Deadlock with trio, when streaming changes

[palkeo]

Hi !

I think I found a deadlock in the library.
It happens when using trio, when iterating over the changes to a table. If the connexion is closed an exception will be raised, we will enter the aexit of the r.open() context manager and deadlock in there.

Here is the sequence of events:

• exception emitted by the async for change in changes line (internally it's emitted by _get_next in net_trio.py).
• exception propagated, so we enter AsyncTrioConnectionContextManager.__aexit__()
• we enter ConnectionInstance.close()

Then it waits infinitely in the last line of this code:

        # We must not wait for the _reader_task if we got an exception, because that
        # means that we were called from it. Waiting would lead to a deadlock.
        if self._reader_ended_event:
            await self._reader_ended_event.wait()

I added logs and was able to confirm it blocks there indefinitely. And it matches the comments as well.

Here is how to reproduce:

    async def print_changes(self, table):
        async with trio.open_nursery() as nursery, r.open(host=self.host, db=self.db, port=self.port, nursery=nursery) as conn:
            changes = await r.table(table).changes().run(conn)
            async for change in changes:
                logging.info({'name': table, **change})

Now break the connection to rethinkdb, and you will see it will never raise an exception nor terminate.

Thank you!