cmd/syncthing: Work around binary in current dir restriction (fixes #8499) (#8500)

cmd/syncthing/monitor.go

@@ -83,6 +83,15 @@ func monitorMain(options serveOptions) {
 	}
 
 	args := os.Args
+	binary := args[0]
+	if build.IsWindows {
+		var err error
+		binary, err = expandExecutableInCurrentDirectory(binary)
+		if err != nil {
+			l.Warnln("Error starting the main Syncthing process:", err)
+			panic("Error starting the main Syncthing process")
+		}
+	}
 	var restarts [restartCounts]time.Time
 
 	stopSign := make(chan os.Signal, 1)
@@ -104,7 +113,7 @@ func monitorMain(options serveOptions) {
 		copy(restarts[0:], restarts[1:])
 		restarts[len(restarts)-1] = time.Now()
 
-		cmd := exec.Command(args[0], args[1:]...)
+		cmd := exec.Command(binary, args[1:]...)
 		cmd.Env = childEnv
 
 		stderr, err := cmd.StderrPipe()
@@ -180,7 +189,7 @@ func monitorMain(options serveOptions) {
 				// Restart the monitor process to release the .old
 				// binary as part of the upgrade process.
 				l.Infoln("Restarting monitor...")
-				if err = restartMonitor(args); err != nil {
+				if err = restartMonitor(binary, args); err != nil {
 					l.Warnln("Restart:", err)
 				}
 				os.Exit(exitCode)
@@ -203,6 +212,21 @@ func monitorMain(options serveOptions) {
 	}
 }
 
+func expandExecutableInCurrentDirectory(args0 string) (string, error) {
+	// Works around a restriction added in go1.19 that executables in the
+	// current directory are not resolved when specifying just an executable
+	// name (like e.g. "syncthing")
+	if !strings.ContainsRune(args0, os.PathSeparator) {
+		// Check if it's in PATH
+		_, err := exec.LookPath(args0)
+		if err != nil {
+			// Try to get the path to the current executable
+			return os.Executable()
+		}
+	}
+	return args0, nil
+}
+
 func copyStderr(stderr io.Reader, dst io.Writer) {
 	br := bufio.NewReader(stderr)
 
@@ -309,7 +333,7 @@ func copyStdout(stdout io.Reader, dst io.Writer) {
 	}
 }
 
-func restartMonitor(args []string) error {
+func restartMonitor(binary string, args []string) error {
 	// Set the STRESTART environment variable to indicate to the next
 	// process that this is a restart and not initial start. This prevents
 	// opening the browser on startup.
@@ -319,19 +343,20 @@ func restartMonitor(args []string) error {
 		// 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)
+		return restartMonitorUnix(binary, args)
 	}
 
 	// but it isn't supported on Windows, so there we start a normal
 	// exec.Command and return.
-	return restartMonitorWindows(args)
+	return restartMonitorWindows(binary, args)
 }
 
-func restartMonitorUnix(args []string) error {
-	if !strings.ContainsRune(args[0], os.PathSeparator) {
+func restartMonitorUnix(binary string, args []string) error {
+	if !strings.ContainsRune(binary, 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])
+		var err error
+		binary, err = exec.LookPath(binary)
 		if err != nil {
 			return err
 		}
@@ -341,8 +366,8 @@ func restartMonitorUnix(args []string) error {
 	return syscall.Exec(args[0], args, os.Environ())
 }
 
-func restartMonitorWindows(args []string) error {
-	cmd := exec.Command(args[0], args[1:]...)
+func restartMonitorWindows(binary string, args []string) error {
+	cmd := exec.Command(binary, args[1:]...)
 	// Retain the standard streams
 	cmd.Stderr = os.Stderr
 	cmd.Stdout = os.Stdout