Add _peek_bufsize to NetlinkSocketBase so recv can use it

NetlinkSocketBase.recv uses DEFAULT_RCVBUF to fetch things like ip links
via netlink sockets. It happed in the last cycles that with new NICs
with higher VF numbers or similar we have to change the buffer size
(see svinota#751 or svinota#813).
A better solution (thanks to one or our experts) can be to check the
necessary buffer size before receiving the sockets.

Fixes svinota#1044

Change-Id: I87c75e4d424653e5a29408b3ac2ba8504cb2db49

pyroute2/netlink/nlsocket.py

@@ -92,7 +92,7 @@
 import traceback
 import warnings
 from functools import partial
-from socket import MSG_PEEK, SO_RCVBUF, SO_SNDBUF, SOCK_DGRAM, SOL_SOCKET
+from socket import MSG_PEEK, MSG_TRUNC, MSG_DONTWAIT, SO_RCVBUF, SO_SNDBUF, SOCK_DGRAM, SOL_SOCKET
 
 from pyroute2 import config
 from pyroute2.common import DEFAULT_RCVBUF, AddrPool
@@ -1002,6 +1002,12 @@ def get_policy_map(self, policy=None):
 
         return ret
 
+    def _peek_bufsize(self, socket_descriptor):
+        data = bytearray()
+        bufsize, _ = socket_descriptor.recvfrom_into(
+            data, 0, MSG_DONTWAIT | MSG_PEEK | MSG_TRUNC)
+        return bufsize
+
     def sendto(self, *argv, **kwarg):
         return self._sendto(*argv, **kwarg)
 
@@ -1011,7 +1017,7 @@ def recv(self, *argv, **kwarg):
             if isinstance(data_in, Exception):
                 raise data_in
             return data_in
-        return self._sock.recv(*argv, **kwarg)
+        return self._sock.recv(self._peek_bufsize(self._sock), **kwarg)
 
     def recv_into(self, data, *argv, **kwarg):
         if self.input_from_buffer_queue: