UDS: use lock file

[om26er]

If nexus is running a unix domain socket transport and the system crashes, on next boot, the sock file exist and nexus fails to start.

Nexus should

1. Create a lock file
2. Start the unix transport

Some reading https://gavv.net/articles/unix-socket-reuse/

[om26er]

As a PoC the below change achieved what I wanted. But of course below code would only run on UNIX based systems

diff --git a/router/rawsocketserver.go b/router/rawsocketserver.go
index 23c2f67..ea310f5 100644
--- a/router/rawsocketserver.go
+++ b/router/rawsocketserver.go
@@ -5,9 +5,11 @@ import (
        "fmt"
        "io"
        "net"
+       "os"
        "time"
 
        "github.com/gammazero/nexus/v3/transport"
+       "golang.org/x/sys/unix"
 )
 
 // RawSocketServer handles socket connections.
@@ -36,6 +38,18 @@ func NewRawSocketServer(r Router) *RawSocketServer {
 // ListenAndServe listens on the specified endpoint and starts a goroutine that
 // accepts new client connections until the returned io.closer is closed.
 func (s *RawSocketServer) ListenAndServe(network, address string) (io.Closer, error) {
+       file, err := unix.Open(address+".lock", os.O_RDONLY|os.O_CREATE, 0600)
+       if err != nil {
+               return nil, err
+       }
+
+       err = unix.Flock(file, unix.LOCK_EX|unix.LOCK_NB)
+       if err != nil {
+               return nil, err
+       }
+
+       _ = unix.Unlink(address)
+
        l, err := net.Listen(network, address)
        if err != nil {
                return nil, err