Implement and turn on the osnet demux

This change introduces a demux that uses the kernel's network stack for UDP
datagram-to-socket assignment based on packet source address (as opposed to the
forwarding strategy of the routing demux). The osnet demux is used by default
on non-Windows platforms. When possible, use of the osnet demux is preferred
over the routing demux, since it can be expected to perform better.

The unit test suite has been extended to run all tests first with the demux
selected by default for the current platform, and then with the routing demux,
if the latter differs from the former. Tests were already being run twice, first
with IPv4 and then with IPv6, and thus we now run each test four times on
Linux, twice on Windows.

All unit tests pass with both demux types.

dtls/demux/__init__.py

@@ -20,8 +20,28 @@ import sys
 
 if sys.platform.startswith('win') or sys.platform.startswith('cygwin'):
     from router import UDPDemux
+    _routing = True
 else:
-    #from osnet import UDPDemux
-    from router import UDPDemux
+    from osnet import UDPDemux
+    _routing = False
+_default_demux = None
 
-__all__ = ["UDPDemux"]
+def force_routing_demux():
+    global _routing
+    if _routing:
+        return False  # no change - already loaded
+    global UDPDemux, _default_demux
+    import router
+    _default_demux = UDPDemux
+    UDPDemux = router.UDPDemux
+    _routing = True
+    return True  # new router loaded and switched
+
+def reset_default_demux():
+    global UDPDemux, _routing, _default_demux
+    if _default_demux:
+        UDPDemux = _default_demux
+        _default_demux = None
+        _routing = not _routing
+
+__all__ = ["UDPDemux", "force_routing_demux", "reset_default_demux"]

dtls/demux/osnet.py

@@ -17,9 +17,16 @@ Classes:
 
 Exceptions:
 
+  InvalidSocketError -- exception raised for improper socket objects
   KeyError -- raised for unknown peer addresses
 """
 
+import socket
+from logging import getLogger
+from ..err import InvalidSocketError
+
+_logger = getLogger(__name__)
+
 
 class UDPDemux(object):
     """OS network stack configuring demux
@@ -31,10 +38,34 @@ class UDPDemux(object):
     Methods:
 
       get_connection -- create a new connection or retrieve an existing one
-      remove_connection -- remove an existing connection
       service -- this method does nothing for this type of demux
     """
 
+    def __init__(self, datagram_socket):
+        """Constructor
+
+        Arguments:
+        datagram_socket -- the root socket; this must be a bound, unconnected
+                           datagram socket
+        """
+
+        if datagram_socket.type != socket.SOCK_DGRAM:
+            raise InvalidSocketError("datagram_socket is not of " +
+                                     "type SOCK_DGRAM")
+        try:
+            datagram_socket.getsockname()
+        except:
+            raise InvalidSocketError("datagram_socket is unbound")
+        try:
+            datagram_socket.getpeername()
+        except:
+            pass
+        else:
+            raise InvalidSocketError("datagram_socket is connected")
+
+        datagram_socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+        self._datagram_socket = datagram_socket
+
     def get_connection(self, address):
         """Create or retrieve a muxed connection
 
@@ -47,16 +78,27 @@ class UDPDemux(object):
         in case address was None
         """
 
-    def remove_connection(self, address):
-        """Remove a muxed connection
+        if not address:
+            return self._datagram_socket
 
-        Arguments:
-        address -- an address for which a muxed connection was previously
-                   retrieved through get_connection, which has not yet
-                   been removed
+        # Create a new datagram socket bound to the same interface and port as
+        # the root socket, but connected to the given peer
+        conn = socket.socket(self._datagram_socket.family,
+                             self._datagram_socket.type,
+                             self._datagram_socket.proto)
+        conn.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+        conn.bind(self._datagram_socket.getsockname())
+        conn.connect(address)
+        _logger.debug("Created new connection for address: %s", address)
+        return conn
 
-        Return:
-        the socket object whose connection has been removed
+    @staticmethod
+    def service():
+        """Service the root socket
+
+        This type of demux performs no servicing work on the root socket,
+        and instead advises the caller to proceed to listening on the root
+        socket.
         """
 
-        return self.connections.pop(address)
+        return True