Bug 638330 (CVE-2010-3492)

From the upstream bug: http://bugs.python.org/issue6706

An old bad design choice in asyncore is how it forces the user to override
handle_accept() and then call self.accept() to obtain a socket pair.

    def handle_accept(self):
        conn, addr = self.accept()

The documentation itself shows the code above as an example of how an
asyncore-based server should handle an incoming connection.  What the doc
doesn't say is that the user calling self.accept() is exposed to different
risks:

- self.accept() can return None instead of a socket pair in which case
TypeError is raised (see pyftpdlib bug: 
http://code.google.com/p/pyftpdlib/issues/detail?id=91)

- ECONNABORTED can be raised. This is reproducible on Linux by hammering
the server with nmap (see pyftpdlib bug: 
http://code.google.com/p/pyftpdlib/issues/detail?id=105)

- EAGAIN can be raised too. I never experienced this error condition 
myself in pyftpdlib but by looking at twisted/internet/tcp.py it seems 
reasonable to catch EAGAIN too.

- The resulting address can be None, which means that the connection didn't
take place.  This is reproducible by hammering the server with nmap (see
pyftpdlib 
bug http://code.google.com/p/pyftpdlib/issues/detail?id=104).


The right thing to do when one of such events occur is to "return" as no
connection has really been established between client and server.

Now, altough handling these kind of conditions is quite easy, the API
design remains fundamentally wrong, as it forces the user to call accept().
As asyncore.py is structured right now the only chance the user has to
properly accepting a connection is manually handling all these cases in his
subclass.