Fix unix domain socket creation/cleanup logic

* If there is another instance still listening on the same
    Unix domain socket, bail out

  * If there is a leftover from crashed yggstack etc,
    clean the socket file and proceed

Signed-off-by: Vasyl Gello <vasek.gello@gmail.com>

cmd/yggstack/main.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"os/signal"
 	"regexp"
+	"runtime"
 	"strings"
 	"syscall"
 
@@ -273,44 +274,48 @@ func main() {
 
 	// Create SOCKS server
 	{
-		if socks != nil {
+		if socks != nil && *socks != "" {
-			if nameserver != nil {
-				if strings.Contains(*socks, ":") {
-					logger.Infof("Starting SOCKS server on %s", *socks)
-					resolver := types.NewNameResolver(s, *nameserver)
 			socksOptions := []socks5.Option{
 				socks5.WithDial(s.DialContext),
-						socks5.WithResolver(resolver),
+			}
+			if nameserver != nil && *nameserver != "" {
+				resolver := types.NewNameResolver(s, *nameserver)
+				socksOptions = append(socksOptions, socks5.WithResolver(resolver))
+			} else {
+				logger.Warningf("DNS nameserver is not set!")
+				logger.Warningf("SOCKS server will not be able to resolve hostnames other than .pk.ygg !")
 			}
 			if logger.GetLevel("debug") {
 				socksOptions = append(socksOptions, socks5.WithLogger(logger))
 			}
 			server := socks5.NewServer(socksOptions...)
+			if strings.Contains(*socks, ":") {
+				logger.Infof("Starting SOCKS server on %s", *socks)
 				go server.ListenAndServe("tcp", *socks) // nolint:errcheck
 			} else {
 				logger.Infof("Starting SOCKS server with socket file %s", *socks)
-					_, err := os.Stat(*socks)
-					if os.IsNotExist(err) {
 				n.socks5Listener, err = net.Listen("unix", *socks)
+				if err != nil {
+					// If address in use, try connecting to
+					// the socket to see if other yggstack
+					// instance is listening on it
+
+					if isErrorAddressAlreadyInUse(err) {
+						_, err = net.Dial("unix", *socks)
+						if err != nil {
+							// Unlink dead socket if not connected
+							err = os.RemoveAll(*socks)
 							if err != nil {
 								panic(err)
 							}
-						resolver := types.NewNameResolver(s, *nameserver)
-						socksOptions := []socks5.Option{
-							socks5.WithDial(s.DialContext),
-							socks5.WithResolver(resolver),
-						}
-						if logger.GetLevel("debug") {
-							socksOptions = append(socksOptions, socks5.WithLogger(logger))
-						}
-						server := socks5.NewServer(socksOptions...)
-						go server.Serve(n.socks5Listener) // nolint:errcheck
-					} else if err != nil {
-						logger.Errorf("Cannot create socket file %s: %s", *socks, err)
 						} else {
-						panic(errors.New(fmt.Sprintf("Socket file %s already exists", *socks)))
+							panic(fmt.Errorf("Another yggstack instance is listening on socket '%s'", *socks))
+						}
+					} else {
+						panic(err)
 					}
 				}
+				go server.Serve(n.socks5Listener) // nolint:errcheck
 			}
 		}
 	}
@@ -349,11 +354,34 @@ func main() {
 	_ = n.multicast.Stop()
 	if n.socks5Listener != nil {
 		_ = n.socks5Listener.Close()
+		_ = os.RemoveAll(*socks)
 		logger.Infof("Stopped UNIX socket listener")
 	}
 	n.core.Stop()
 }
 
+// Helper to detect if socket address is in use
+// https://stackoverflow.com/a/52152912
+func isErrorAddressAlreadyInUse(err error) bool {
+	var eOsSyscall *os.SyscallError
+	if !errors.As(err, &eOsSyscall) {
+		return false
+	}
+	var errErrno syscall.Errno // doesn't need a "*" (ptr) because it's already a ptr (uintptr)
+	if !errors.As(eOsSyscall, &errErrno) {
+		return false
+	}
+	if errors.Is(errErrno, syscall.EADDRINUSE) {
+		return true
+	}
+	const WSAEADDRINUSE = 10048
+	if runtime.GOOS == "windows" && errErrno == WSAEADDRINUSE {
+		return true
+	}
+	return false
+}
+
+// Helper to set logging level
 func setLogLevel(loglevel string, logger *log.Logger) {
 	levels := [...]string{"error", "warn", "info", "debug", "trace"}
 	loglevel = strings.ToLower(loglevel)