Don't poll for disconnects in BaseHTTPMiddleware via StreamingResponse

Fixes #2516

starlette/middleware/base.py

@@ -6,9 +6,8 @@
 from anyio.abc import ObjectReceiveStream, ObjectSendStream
 
 from starlette._utils import collapse_excgroups
-from starlette.background import BackgroundTask
 from starlette.requests import ClientDisconnect, Request
-from starlette.responses import ContentStream, Response, StreamingResponse
+from starlette.responses import AsyncContentStream, Response
 from starlette.types import ASGIApp, Message, Receive, Scope, Send
 
 RequestResponseEndpoint = typing.Callable[[Request], typing.Awaitable[Response]]
@@ -198,20 +197,33 @@ async def dispatch(
         raise NotImplementedError()  # pragma: no cover
 
 
-class _StreamingResponse(StreamingResponse):
+class _StreamingResponse(Response):
     def __init__(
         self,
-        content: ContentStream,
+        content: AsyncContentStream,
         status_code: int = 200,
         headers: typing.Mapping[str, str] | None = None,
         media_type: str | None = None,
-        background: BackgroundTask | None = None,
         info: typing.Mapping[str, typing.Any] | None = None,
     ) -> None:
-        self._info = info
-        super().__init__(content, status_code, headers, media_type, background)
+        self.info = info
+        self.body_iterator = content
+        self.status_code = status_code
+        self.media_type = media_type
+        self.init_headers(headers)
 
-    async def stream_response(self, send: Send) -> None:
-        if self._info:
-            await send({"type": "http.response.debug", "info": self._info})
-        return await super().stream_response(send)
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        if self.info is not None:
+            await send({"type": "http.response.debug", "info": self.info})
+        await send(
+            {
+                "type": "http.response.start",
+                "status": self.status_code,
+                "headers": self.raw_headers,
+            }
+        )
+
+        async for chunk in self.body_iterator:
+            await send({"type": "http.response.body", "body": chunk, "more_body": True})
+
+        await send({"type": "http.response.body", "body": b"", "more_body": False})