API Gateway does not provide Content-Length header, breaks strict checking

[antdking]

We're having some issues at the moment, where running Flask via the Starlette WSGIMiddleware ends up dropping the Body due to no Content-Length being present.

API Gateway doesn't provide the Content-Length header, and the main lambda -> wsgi adaptors handle this by setting the header based on the length of the decoded body:

• apig-wsgi
• serverless-wsgi

Looking at this commit, it seemed to be an active decision to move to this method, and completely override the provided Content-Length (I'm assuming this was either historic APIGW behaviour, or someone manually mapped the header).

Can the same functionality be added to mangum, to add support for frameworks that have stricter checking on Content-Length please?

[antdking]

We're happy to contribute the fix if this is something you're happy to accommodate.

Are there any contributor guidelines we should be aware of?

[jordaneremieff]

@cybojenix sure, happy for you to PR this change. 👍

I'm not very strict on contributions, just so long as there is some test coverage and the changes conform to black formatting, then it should be fine. Mypy is run in the build, but don't worry too much about type hints, sprinkling # type: ignore is acceptable in cases where the correct typing isn't obvious.

This reminds me I should add a contributor guidelines to the repo at some point. :)

[antdking]

This is still in our internal issue tracker, however during the porting, we implemented a small hack to patch the functionality.

This is only tested with Flask/Werkzeug 1.x, so your mileage may vary:

def create_app():
    flask_app = create_flask_app()

    app = FastAPI()

    # This mount must always go last, so that FastAPI routes take precedence
    app.mount("/", WSGIMiddleware(_patch_input_stream(flask_app)))

    return app, flask_app


def _patch_input_stream(app):
    """
    Support requests that don't provide a Content-Length.

    This happens with API-Gateway.

    We're safe to disable the protection Werkzeug provides, as
    Starlette unpacks the entire request for us already.

    It's also a nice little speedup, as extra checks are skipped, and
    we don't rewrap the input.

    https://github.com/pallets/werkzeug/blob/f3b081634c744ae1e0ae3f76034ee90bfdc2a808/src/werkzeug/wsgi.py#L217-L221
    """

    def inner(environ, start_response):
        # safe to change directly, it's been cloned.
        environ["wsgi.input_terminated"] = True

        return app(environ, start_response)

    return inner

[druska]

If anyone is Googling for this, it also breaks parsing request bodies with Django ASGI. This is using the API Gateway lambda-proxy integration.

A couple of workarounds are here: https://stackoverflow.com/a/62382607/403844

[dgilmanAIDENTIFIED]

I'm having success with just intercepting the Lambda payload before it makes it to Mangum:

asgi_handler = mangum.Mangum(app)

def handler(event, context):
    if event.get("body") is not None:
        content_length = str(len(event["body"]))
        if "multiValueHeaders" in event:
            event["multiValueHeaders"]["Content-Length"] = [content_length]
        if "headers" in event:
            event["headers"]["Content-Length"] = content_length

    return asgi_handler(event, context)