Fix BackgroundTasks with BaseHTTPMiddleware

[adriangb]

Alternative to #2176

[dmitry-mli]

FYI, The streaming response used to also do:

            if not isinstance(chunk, (bytes, memoryview)):
                chunk = chunk.encode(self.charset)

It itsn't covered so when it was removed in https://github.com/encode/starlette/pull/2620/files it didn't break

[dmitry-mli]

For full backward compatibility add it to constructor also:

class _StreamingResponse(Response):
    def __init__(
        self,
        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
        self.body_iterator = content
        self.status_code = status_code
        self.media_type = media_type
        self.background = background                             <<<
        self.init_headers(headers)

[dmitry-mli]

This is because some middleware may be referring to that field. Or below will work also

self.background = None

[adriangb]

No users should be calling the constructor unless they do something like type(response)(...) which is not something we need to support.

[dmitry-mli]

Agree, this aligns with the class being marked as private. Then please include

self.background = None

For full backward compatibility, any response object injected into BaseHTTPMiddleware based middleware needs to have the background field (default None)

[adriangb]

Ok I see your point now with the edit. Good point. Added.

@Kludex this is another reason to move the background task logic out of responses and a good reason to rework our Response inheritance so that Response and StreamingResponse both inherit from a BaseResponse that has the API and initialization of common bits.

[Kludex]

Can we add a test here?

[adriangb]

There is a test already, am I missing something?