source port may take a bit to become available again, so retry binding three times

examples/app/app.nim

@@ -166,10 +166,18 @@ proc punchHole(punchdConn: PunchdConnection, serverConn: ServerConnection,
 
 proc initServerConnection(serverHostname: string, serverPort: Port,
                           probingPort: Port): Future[ServerConnection] {.async.} =
-  for i in 0 .. 1:
+  var failCount = 0
+  while result.probedPorts.len < 2:
     # FIXME: error handling
     let sock = newAsyncSocket()
+    try:
       sock.bindAddr(probingPort)
+    except OSError as e:
+      if failCount == 3:
+        raise e
+      failCount.inc
+      await sleepAsync(1000)
+      continue
     await sock.connect(serverHostname, serverPort)
     let id = rand(uint32)
     await sock.send(&"get-endpoint|{id}\n")