Stream response body in ASGITransport

Fixes encode#2186

httpx/_transports/asgi.py

@@ -1,3 +1,4 @@
+import types
 import typing
 
 import sniffio
@@ -33,12 +34,76 @@ def create_event() -> "Event":
         return asyncio.Event()
 
 
+class _AwaitableRunner:
+    def __init__(self, awaitable: typing.Awaitable[typing.Any]):
+        self._generator = awaitable.__await__()
+        self._started = False
+        self._next_item: typing.Any = None
+        self._finished = False
+
+    @types.coroutine
+    def __call__(
+        self, *, until: typing.Optional[typing.Callable[[], bool]] = None
+    ) -> typing.Generator[typing.Any, typing.Any, typing.Any]:
+        while not self._finished and (until is None or not until()):
+            send_value, throw_value = None, None
+            if self._started:
+                try:
+                    send_value = yield self._next_item
+                except BaseException as e:
+                    throw_value = e
+
+            self._started = True
+            try:
+                if throw_value is not None:
+                    self._next_item = self._generator.throw(throw_value)
+                else:
+                    self._next_item = self._generator.send(send_value)
+            except StopIteration as e:
+                self._exception = None
+                self._finished = True
+                return e.value
+            except BaseException:
+                self._generator.close()
+                self._finished = True
+                raise
+
+
 class ASGIResponseStream(AsyncByteStream):
-    def __init__(self, body: typing.List[bytes]) -> None:
+    def __init__(
+        self,
+        body: typing.List[bytes],
+        raise_app_exceptions: bool,
+        response_complete: "Event",
+        app_runner: _AwaitableRunner,
+    ) -> None:
         self._body = body
+        self._raise_app_exceptions = raise_app_exceptions
+        self._response_complete = response_complete
+        self._app_runner = app_runner
 
     async def __aiter__(self) -> typing.AsyncIterator[bytes]:
-        yield b"".join(self._body)
+        try:
+            while bool(self._body) or not self._response_complete.is_set():
+                if self._body:
+                    yield b"".join(self._body)
+                    self._body.clear()
+                await self._app_runner(
+                    until=lambda: bool(self._body) or self._response_complete.is_set()
+                )
+        except Exception:  # noqa: PIE786
+            if self._raise_app_exceptions:
+                raise
+        finally:
+            await self.aclose()
+
+    async def aclose(self) -> None:
+        self._response_complete.set()
+        try:
+            await self._app_runner()
+        except Exception:  # noqa: PIE786
+            if self._raise_app_exceptions:
+                raise
 
 
 class ASGITransport(AsyncBaseTransport):
@@ -145,8 +210,10 @@ async def send(message: _Message) -> None:
                 response_headers = message.get("headers", [])
                 response_started = True
 
-            elif message["type"] == "http.response.body":
-                assert not response_complete.is_set()
+            elif (
+                message["type"] == "http.response.body"
+                and not response_complete.is_set()
+            ):
                 body = message.get("body", b"")
                 more_body = message.get("more_body", False)
 
@@ -156,9 +223,11 @@ async def send(message: _Message) -> None:
                 if not more_body:
                     response_complete.set()
 
+        app_runner = _AwaitableRunner(self.app(scope, receive, send))
+
         try:
-            await self.app(scope, receive, send)
-        except Exception:  # noqa: PIE-786
+            await app_runner(until=lambda: response_started)
+        except Exception:  # noqa: PIE786
             if self.raise_app_exceptions:
                 raise
 
@@ -168,10 +237,11 @@ async def send(message: _Message) -> None:
             if response_headers is None:
                 response_headers = {}
 
-        assert response_complete.is_set()
         assert status_code is not None
         assert response_headers is not None
 
-        stream = ASGIResponseStream(body_parts)
+        stream = ASGIResponseStream(
+            body_parts, self.raise_app_exceptions, response_complete, app_runner
+        )
 
         return Response(status_code, headers=response_headers, stream=stream)

tests/test_asgi.py

@@ -1,5 +1,6 @@
 import json
 
+import anyio
 import pytest
 
 import httpx
@@ -60,13 +61,24 @@ async def raise_exc(scope, receive, send):
     raise RuntimeError()
 
 
+async def raise_exc_after_response_start(scope, receive, send):
+    status = 200
+    output = b"Hello, World!"
+    headers = [(b"content-type", "text/plain"), (b"content-length", str(len(output)))]
+
+    await send({"type": "http.response.start", "status": status, "headers": headers})
+    await anyio.sleep(0)
+    raise RuntimeError()
+
+
 async def raise_exc_after_response(scope, receive, send):
     status = 200
     output = b"Hello, World!"
     headers = [(b"content-type", "text/plain"), (b"content-length", str(len(output)))]
 
     await send({"type": "http.response.start", "status": status, "headers": headers})
     await send({"type": "http.response.body", "body": output})
+    await anyio.sleep(0)
     raise RuntimeError()
 
 
@@ -165,6 +177,13 @@ async def test_asgi_exc():
             await client.get("http://www.example.org/")
 
 
+@pytest.mark.anyio
+async def test_asgi_exc_after_response_start():
+    async with httpx.AsyncClient(app=raise_exc_after_response_start) as client:
+        with pytest.raises(RuntimeError):
+            await client.get("http://www.example.org/")
+
+
 @pytest.mark.anyio
 async def test_asgi_exc_after_response():
     async with httpx.AsyncClient(app=raise_exc_after_response) as client:
@@ -213,3 +232,63 @@ async def test_asgi_exc_no_raise():
         response = await client.get("http://www.example.org/")
 
         assert response.status_code == 500
+
+
+@pytest.mark.anyio
+async def test_asgi_exc_no_raise_after_response_start():
+    transport = httpx.ASGITransport(
+        app=raise_exc_after_response_start, raise_app_exceptions=False
+    )
+    async with httpx.AsyncClient(transport=transport) as client:
+        response = await client.get("http://www.example.org/")
+
+        assert response.status_code == 200
+
+
+@pytest.mark.anyio
+async def test_asgi_exc_no_raise_after_response():
+    transport = httpx.ASGITransport(
+        app=raise_exc_after_response, raise_app_exceptions=False
+    )
+    async with httpx.AsyncClient(transport=transport) as client:
+        response = await client.get("http://www.example.org/")
+
+        assert response.status_code == 200
+
+
+@pytest.mark.anyio
+async def test_asgi_stream_returns_before_waiting_for_body():
+    start_response_body = anyio.Event()
+
+    async def send_response_body_after_event(scope, receive, send):
+        status = 200
+        headers = [(b"content-type", b"text/plain")]
+        await send(
+            {"type": "http.response.start", "status": status, "headers": headers}
+        )
+        await start_response_body.wait()
+        await send({"type": "http.response.body", "body": b"body", "more_body": False})
+
+    async with httpx.AsyncClient(app=send_response_body_after_event) as client:
+        async with client.stream("GET", "http://www.example.org/") as response:
+            assert response.status_code == 200
+            start_response_body.set()
+            await response.aread()
+            assert response.text == "body"
+
+
+@pytest.mark.anyio
+async def test_asgi_can_be_canceled():
+    # This test exists to cover transmission of the cancellation exception through
+    # _AwaitableRunner
+    app_started = anyio.Event()
+
+    async def never_return(scope, receive, send):
+        app_started.set()
+        await anyio.sleep_forever()
+
+    async with httpx.AsyncClient(app=never_return) as client:
+        async with anyio.create_task_group() as task_group:
+            task_group.start_soon(client.get, "http://www.example.org/")
+            await app_started.wait()
+            task_group.cancel_scope.cancel()