allow LISTEN_FDS to be empty: support older systemd version

activation/activation.go

@@ -44,7 +44,27 @@ func Listen(addr string) (net.Listener, error) {
 }
 
 func activationListeners() ([]net.Listener, error) {
-	if err := checkEnv(); err != nil {
+	files, err := getFiles()
+	if err != nil {
+		return nil, err
+	}
+	listeners := make([]net.Listener, len(files))
+	for i, f := range files {
+		listener, err := net.FileListener(f)
+		if err != nil {
+			return nil, fmt.Errorf("failed to init new file listener: %v", err)
+		}
+		if err = f.Close(); err != nil {
+			return nil, fmt.Errorf("failed to close %s file: %v", f.Name(), err)
+		}
+		listeners[i] = listener
+	}
+	return listeners, nil
+}
+
+func getFiles() ([]*os.File, error) {
+	err := checkEnv()
+	if err != nil {
 		return nil, err
 	}
 	listenPid, err := strconv.Atoi(os.Getenv("LISTEN_PID"))
@@ -58,9 +78,12 @@ func activationListeners() ([]net.Listener, error) {
 	if err != nil {
 		return nil, fmt.Errorf("failed to parse LISTEN_FDS as int: %v", err)
 	}
-	listenFdNames := strings.Split(os.Getenv("LISTEN_FDNAMES"), ":")
-	if err = os.Unsetenv("LISTEN_PID"); err != nil {
-		return nil, fmt.Errorf("failed to unset env LISTEN_PID: %v", err)
+	var listenFdNames []string
+	if _, ok := os.LookupEnv("LISTEN_FDS"); ok {
+		listenFdNames = strings.Split(os.Getenv("LISTEN_FDNAMES"), ":")
+		if err = os.Unsetenv("LISTEN_FDNAMES"); err != nil {
+			return nil, fmt.Errorf("failed to unset env LISTEN_FDNAMES: %v", err)
+		}
 	}
 	if err = os.Unsetenv("LISTEN_FDS"); err != nil {
 		return nil, fmt.Errorf("failed to unset env LISTEN_PID: %v", err)
@@ -73,24 +96,15 @@ func activationListeners() ([]net.Listener, error) {
 		// http://man7.org/linux/man-pages/man2/fcntl.2.html
 		syscall.CloseOnExec(fd)
 		name := "LISTEN_FD_" + strconv.Itoa(fd)
-		offset := fd - listenFdsStart
-		if offset < len(listenFdNames) && len(listenFdNames[offset]) > 0 {
-			name = listenFdNames[offset]
+		if listenFdNames != nil {
+			offset := fd - listenFdsStart
+			if offset < len(listenFdNames) && len(listenFdNames[offset]) > 0 {
+				name = listenFdNames[offset]
+			}
 		}
 		files = append(files, os.NewFile(uintptr(fd), name))
 	}
-	listeners := make([]net.Listener, len(files))
-	for i, f := range files {
-		listener, err := net.FileListener(f)
-		if err != nil {
-			return nil, fmt.Errorf("failed to init new file listener: %v", err)
-		}
-		if err = f.Close(); err != nil {
-			return nil, fmt.Errorf("failed to close %s file: %v", f.Name(), err)
-		}
-		listeners[i] = listener
-	}
-	return listeners, nil
+	return files, nil
 }
 
 func checkEnv() error {
@@ -101,8 +115,5 @@ func checkEnv() error {
 	if _, ok = os.LookupEnv("LISTEN_FDS"); !ok {
 		return errors.New("LISTEN_FDS not set")
 	}
-	if _, ok = os.LookupEnv("LISTEN_FDNAMES"); !ok {
-		return errors.New("LISTEN_FDNAMES not set")
-	}
 	return nil
 }