Raising trio.WouldBlock from within task creation

[David R. MacIver]

When running redispy's asyncio support on top of trio-asyncio, we occasionally see the following error:

future: <Task finished name='Task-84858' coro=<ClusterNode.disconnect() done, defined at site-packages/redis/asyncio/cluster.py:992> exception=WouldBlock()>Traceback (most recent call last):
 site-packages/redis/asyncio/cluster.py", line 993, in disconnect
    ret = await asyncio.gather(
                ^^^^^^^^^^^^^^^
 site-packages/redis/asyncio/cluster.py", line 995, in <genexpr>
    asyncio.create_task(connection.disconnect())
  File "/usr/lib64/python3.11/asyncio/tasks.py", line 374, in create_task
    task = loop.create_task(coro)
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.11/asyncio/base_events.py", line 436, in create_task
    task = tasks.Task(coro, loop=self, name=name, context=context)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 site-packages/trio_asyncio/_base.py", line 320, in call_soon
    return self._queue_handle(asyncio.Handle(callback, args, self, **context))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 site-packages/trio_asyncio/_async.py", line 14, in _queue_handle
    self._q_send.send_nowait(handle)
 site-packages/trio/_core/_ki.py", line 181, in wrapper
    return fn(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^
 site-packages/trio/_channel.py", line 183, in send_nowait
    raise trio.WouldBlock
trio.WouldBlock
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
  File "application.py", line 256, in run
    trio_asyncio.run(self.main)
 site-packages/trio_asyncio/_loop.py", line 530, in run
    return trio.run(_run_task, proc, args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 site-packages/trio/_core/_run.py", line 2251, in run
    timeout = gen.send(next_send)
              ^^^^^^^^^^^^^^^^^^^
 site-packages/trio/_core/_run.py", line 2644, in unrolled_run
    raise TrioInternalError("internal error in Trio - please file a bug!") from exc
trio.TrioInternalError: internal error in Trio - please file a bug!

I'm afraid I don't have anything resembling a reproducible test case for this - sorry. It's a thing that we hit occasionally in a production run, and I have no idea how to produce it reliably on our systems, let alone produce a simple test case. I suspect it's something to do with the scale we're running at.

[Joshua Oreman]

Wow, I have no idea how that was ever supposed to work. The best kind of backpressure: it just crashes your program if the queue gets too long. The default max queue length is 10000 so I'm not surprised it only shows up under load.

You can specify a different length as an argument to open_loop(). I bet open_loop(math.inf) will solve your problem. It's kind of silly to accept a limit at all; the asyncio methods to enqueue callbacks are synchronous so there's no way to enforce backpressure if the queue fills up.