Python socket.MSG_TRUNC Examples

def _read_frame(self, ts_mono_ns: int) -> _media.TimestampedDataFrame:
        while True:
            data, ancdata, msg_flags, _addr = self._sock.recvmsg(self._native_frame_size,
                                                                 self._ancillary_data_buffer_size)
            assert msg_flags & socket.MSG_TRUNC == 0, 'The data buffer is not large enough'
            assert msg_flags & socket.MSG_CTRUNC == 0, 'The ancillary data buffer is not large enough'

            loopback = bool(msg_flags & socket.MSG_CONFIRM)
            ts_system_ns = 0
            for cmsg_level, cmsg_type, cmsg_data in ancdata:
                if cmsg_level == socket.SOL_SOCKET and cmsg_type == _SO_TIMESTAMP:
                    sec, usec = _TIMEVAL_STRUCT.unpack(cmsg_data)
                    ts_system_ns = (sec * 1_000_000 + usec) * 1000
                else:
                    assert False, f'Unexpected ancillary data: {cmsg_level}, {cmsg_type}, {cmsg_data!r}'

            assert ts_system_ns > 0, 'Missing the timestamp; does the driver support timestamping?'
            timestamp = pyuavcan.transport.Timestamp(system_ns=ts_system_ns, monotonic_ns=ts_mono_ns)

            out = SocketCANMedia._parse_native_frame(data, loopback=loopback, timestamp=timestamp)
            if out is not None:
                return out 

def _testRecvmsgShorter(self):
        self.sendToServer(MSG)

    # FreeBSD < 8 doesn't always set the MSG_TRUNC flag when a truncated
    # datagram is received (issue #13001). 

def testRecvmsgPeek(self):
        # Check that MSG_PEEK in flags enables examination of pending
        # data without consuming it.

        # Receive part of data with MSG_PEEK.
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG) - 3, 0,
                                                   socket.MSG_PEEK)
        self.assertEqual(msg, MSG[:-3])
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.assertEqual(ancdata, [])
        # Ignoring MSG_TRUNC here (so this test is the same for stream
        # and datagram sockets).  Some wording in POSIX seems to
        # suggest that it needn't be set when peeking, but that may
        # just be a slip.
        self.checkFlags(flags, eor=False,
                        ignore=getattr(socket, "MSG_TRUNC", 0))

        # Receive all data with MSG_PEEK.
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG), 0,
                                                   socket.MSG_PEEK)
        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.assertEqual(ancdata, [])
        self.checkFlags(flags, eor=True)

        # Check that the same data can still be received normally.
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock, len(MSG))
        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.assertEqual(ancdata, [])
        self.checkFlags(flags, eor=True) 

def _testRecvmsgShorter(self):
        self.sendToServer(MSG)

    # FreeBSD < 8 doesn't always set the MSG_TRUNC flag when a truncated
    # datagram is received (issue #13001). 

def testRecvmsgPeek(self):
        # Check that MSG_PEEK in flags enables examination of pending
        # data without consuming it.

        # Receive part of data with MSG_PEEK.
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG) - 3, 0,
                                                   socket.MSG_PEEK)
        self.assertEqual(msg, MSG[:-3])
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.assertEqual(ancdata, [])
        # Ignoring MSG_TRUNC here (so this test is the same for stream
        # and datagram sockets).  Some wording in POSIX seems to
        # suggest that it needn't be set when peeking, but that may
        # just be a slip.
        self.checkFlags(flags, eor=False,
                        ignore=getattr(socket, "MSG_TRUNC", 0))

        # Receive all data with MSG_PEEK.
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG), 0,
                                                   socket.MSG_PEEK)
        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.assertEqual(ancdata, [])
        self.checkFlags(flags, eor=True)

        # Check that the same data can still be received normally.
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock, len(MSG))
        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.assertEqual(ancdata, [])
        self.checkFlags(flags, eor=True) 

def _testRecvmsgShorter(self):
        self.sendToServer(MSG)

    # FreeBSD < 8 doesn't always set the MSG_TRUNC flag when a truncated
    # datagram is received (issue #13001). 

def testRecvmsgPeek(self):
        # Check that MSG_PEEK in flags enables examination of pending
        # data without consuming it.

        # Receive part of data with MSG_PEEK.
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG) - 3, 0,
                                                   socket.MSG_PEEK)
        self.assertEqual(msg, MSG[:-3])
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.assertEqual(ancdata, [])
        # Ignoring MSG_TRUNC here (so this test is the same for stream
        # and datagram sockets).  Some wording in POSIX seems to
        # suggest that it needn't be set when peeking, but that may
        # just be a slip.
        self.checkFlags(flags, eor=False,
                        ignore=getattr(socket, "MSG_TRUNC", 0))

        # Receive all data with MSG_PEEK.
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG), 0,
                                                   socket.MSG_PEEK)
        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.assertEqual(ancdata, [])
        self.checkFlags(flags, eor=True)

        # Check that the same data can still be received normally.
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock, len(MSG))
        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.assertEqual(ancdata, [])
        self.checkFlags(flags, eor=True) 

def recv(self):
        """Receive a Message

        This receives the next pending message from the socket. This operation
        is synchronous.

        A tuple consisting of the deserialized message payload, the auxiliary
        file-descriptor set, and the socket-address of the sender is returned.
        """

        # On `SOCK_DGRAM`, packets might be arbitrarily sized. There is no
        # hard-coded upper limit, since it is only restricted by the size of
        # the kernel write buffer on sockets (which itself can be modified via
        # sysctl). The only real maximum is probably something like 2^31-1,
        # since that is the maximum of that sysctl datatype.
        # Anyway, `MSG_TRUNC+MSG_PEEK` usually allows us to easily peek at the
        # incoming buffer. Unfortunately, the python `recvmsg()` wrapper
        # discards the return code and we cannot use that. Instead, we simply
        # loop until we know the size. This is slightly awkward, but seems fine
        # as long as you do not put this into a hot-path.
        size = 4096
        while True:
            peek = self._socket.recvmsg(size, 0, socket.MSG_PEEK)
            if not (peek[2] & socket.MSG_TRUNC):
                break
            size *= 2

        # Fetch a packet from the socket. On linux, the maximum SCM_RIGHTS array
        # size is hard-coded to 253. This allows us to size the ancillary buffer
        # big enough to receive any possible message.
        fds = array.array("i")
        msg = self._socket.recvmsg(size, socket.CMSG_LEN(253 * fds.itemsize))

        # First thing we do is always to fetch the CMSG FDs into an FdSet. This
        # guarantees that we do not leak FDs in case the message handling fails
        # for other reasons.
        for level, ty, data in msg[1]:
            if level == socket.SOL_SOCKET and ty == socket.SCM_RIGHTS:
                assert len(data) % fds.itemsize == 0
                fds.frombytes(data)
        fdset = FdSet(rawfds=fds)

        # Check the returned message flags. If the message was truncated, we
        # have to discard it. This shouldn't happen, but there is no harm in
        # handling it. However, `CTRUNC` can happen, since it is also triggered
        # when LSMs reject FD transmission. Treat it the same as a parser error.
        flags = msg[2]
        if flags & (socket.MSG_TRUNC | socket.MSG_CTRUNC):
            raise BufferError

        try:
            payload = json.loads(msg[0])
        except json.JSONDecodeError:
            raise BufferError

        return (payload, fdset, msg[3])