPath := dstPath + id.partialSuffix()
-	inFileKey := incomingFileKey{id, baseName}
-	inFile, loaded := m.incomingFiles.LoadOrInit(inFileKey, func() *incomingFile {
-		inFile := &incomingFile{
+	partialFileKey := incomingFileKey{id, baseName}
+	inFile, loaded := m.incomingFiles.LoadOrInit(partialFileKey, func() *incomingFile {
+		return &incomingFile{
 			clock:          m.opts.Clock,
 			started:        m.opts.Clock.Now(),
 			size:           length,
 			sendFileNotify: m.opts.SendFileNotify,
 		}
-		if m.opts.DirectFileMode {
-			inFile.partialPath = partialPath
-			inFile.finalPath = dstPath
-		}
-		return inFile
 	})
 	if loaded {
 		return 0, ErrFileExists
 	}
-	defer m.incomingFiles.Delete(inFileKey)
-	m.deleter.Remove(filepath.Base(partialPath)) // avoid deleting the partial file while receiving
+	defer m.incomingFiles.Delete(partialFileKey)
 
-	// Create (if not already) the partial file with read-write permissions.
-	f, err := os.OpenFile(partialPath, os.O_CREATE|os.O_RDWR, 0666)
+	// Open writer & populate inFile paths
+	wc, partialPath, err := m.openWriterAndPaths(id, m.opts.Mode, inFile, baseName, dstPath, offset)
 	if err != nil {
-		return 0, redactAndLogError("Create", err)
+		return 0, m.redactAndLogError("Create", err)
 	}
 	defer func() {
-		f.Close() // best-effort to cleanup dangling file handles
+		wc.Close()
 		if err != nil {
 			m.deleter.Insert(filepath.Base(partialPath)) // mark partial file for eventual deletion
 		}
 	}()
-	inFile.w = f
 
 	// Record that we have started to receive at least one file.
 	// This is used by the deleter upon a cold-start to scan the directory
 	// for any files that need to be deleted.
-	if m.opts.State != nil {
-		if b, _ := m.opts.State.ReadState(ipn.TaildropReceivedKey); len(b) == 0 {
-			if err := m.opts.State.WriteState(ipn.TaildropReceivedKey, []byte{1}); err != nil {
-				m.opts.Logf("WriteState error: %v", err) // non-fatal error
+	if st := m.opts.State; st != nil {
+		if b, _ := st.ReadState(ipn.TaildropReceivedKey); len(b) == 0 {
+			if werr := st.WriteState(ipn.TaildropReceivedKey, []byte{1}); werr != nil {
+				m.opts.Logf("WriteState error: %v", werr) // non-fatal error
 			}
 		}
 	}
 
-	// A positive offset implies that we are resuming an existing file.
-	// Seek to the appropriate offset and truncate the file.
-	if offset != 0 {
-		currLength, err := f.Seek(0, io.SeekEnd)
-		if err != nil {
-			return 0, redactAndLogError("Seek", err)
-		}
-		if offset < 0 || offset > currLength {
-			return 0, redactAndLogError("Seek", err)
-		}
-		if _, err := f.Seek(offset, io.SeekStart); err != nil {
-			return 0, redactAndLogError("Seek", err)
-		}
-		if err := f.Truncate(offset); err != nil {
-			return 0, redactAndLogError("Truncate", err)
-		}
-	}
-
-	// Copy the contents of the file.
-	copyLength, err := io.Copy(inFile, r)
+	// Copy the contents of the file to the writer.
+	copyLength, err := io.Copy(wc, r)
 	if err != nil {
-		return 0, redactAndLogError("Copy", err)
+		return 0, m.redactAndLogError("Copy", err)
 	}
 	if length >= 0 && copyLength != length {
-		return 0, redactAndLogError("Copy", errors.New("copied an unexpected number of bytes"))
+		return 0, m.redactAndLogError("Copy", fmt.Errorf("copied %d bytes; expected %d", copyLength, length))
 	}
-	if err := f.Close(); err != nil {
-		return 0, redactAndLogError("Close", err)
+	if err := wc.Close(); err != nil {
+		return 0, m.redactAndLogError("Close", err)
 	}
+
 	fileLength := offset + copyLength
 
 	inFile.mu.Lock()
 	inFile.done = true
 	inFile.mu.Unlock()
 
-	// File has been successfully received, rename the partial file
-	// to the final destination filename. If a file of that name already exists,
-	// then try multiple times with variations of the filename.
-	computePartialSum := sync.OnceValues(func() ([sha256.Size]byte, error) {
-		return sha256File(partialPath)
-	})
-	maxRetries := 10
-	for ; maxRetries > 0; maxRetries-- {
+	// Finalize rename
+	switch m.opts.Mode {
+	case PutModeDirect:
+		var finalDst string
+		finalDst, err = m.finalizeDirect(inFile, partialPath, dstPath, fileLength)
+		if err != nil {
+			return 0, m.redactAndLogError("Rename", err)
+		}
+		inFile.finalPath = finalDst
+
+	case PutModeAndroidSAF:
+		if err = m.finalizeSAF(partialPath, baseName); err != nil {
+			return 0, m.redactAndLogError("Rename", err)
+		}
+	}
+
+	m.totalReceived.Add(1)
+	m.opts.SendFileNotify()
+	return fileLength, nil
+}
+
+// openWriterAndPaths opens the correct writer, seeks/truncates if needed,
+// and sets inFile.partialPath & inFile.finalPath for later cleanup/rename.
+// The caller is responsible for closing the file on completion.
+func (m *manager) openWriterAndPaths(
+	id clientID,
+	mode PutMode,
+	inFile *incomingFile,
+	baseName string,
+	dstPath string,
+	offset int64,
+) (wc io.WriteCloser, partialPath string, err error) {
+	switch mode {
+
+	case PutModeDirect:
+		partialPath = dstPath + id.partialSuffix()
+		f, err := os.OpenFile(partialPath, os.O_CREATE|os.O_RDWR, 0o666)
+		if err != nil {
+			return nil, "", m.redactAndLogError("Create", err)
+		}
+		if offset != 0 {
+			curr, err := f.Seek(0, io.SeekEnd)
+			if err != nil {
+				f.Close()
+				return nil, "", m.redactAndLogError("Seek", err)
+			}
+			if offset < 0 || offset > curr {
+				f.Close()
+				return nil, "", m.redactAndLogError("Seek", fmt.Errorf("offset %d out of range", offset))
+			}
+			if _, err := f.Seek(offset, io.SeekStart); err != nil {
+				f.Close()
+				return nil, "", m.redactAndLogError("Seek", err)
+			}
+			if err := f.Truncate(offset); err != nil {
+				f.Close()
+				return nil, "", m.redactAndLogError("Truncate", err)
+			}
+		}
+		inFile.w = f
+		wc = f
+		inFile.partialPath = partialPath
+		inFile.finalPath = dstPath
+		return wc, partialPath, nil
+
+	case PutModeAndroidSAF:
+		if m.opts.FileOps == nil {
+			return nil, "", m.redactAndLogError("Create (SAF)", fmt.Errorf("missing FileOps"))
+		}
+		writer, uri, err := m.opts.FileOps.OpenFileWriter(baseName)
+		if err != nil {
+			return nil, "", m.redactAndLogError("Create (SAF)", fmt.Errorf("failed to open file for writing via SAF"))
+		}
+		if writer == nil || uri == "" {
+			return nil, "", fmt.Errorf("invalid SAF writer or URI")
+		}
+		// SAF mode does not support resuming, so enforce offset == 0.
+		if offset != 0 {
+			writer.Close()
+			return nil, "", m.redactAndLogError("Seek", fmt.Errorf("resuming is not supported in SAF mode"))
+		}
+		inFile.w = writer
+		wc = writer
+		partialPath = uri
+		inFile.partialPath = uri
+		inFile.finalPath = baseName
+		return wc, partialPath, nil
+
+	default:
+		return nil, "", fmt.Errorf("unsupported PutMode: %v", mode)
+	}
+}
+
+// finalizeDirect atomically renames or dedups the partial file, retrying
+// under new names up to 10 times. It returns the final path that succeeded.
+func (m *manager) finalizeDirect(
+	inFile *incomingFile,
+	partialPath string,
+	initialDst string,
+	fileLength int64,
+) (string, error) {
+	var (
+		once       sync.Once
+		cachedSum  [sha256.Size]byte
+		cacheErr   error
+		computeSum = func() ([sha256.Size]byte, error) {
+			once.Do(func() { cachedSum, cacheErr = sha256File(partialPath) })
+			return cachedSum, cacheErr
+		}
+	)
+
+	dstPath := initialDst
+	const maxRetries = 10
+	for i := 0; i < maxRetries; i++ {
 		// Atomically rename the partial file as the destination file if it doesn't exist.
 		// Otherwise, it returns the length of the current destination file.
 		// The operation is atomic.
-		dstLength, err := func() (int64, error) {
+		lengthOnDisk, err := func() (int64, error) {
 			m.renameMu.Lock()
 			defer m.renameMu.Unlock()
-			switch fi, err := os.Stat(dstPath); {
-			case os.IsNotExist(err):
+			fi, statErr := os.Stat(dstPath)
+			if os.IsNotExist(statErr) {
+				// dst missing → rename partial into place
 				return -1, os.Rename(partialPath, dstPath)
-			case err != nil:
-				return -1, err
-			default:
-				return fi.Size(), nil
 			}
+			if statErr != nil {
+				return -1, statErr
+			}
+			return fi.Size(), nil
 		}()
 		if err != nil {
-			return 0, redactAndLogError("Rename", err)
+			return "", err
 		}
-		if dstLength < 0 {
-			break // we successfully renamed; so stop
+		if lengthOnDisk < 0 {
+			// successfully moved
+			inFile.finalPath = dstPath
+			return dstPath, nil
 		}
 
 		// Avoid the final rename if a destination file has the same contents.
@@ -209,33 +298,59 @@ func (m *manager) PutFile(id clientID, baseName string, r io.Reader, offset, len
 		// Note: this is best effort and copying files from iOS from the Media Library
 		// results in processing on the iOS side which means the size and shas of the
 		// same file can be different.
-		if dstLength == fileLength {
-			partialSum, err := computePartialSum()
+		if lengthOnDisk == fileLength {
+			partSum, err := computeSum()
 			if err != nil {
-				return 0, redactAndLogError("Rename", err)
+				return "", err
 			}
 			dstSum, err := sha256File(dstPath)
 			if err != nil {
-				return 0, redactAndLogError("Rename", err)
+				return "", err
 			}
-			if dstSum == partialSum {
+			if partSum == dstSum {
+				// same content → drop the partial
 				if err := os.Remove(partialPath); err != nil {
-					return 0, redactAndLogError("Remove", err)
+					return "", err
 				}
-				break // we successfully found a content match; so stop
+				inFile.finalPath = dstPath
+				return dstPath, nil
 			}
 		}
 
 		// Choose a new destination filename and try again.
 		dstPath = nextFilename(dstPath)
-		inFile.finalPath = dstPath
 	}
-	if maxRetries <= 0 {
-		return 0, errors.New("too many retries trying to rename partial file")
+
+	return "", fmt.Errorf("too many retries trying to rename a partial file %q", initialDst)
+}
+
+// finalizeSAF retries RenamePartialFile up to 10 times, generating a new
+// name on each failure until the SAF URI changes.
+func (m *manager) finalizeSAF(
+	partialPath, finalName string,
+) error {
+	if m.opts.FileOps == nil {
+		return fmt.Errorf("missing FileOps for SAF finalize")
 	}
-	m.totalReceived.Add(1)
-	m.opts.SendFileNotify()
-	return fileLength, nil
+	const maxTries = 10
+	name := finalName
+	for i := 0; i < maxTries; i++ {
+		newURI, err := m.opts.FileOps.RenamePartialFile(partialPath, m.opts.Dir, name)
+		if err != nil {
+			return err
+		}
+		if newURI != "" && newURI != name {
+			return nil
+		}
+		name = nextFilename(name)
+	}
+	return fmt.Errorf("failed to finalize SAF file after %d retries", maxTries)
+}
+
+func (m *manager) redactAndLogError(stage string, err error) error {
+	err = redactError(err)
+	m.opts.Logf("put %s error: %v", stage, err)
+	return err
 }
 
 func sha256File(file string) (out [sha256.Size]byte, err error) {

feature/taildrop/send_test.go

@@ -0,0 +1,128 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package taildrop
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"tailscale.com/tstime"
+)
+
+// nopWriteCloser is a no-op io.WriteCloser wrapping a bytes.Buffer.
+type nopWriteCloser struct{ *bytes.Buffer }
+
+func (nwc nopWriteCloser) Close() error { return nil }
+
+// mockFileOps implements just enough of the FileOps interface for SAF tests.
+type mockFileOps struct {
+	writes   *bytes.Buffer
+	renameOK bool
+}
+
+func (m *mockFileOps) OpenFileWriter(name string) (io.WriteCloser, string, error) {
+	m.writes = new(bytes.Buffer)
+	return nopWriteCloser{m.writes}, "uri://" + name + ".partial", nil
+}
+
+func (m *mockFileOps) RenamePartialFile(partialPath, dir, finalName string) (string, error) {
+	if !m.renameOK {
+		m.renameOK = true
+		return "uri://" + finalName, nil
+	}
+	return "", io.ErrUnexpectedEOF
+}
+
+func TestPutFile(t *testing.T) {
+	const content = "hello, world"
+
+	tests := []struct {
+		name     string
+		mode     PutMode
+		setup    func(t *testing.T) (*manager, string, *mockFileOps)
+		wantFile string
+	}{
+		{
+			name: "PutModeDirect",
+			mode: PutModeDirect,
+			setup: func(t *testing.T) (*manager, string, *mockFileOps) {
+				dir := t.TempDir()
+				opts := managerOptions{
+					Logf:           t.Logf,
+					Clock:          tstime.DefaultClock{},
+					State:          nil,
+					Dir:            dir,
+					Mode:           PutModeDirect,
+					DirectFileMode: true,
+					SendFileNotify: func() {},
+				}
+				mgr := opts.New()
+				return mgr, dir, nil
+			},
+			wantFile: "file.txt",
+		},
+		{
+			name: "PutModeAndroidSAF",
+			mode: PutModeAndroidSAF,
+			setup: func(t *testing.T) (*manager, string, *mockFileOps) {
+				// SAF still needs a non-empty Dir to pass the guard.
+				dir := t.TempDir()
+				mops := &mockFileOps{}
+				opts := managerOptions{
+					Logf:           t.Logf,
+					Clock:          tstime.DefaultClock{},
+					State:          nil,
+					Dir:            dir,
+					Mode:           PutModeAndroidSAF,
+					FileOps:        mops,
+					DirectFileMode: true,
+					SendFileNotify: func() {},
+				}
+				mgr := opts.New()
+				return mgr, dir, mops
+			},
+			wantFile: "file.txt",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			mgr, dir, mops := tc.setup(t)
+			id := clientID(fmt.Sprint(0))
+			reader := bytes.NewReader([]byte(content))
+
+			n, err := mgr.PutFile(id, "file.txt", reader, 0, int64(len(content)))
+			if err != nil {
+				t.Fatalf("PutFile(%s) error: %v", tc.name, err)
+			}
+			if n != int64(len(content)) {
+				t.Errorf("wrote %d bytes; want %d", n, len(content))
+			}
+
+			switch tc.mode {
+			case PutModeDirect:
+				path := filepath.Join(dir, tc.wantFile)
+				data, err := os.ReadFile(path)
+				if err != nil {
+					t.Fatalf("ReadFile error: %v", err)
+				}
+				if got := string(data); got != content {
+					t.Errorf("file contents = %q; want %q", got, content)
+				}
+
+			case PutModeAndroidSAF:
+				if mops.writes == nil {
+					t.Fatal("SAF writer was never created")
+				}
+				if got := mops.writes.String(); got != content {
+					t.Errorf("SAF writes = %q; want %q", got, content)
+				}
+			}
+		})
+	}
+}

feature/taildrop/taildrop.go

@@ -91,6 +91,10 @@ type managerOptions struct {
 	// copy them out, and then delete them.
 	DirectFileMode bool
 
+	FileOps FileOps
+
+	Mode PutMode
+
 	// SendFileNotify is called periodically while a file is actively
 	// receiving the contents for the file. There is a final call
 	// to the function when reception completes.