Add support for emitting signals to host.Service which can be used to
transmit data back to the client during an ongoing method call. This
provides the possibility for the services to send information to their
client counterpart while running. The signal can take file descriptors
as extra parameters to send data on separate files.
If there are fds to send back to the client, do a check that none
of them are invalid, so that we do not raise an exception in send
later. This allows us to send a proper RemoteError instead of no
reply at all.
When decoding a message, first check that it is not empty and
raise a `ProtocolError` otherwise. This prevent a more obscure
error like "NoneType has no get method".
On the service server side, i.e. the actual host service binary,
when we receive a message that contains file descriptors, clean
then up eagerly, instead relying on the garbage collector.
More importantly, the fds that we get from as a reply, if any,
need to be closed since in the current model the ownership is
transferred to the caller of `dispatch`.
Catch the BrokenPipeError exception when sending a reply. This will
happen when the other side closes their side of the connection/pipe
so in that case we just break out of the serve loop.
Host services are a way to provide functionality to stages that is
restricted to the host and not directly available in the container,
such as providing input to stages, devices access and mounting.
This commit introduces a `ServiceManager` class that can be used to
start and (automatically) stop host service, as well as a `Service`
base class together with a `ServiceClient` class that be used to
implement host services and communicate with them. Refer to the doc
string of the module for more information.
