Further improve handling of task cancellation in SCP

This commit is a follow-on to commit 676534b to better support
KeyboardInterrupt or other forms of task cancellation during a
large SCP transfer.

This fix could also apply in other cases where `close()` or `abort()`
are called on a channel when the peer either delays or fails to send
back a MSG_CHANNEL_CLOSE, leaving the channel half-open. With this
fix, a client can close a channel and immediately wake up any blocked
tasks associated with that channel, without waiting to receive a close
message from the peer.

asyncssh/channel.py

@@ -229,7 +229,7 @@ def _cleanup(self, exc: Optional[Exception] = None) -> None:
 
         self._close_event.set()
 
-        if self._conn: # pragma: no branch
+        if self._conn and self._recv_state == 'closed': # pragma: no branch
             self.logger.info('Channel closed%s',
                              ': ' + str(exc) if exc else '')
 
@@ -263,7 +263,8 @@ def _discard_recv(self) -> None:
         # If recv is close_pending, we know send is already closed
         if self._recv_state == 'close_pending':
             self._recv_state = 'closed'
-            self._loop.call_soon(self._cleanup)
+
+        self._loop.call_soon(self._cleanup)
 
     async def _start_reading(self) -> None:
         """Start processing data on a new connection"""