ipnlocal, taildrop: use SAF to open Android files

ipn/ipnlocal/local.go

@@ -406,6 +406,9 @@ type LocalBackend struct {
 	// outgoingFiles keeps track of Taildrop outgoing files keyed to their OutgoingFile.ID
 	outgoingFiles map[string]*ipn.OutgoingFile
 
+	// getSafFd gets the Storage Access Framework file descriptor for writing Taildrop files to
+	GetSafFd func(filename string) int32
+
 	// lastSuggestedExitNode stores the last suggested exit node suggestion to
 	// avoid unnecessary churn between multiple equally-good options.
 	lastSuggestedExitNode tailcfg.StableNodeID
@@ -5303,7 +5306,7 @@ func (b *LocalBackend) initPeerAPIListener() {
 			Dir:            fileRoot,
 			DirectFileMode: b.directFileRoot != "",
 			SendFileNotify: b.sendFileNotify,
-		}.New(),
+		}.New(b.getSafFd),
 	}
 	if dm, ok := b.sys.DNSManager.GetOK(); ok {
 		ps.resolver = dm.Resolver()

taildrop/send.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strings"
 	"sync"
 	"time"
 
@@ -82,9 +83,17 @@ 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
+	var dstPath string
+	var err error
+	if !(m.opts.DirectFileMode && strings.HasPrefix(m.opts.Dir, "content://")) {
+		dstPath, err = joinDir(m.opts.Dir, baseName)
+		if err != nil {
+			return 0, err
+		}
+	} else {
+		// For Android which uses SAF mode, we can simply use the baseName as our destination "path"
+		// (since we won't be using traditional file paths).
+		dstPath = baseName
 	}
 
 	redactAndLogError := func(action string, err error) error {
@@ -116,9 +125,21 @@ func (m *Manager) PutFile(id ClientID, baseName string, r io.Reader, offset, len
 	m.deleter.Remove(filepath.Base(partialPath)) // avoid deleting the partial file while receiving
 
 	// Create (if not already) the partial file with read-write permissions.
-	f, err := os.OpenFile(partialPath, os.O_CREATE|os.O_RDWR, 0666)
-	if err != nil {
-		return 0, redactAndLogError("Create", err)
+	var f *os.File
+	if m.opts.DirectFileMode && strings.HasPrefix(m.opts.Dir, "content://") {
+		// SAF mode: open the file using Android's SAF.
+		fd := m.GetSafFd(m.opts.Dir, baseName)
+		if err != nil {
+			return 0, redactAndLogError("Create (SAF)", err)
+		}
+		f = os.NewFile(uintptr(fd), baseName)
+		// In SAF mode, we don't have a traditional filesystem path; partialPath remains only for bookkeeping.
+	} else {
+		// Non-SAF (traditional filesystem) mode.
+		f, err = os.OpenFile(partialPath, os.O_CREATE|os.O_RDWR, 0666)
+		if err != nil {
+			return 0, redactAndLogError("Create", err)
+		}
 	}
 	defer func() {
 		f.Close() // best-effort to cleanup dangling file handles

taildrop/taildrop.go

@@ -101,6 +101,8 @@ type ManagerOptions struct {
 type Manager struct {
 	opts ManagerOptions
 
+	getSafFd func() int
+
 	// incomingFiles is a map of files actively being received.
 	incomingFiles syncs.Map[incomingFileKey, *incomingFile]
 	// deleter managers asynchronous deletion of files.
@@ -119,14 +121,14 @@ type Manager struct {
 // New initializes a new taildrop manager.
 // It may spawn asynchronous goroutines to delete files,
 // so the Shutdown method must be called for resource cleanup.
-func (opts ManagerOptions) New() *Manager {
+func (opts ManagerOptions) New(getSafFd func() int) *Manager {
 	if opts.Logf == nil {
 		opts.Logf = logger.Discard
 	}
 	if opts.SendFileNotify == nil {
 		opts.SendFileNotify = func() {}
 	}
-	m := &Manager{opts: opts}
+	m := &Manager{opts: opts, getSafFd: getSafFd}
 	m.deleter.Init(m, func(string) {})
 	m.emptySince.Store(-1) // invalidate this cache
 	return m