Retain standard streams over restart (fixes #2155)

cmd/syncthing/monitor.go

@@ -146,9 +146,8 @@ func monitorMain() {
 						// binary as part of the upgrade process.
 						l.Infoln("Restarting monitor...")
 						os.Setenv("STNORESTART", "")
-						err := exec.Command(args[0], args[1:]...).Start()
-						if err != nil {
-							l.Warnln("restart:", err)
+						if err = restartMonitor(args); err != nil {
+							l.Warnln("Restart:", err)
 						}
 						return
 					}
@@ -227,3 +226,39 @@ func copyStdout(stdout io.Reader, dst io.Writer) {
 		dst.Write([]byte(line))
 	}
 }
+
+func restartMonitor(args []string) error {
+	if runtime.GOOS != "windows" {
+		// syscall.Exec is the cleanest way to restart on Unixes as it
+		// replaces the current process with the new one, keeping the pid and
+		// controlling terminal and so on
+		return restartMonitorUnix(args)
+	}
+
+	// but it isn't supported on Windows, so there we start a normal
+	// exec.Command and return.
+	return restartMonitorWindows(args)
+}
+
+func restartMonitorUnix(args []string) error {
+	if !strings.ContainsRune(args[0], os.PathSeparator) {
+		// The path to the binary doesn't contain a slash, so it should be
+		// found in $PATH.
+		binary, err := exec.LookPath(args[0])
+		if err != nil {
+			return err
+		}
+		args[0] = binary
+	}
+
+	return syscall.Exec(args[0], args, os.Environ())
+}
+
+func restartMonitorWindows(args []string) error {
+	cmd := exec.Command(args[0], args[1:]...)
+	// Retain the standard streams
+	cmd.Stderr = os.Stderr
+	cmd.Stdout = os.Stdout
+	cmd.Stdin = os.Stdin
+	return cmd.Start()
+}