Any use of asyncio.run(request.json()) fails when TestClient is used after v0.21.0 (post-httpx)

[nicklyra]

We have a handful of methods which we are unable to convert to async due to the third-party dependencies which are synchronous. In these methods, we tend to retrieve a json body using something like body = asyncio.run(request.json()).

This works the way you would expect when running the actual server and pointing a real client at it; however, when we try to test using TestClient, this results in the following exception when we hit the asyncio.run line in the server code:

RuntimeError: <asyncio.locks.Event object at 0x<some id> [unset, waiters:1]> is bound to a different event loop

The bottom of the stack looks like this:

my_api/api/routers/my_router.py:50: in post
    data = asyncio.run(request.json())
python3.10/asyncio/runners.py:44: in run
    return loop.run_until_complete(main)
python3.10/asyncio/base_events.py:649: in run_until_complete
    return future.result()
starlette/requests.py:243: in json
    body = await self.body()
starlette/requests.py:236: in body
    async for chunk in self.stream():
starlette/requests.py:221: in stream
    message = await self._receive()
starlette/middleware/base.py:39: in receive_or_disconnect
    async with anyio.create_task_group() as task_group:

and then inside the task group we blow up:

starlette/middleware/base.py, line 42, in wrap
  result = await func()
anyio/_backends/_asyncio.py, line 1842, in wait
  if await self._event.wait():
asyncio/locks.py, line 211, in wait
  fut = self._get_loop().create_future()
asyncio/mixins.py, line 30, in _get_loop
  raise RuntimeError(f'{self!r} is bound to a different event loop')

I don't know a great deal about the internals of Starlette's request & response classes, but presumably the issue is that whatever TestClient is doing mixed with what our server code is doing causes a different event loop to be trying to consume the response than the event loop that the TestClient is using to try to generate the response, and then badness ensues.

If the answer is "don't use TestClient" then I'm happy to drop it for something else like async-asgi-testclient, but it might be nice to find a way to make this work properly if one exists.