Make error reproducible by emulating 0.15.0 logic

`RuntimeError: No response returned.`

starlette/concurrency.py

@@ -18,10 +18,21 @@
 
 
 async def run_until_first_complete(*args: typing.Tuple[typing.Callable, dict]) -> None:
-    tasks = [create_task(handler(**kwargs)) for handler, kwargs in args]
+    async def run(func: typing.Callable[[], typing.Coroutine]) -> None:
+        await func()
+        # (starlette 0.15.0) starlette.concurrency.run_until_first_complete `task_group.cancel_scope.cancel()`
+        for task in tasks:
+            if not task.done() and task != asyncio.current_task():
+                task.cancel()
+
+    tasks = [create_task(run(functools.partial(handler, **kwargs))) for handler, kwargs in args]
     (done, pending) = await asyncio.wait(tasks, return_when=asyncio.FIRST_COMPLETED)
     [task.cancel() for task in pending]
-    [task.result() for task in done]
+    for task in done:
+        try:
+            task.result()
+        except asyncio.CancelledError:
+            pass
 
 
 async def run_in_threadpool(

starlette/middleware/base.py

@@ -31,7 +31,10 @@ async def call_next(self, request: Request) -> Response:
 
         scope = request.scope
         receive = request.receive
-        send = queue.put
+
+        async def send(item: typing.Any) -> None:
+            await asyncio.sleep(0)  # anyio.streams.memory.MemoryObjectSendStream.send `await checkpoint()`
+            await queue.put(item)
 
         async def coro() -> None:
             try: