Fix accept defect (#206)

* Fix accept() FutureDefect on cancellation.

* Do not close pipe twice.

* Check for retFuture state before changing it state.

chronos/transports/stream.nim

@@ -1032,9 +1032,10 @@ when defined(windows):
       var server = cast[StreamServer](ovl.data.udata)
 
       server.apending = false
+      if not(retFuture.finished()):
         if server.status in {ServerStatus.Stopped, ServerStatus.Closed}:
-        server.asock.closeSocket()
           retFuture.fail(getServerUseClosedError())
+          server.asock.closeSocket()
           server.clean()
         else:
           if ovl.data.errCode == OSErrorCode(-1):
@@ -1079,8 +1080,10 @@ when defined(windows):
       var server = cast[StreamServer](ovl.data.udata)
 
       server.apending = false
+      if not(retFuture.finished()):
         if server.status in {ServerStatus.Stopped, ServerStatus.Closed}:
           retFuture.fail(getServerUseClosedError())
+          server.sock.closeHandle()
           server.clean()
         else:
           if ovl.data.errCode == OSErrorCode(-1):
@@ -1698,6 +1701,7 @@ else:
         saddr: Sockaddr_storage
         slen: SockLen
 
+      if not(retFuture.finished()):
         if server.status in {ServerStatus.Stopped, ServerStatus.Closed}:
           retFuture.fail(getServerUseClosedError())
         else:
@@ -1705,7 +1709,9 @@ else:
             let res = posix.accept(SocketHandle(server.sock),
                                    cast[ptr SockAddr](addr saddr), addr slen)
             if int(res) > 0:
-            let sock = try: wrapAsyncSocket(res)
+              let sock =
+                try:
+                  wrapAsyncSocket(res)
                 except CatchableError as exc:
                   close(res)
                   retFuture.fail(exc)
@@ -1718,7 +1724,8 @@ else:
                   ntransp = newStreamSocketTransport(sock, server.bufferSize,
                                                      transp)
                 else:
-                ntransp = newStreamSocketTransport(sock, server.bufferSize, nil)
+                  ntransp = newStreamSocketTransport(sock, server.bufferSize,
+                                                     nil)
                 # Start tracking transport
                 trackStream(ntransp)
                 retFuture.complete(ntransp)
@@ -1737,6 +1744,7 @@ else:
               else:
                 retFuture.fail(getTransportOsError(err))
             break
+
         try:
           removeReader(server.sock)
         except IOSelectorsException as exc: