[Twisted-Python] If lineReceived returns deferred, then connection is dropped and this deferred given as error message

[Marcin Kasperski]

I am observing the twisted behaviour which seems rather strange to me. 

In short: if lineReceived returns some deferred, then the whole connection is 
aborted, moreover, this deferred is returned as error message.

Longer story (the code pieces extracted from quite large app, I have not
yet tried writing small example)

### Crucial code fragments

class SomeProtocol(basic.LineReceiver):
   (...)
   def lineReceived(self, line):
	(...)
	return defer.succeed(997)    # magic value I can recognize easily

class SomeFactory(protocol.ReconnectingClientFactory):
   (...)
   def clientConnectionLost(self, connector, reason):
        logger.warn("Connection lost. Reason: %s\nTraceback: %s" % (
            reason.getErrorMessage(), reason.getTraceback()))
        protocol.ReconnectingClientFactory.clientConnectionLost(self, connector, reason)

reactor.connectTCP(HOST, PORT, SomeFactory(...))
reactor.run()

### Things seen in log file

myapp     : WARNING  Connection lost. Reason: <Deferred at 0xB551E1ECL  current result: 997>
Traceback: Traceback (most recent call last):
Failure: <type 'instance'>: <Deferred at 0xB551E1ECL  current result: 997>

(you recognized this 997, didn't you?)

### Extra info

1. Removing return defer.succeed() from lineReceived helps.

2. I wanted to return deferreds from lineReceived because I wanted to write unit tests
    testing my factory object (and in some cases to check whether all worked correctly
    required waiting for some spawned deferred calls).