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/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)
+				}
+			}
+		})
+	}
+}