import requests
from django.conf import settings
from django.http import HttpResponse
from django.utils.decorators import method_decorator
from django.views import View
from django.views.decorators.csrf import csrf_exempt

from utils.http_fallback import request_with_ssl_fallback

# Headers that must not be copied verbatim between hops (RFC 7230 6.1) plus
# framing headers that HttpResponse recomputes itself.
HOP_BY_HOP_HEADERS = {
    "connection",
    "keep-alive",
    "proxy-authenticate",
    "proxy-authorization",
    "te",
    "trailers",
    "transfer-encoding",
    "upgrade",
    "content-encoding",
    "content-length",
}


def _incoming_headers(request):
    headers = {}
    for key, value in request.META.items():
        if key.startswith("HTTP_"):
            name = key[5:].replace("_", "-").title()
        elif key in ("CONTENT_TYPE", "CONTENT_LENGTH") and value:
            name = key.replace("_", "-").title()
        else:
            continue
        headers[name] = value
    headers.pop("Host", None)
    return headers


@method_decorator(csrf_exempt, name="dispatch")
class ServiceProxyView(View):
    """Forwards `/<service>/<rest>` to the configured base URL for `service`."""

    def dispatch(self, request, service, rest="", *args, **kwargs):
        base_url = settings.PROXY_SERVICE_BASE_URLS.get(service)
        if not base_url:
            return HttpResponse(
                f'Unknown or unconfigured service "{service}".',
                status=404,
                content_type="text/plain",
            )

        target_url = f"{base_url.rstrip('/')}/{rest.lstrip('/')}"
        query_string = request.META.get("QUERY_STRING", "")
        if query_string:
            target_url = f"{target_url}?{query_string}"

        try:
            upstream = request_with_ssl_fallback(
                request.method,
                target_url,
                headers=_incoming_headers(request),
                data=request.body,
                timeout=settings.SERVICE_REQUEST_TIMEOUT,
                allow_redirects=False,
            )
        except requests.RequestException as exc:
            return HttpResponse(
                f'Error contacting upstream service "{service}": {exc}',
                status=502,
                content_type="text/plain",
            )

        response = HttpResponse(content=upstream.content, status=upstream.status_code)
        for name, value in upstream.headers.items():
            if name.lower() in HOP_BY_HOP_HEADERS:
                continue
            response[name] = value
        return response