drive: don't allow DELETE on read-only shares

Fixes tailscale/corp#19646

Signed-off-by: Percy Wegmann <percy@tailscale.com>

drive/driveimpl/drive_test.go

@@ -88,11 +88,33 @@ func TestFileManipulation(t *testing.T) {
 	s.checkFileContents(remote1, share11, file112)
 
 	s.addShare(remote1, share12, drive.PermissionReadOnly)
-	s.writeFile("writing file to read-only remote should fail", remote1, share12, file111, "hello world", false)
 	s.writeFile("writing file to non-existent remote should fail", "non-existent", share11, file111, "hello world", false)
 	s.writeFile("writing file to non-existent share should fail", remote1, "non-existent", file111, "hello world", false)
 }
 
+func TestPermissions(t *testing.T) {
+	s := newSystem(t)
+
+	s.addRemote(remote1)
+	s.addShare(remote1, share12, drive.PermissionReadOnly)
+
+	s.writeFile("writing file to read-only remote should fail", remote1, share12, file111, "hello world", false)
+	if err := s.client.Mkdir(path.Join(remote1, share12), 0644); err == nil {
+		t.Error("making directory on read-only remote should fail")
+	}
+
+	// Now, write file directly to file system so that we can test permissions
+	// on other operations.
+	s.write(remote1, share12, file111, "hello world")
+	if err := s.client.Remove(pathTo(remote1, share12, file111)); err == nil {
+		t.Error("deleting file from read-only remote should fail")
+	}
+	if err := s.client.Rename(pathTo(remote1, share12, file111), pathTo(remote1, share12, file112), true); err == nil {
+		t.Error("moving file on read-only remote should fail")
+	}
+
+}
+
 type local struct {
 	l  net.Listener
 	fs *FileSystemForLocal
@@ -324,6 +346,14 @@ func (s *system) read(remoteName, shareName, name string) string {
 	return string(b)
 }
 
+func (s *system) write(remoteName, shareName, name, contents string) {
+	filename := filepath.Join(s.remotes[remoteName].shares[shareName], name)
+	err := os.WriteFile(filename, []byte(contents), 0644)
+	if err != nil {
+		s.t.Fatalf("failed to WriteFile: %s", err)
+	}
+}
+
 func (s *system) readViaWebDAV(remoteName, shareName, name string) string {
 	path := pathTo(remoteName, shareName, name)
 	b, err := s.client.Read(path)

drive/driveimpl/remote_impl.go

@@ -380,6 +380,7 @@ var writeMethods = map[string]bool{
 	"MKCOL":     true,
 	"MOVE":      true,
 	"PROPPATCH": true,
+	"DELETE":    true,
 }
 
 // canSudo checks wether we can sudo -u the configured executable as the