Handling of disconnecting clients in asyncio

[Johannes Bauer]

Hi group,

I'm trying to get into async programming using Python. Concretely I open
a UNIX socket server in my application. The UNIX socket server generates
events and also receives commands/responds to them.

I do this by:

async def _create_local_server(self):
    await asyncio.start_unix_server(self._local_server_tasks, path = "foo"))

And then gather the command/response and event tasks:

async def _local_server_tasks(self, reader, writer):
    await asyncio.gather(
        self._local_server_commands(reader, writer),
        self._local_server_events(reader, writer),
    )

I believe so far this is okay, right? If not, please tell me. Anyways,
the event loop as an example:

async def _local_server_events(self, reader, writer):
    while True:
        await asyncio.sleep(1)
	writer.write(b"event\n")

And the command/response loop, obviously simplified:

async def _local_server_commands(self, reader, writer):
    while True:
        msg = await reader.readline()
        writer.write(msg)

Now I'm having the following issue: A client connects to my server and
then properly disconnects (shutdown/RDWR, close). This causes the await
reader.readline() to return an empty message (after which I can properly
end the _local_server_commands loop).

However, the _local_server_events loop does get no such notificiation.
Nor does writer.write() throw an exception that I could catch (and exit
as a consequence). Instead, I get this on stderr:

socket.send() raised exception.
socket.send() raised exception.
socket.send() raised exception.
socket.send() raised exception.
[...]

My questions are:

1. Is the design generally sane or is this usually done differently?
I.e., am I making any obvious beginner mistakes?

2. What is the proper way of discovering a peer has disconnected and
exiting cleanly?

Thanks in advance,
All the best,
Johannes

-- 
"Performance ist nicht das Problem, es läuft ja nachher beides auf der
selben Hardware." -- Hans-Peter Diettrich in d.s.e.