base.py

import typing

import anyio

from starlette.requests import Request
from starlette.responses import Response, StreamingResponse
from starlette.types import ASGIApp, Message, Receive, Scope, Send

RequestResponseEndpoint = typing.Callable[[Request], typing.Awaitable[Response]]
DispatchFunction = typing.Callable[
    [Request, RequestResponseEndpoint], typing.Awaitable[Response]
]

T = typing.TypeVar("T")


async def _call_and_cancel(
    func: typing.Callable[[], typing.Awaitable[T]], cancel_scope: anyio.CancelScope
) -> T:
    result = await func()
    cancel_scope.cancel()
    return result


class BaseHTTPMiddleware:
    def __init__(
        self, app: ASGIApp, dispatch: typing.Optional[DispatchFunction] = None
    ) -> None:
        self.app = app
        self.dispatch_func = self.dispatch if dispatch is None else dispatch

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
        if scope["type"] != "http":
            await self.app(scope, receive, send)
            return

        app_disconnected = anyio.Event()

        async def call_next(request: Request) -> Response:
            app_exc: typing.Optional[Exception] = None
            send_stream, recv_stream = anyio.create_memory_object_stream()

            async def receive_or_disconnect() -> Message:
                if app_disconnected.is_set():
                    return {"type": "http.disconnect"}

                async with anyio.create_task_group() as task_group:
                    # app_disconnected is set, cancel parent scope to cancel
                    # request.receive
                    task_group.start_soon(
                        _call_and_cancel,
                        app_disconnected.wait,
                        task_group.cancel_scope,
                    )
                    # if request.receive returns a message, cancel the task_group to
                    # exit its block
                    message = await _call_and_cancel(
                        request.receive, task_group.cancel_scope
                    )

                if app_disconnected.is_set():
                    return {"type": "http.disconnect"}

                return message

            async def close_recv_stream_on_disconnect():
                await app_disconnected.wait()
                recv_stream.close()

            async def send_no_error(message: Message):
                try:
                    await send_stream.send(message)
                except anyio.BrokenResourceError:
                    # recv_stream has been closed, i.e. app_disconnected has been set.
                    return

            async def coro() -> None:
                nonlocal app_exc

                async with send_stream:
                    try:
                        await self.app(scope, receive_or_disconnect, send_no_error)
                    except Exception as exc:
                        app_exc = exc

            task_group.start_soon(close_recv_stream_on_disconnect)
            task_group.start_soon(coro)

            try:
                message = await recv_stream.receive()
            except anyio.EndOfStream:
                if app_exc is not None:
                    raise app_exc
                raise RuntimeError("No response returned.")

            assert message["type"] == "http.response.start"

            async def body_stream() -> typing.AsyncGenerator[bytes, None]:
                async with recv_stream:
                    async for message in recv_stream:
                        assert message["type"] == "http.response.body"
                        body = message.get("body", b"")
                        if body:
                            yield body
                        if not message.get("more_body", False):
                            break

                if app_exc is not None:
                    raise app_exc

            response = StreamingResponse(
                status_code=message["status"], content=body_stream()
            )
            response.raw_headers = message["headers"]
            return response

        async with anyio.create_task_group() as task_group:
            request = Request(scope, receive=receive)
            response = await self.dispatch_func(request, call_next)
            await response(scope, receive, send)
            app_disconnected.set()

    async def dispatch(
        self, request: Request, call_next: RequestResponseEndpoint
    ) -> Response:
        raise NotImplementedError()  # pragma: no cover