def __call__(self, *msgs, **metadata):
with self._write_lock:<EOL><INDENT>self._source(*msgs, **metadata)<EOL><DEDENT>
Pass messages to the `source` to be consumed.  Typically this means
the messages will be framed and transmitted via whatever transport
layer is currently active.

Parameters
----------
msgs : SBP messages
  SBP messages to send.
metadata : dict
  Metadata for this batch of messages, passed to the `source`.