add fromFd to asyncutils

asyncutils.nim

@@ -18,6 +18,8 @@ from posix import
   CMSG_NXTHDR,
   CMSG_LEN,
   CMSG_DATA,
+  SOL_SOCKET,
+  SCM_RIGHTS,
   sendmsg,
   recvmsg
 
@@ -39,6 +41,12 @@ type ControlMessage* = object
   msgType*: int
   data*: string
 
+proc fromFd*(fd: AsyncFD): ControlMessage =
+  result = ControlMessage(level: SOL_SOCKET,
+                          msgType: SCM_RIGHTS,
+                          data: newString(sizeof(AsyncFD)))
+  cast[ptr AsyncFD](result.data.cstring)[] = fd
+
 proc asyncSendMsg*(fd: AsyncFD, data: string,
                    cmsgs: seq[ControlMessage] = @[]): Future[void] =
   var retFuture = newFuture[void]("asyncSendMsg")