zulip/zerver/middleware.py

import cProfile
import logging
import tempfile
import time
from typing import Any, Callable, Dict, List, MutableMapping, Optional, Tuple
from urllib.parse import urlencode, urljoin

from django.conf import settings
from django.conf.urls.i18n import is_language_prefix_patterns_used
from django.core import signals
from django.db import connection
from django.http import HttpRequest, HttpResponse, HttpResponseRedirect
from django.http.response import HttpResponseBase
from django.middleware.locale import LocaleMiddleware as DjangoLocaleMiddleware
from django.shortcuts import render
from django.utils import translation
from django.utils.cache import patch_vary_headers
from django.utils.crypto import constant_time_compare
from django.utils.deprecation import MiddlewareMixin
from django.utils.log import log_response
from django.utils.translation import gettext as _
from django_scim.middleware import SCIMAuthCheckMiddleware
from django_scim.settings import scim_settings
from sentry_sdk import set_tag
from typing_extensions import Annotated, Concatenate, ParamSpec, override

from zerver.lib.cache import get_remote_cache_requests, get_remote_cache_time
from zerver.lib.db_connections import reset_queries
from zerver.lib.debug import maybe_tracemalloc_listen
from zerver.lib.exceptions import ErrorCode, JsonableError, MissingAuthenticationError, WebhookError
from zerver.lib.markdown import get_markdown_requests, get_markdown_time
from zerver.lib.per_request_cache import flush_per_request_caches
from zerver.lib.rate_limiter import RateLimitResult
from zerver.lib.request import RequestNotes
from zerver.lib.response import (
    AsynchronousResponse,
    json_response,
    json_response_from_error,
    json_unauthorized,
)
from zerver.lib.subdomains import get_subdomain
from zerver.lib.typed_endpoint import INTENTIONALLY_UNDOCUMENTED, ApiParamConfig, typed_endpoint
from zerver.lib.user_agent import parse_user_agent
from zerver.models import Realm
from zerver.models.realms import get_realm

ParamT = ParamSpec("ParamT")
logger = logging.getLogger("zulip.requests")
slow_query_logger = logging.getLogger("zulip.slow_queries")


def record_request_stop_data(log_data: MutableMapping[str, Any]) -> None:
    log_data["time_stopped"] = time.time()
    log_data["remote_cache_time_stopped"] = get_remote_cache_time()
    log_data["remote_cache_requests_stopped"] = get_remote_cache_requests()
    log_data["markdown_time_stopped"] = get_markdown_time()
    log_data["markdown_requests_stopped"] = get_markdown_requests()
    if settings.PROFILE_ALL_REQUESTS:
        log_data["prof"].disable()


def async_request_timer_stop(request: HttpRequest) -> None:
    log_data = RequestNotes.get_notes(request).log_data
    assert log_data is not None
    record_request_stop_data(log_data)


def record_request_restart_data(log_data: MutableMapping[str, Any]) -> None:
    if settings.PROFILE_ALL_REQUESTS:
        log_data["prof"].enable()
    log_data["time_restarted"] = time.time()
    log_data["remote_cache_time_restarted"] = get_remote_cache_time()
    log_data["remote_cache_requests_restarted"] = get_remote_cache_requests()
    log_data["markdown_time_restarted"] = get_markdown_time()
    log_data["markdown_requests_restarted"] = get_markdown_requests()


def async_request_timer_restart(request: HttpRequest) -> None:
    log_data = RequestNotes.get_notes(request).log_data
    assert log_data is not None
    if "time_restarted" in log_data:
        # Don't destroy data when being called from
        # finish_current_handler
        return
    record_request_restart_data(log_data)


def record_request_start_data(log_data: MutableMapping[str, Any]) -> None:
    if settings.PROFILE_ALL_REQUESTS:
        log_data["prof"] = cProfile.Profile()
        log_data["prof"].enable()

    reset_queries()
    log_data["time_started"] = time.time()
    log_data["remote_cache_time_start"] = get_remote_cache_time()
    log_data["remote_cache_requests_start"] = get_remote_cache_requests()
    log_data["markdown_time_start"] = get_markdown_time()
    log_data["markdown_requests_start"] = get_markdown_requests()


def timedelta_ms(timedelta: float) -> float:
    return timedelta * 1000


def format_timedelta(timedelta: float) -> str:
    if timedelta >= 1:
        return f"{timedelta:.1f}s"
    return f"{timedelta_ms(timedelta):.0f}ms"


def is_slow_query(time_delta: float, path: str) -> bool:
    if time_delta < 1.2:
        return False
    is_exempt = path == "/activity" or path.startswith(("/realm_activity/", "/user_activity/"))
    if is_exempt:
        return time_delta >= 5
    if "webathena_kerberos" in path:
        return time_delta >= 10
    return True


def write_log_line(
    log_data: MutableMapping[str, Any],
    path: str,
    method: str,
    remote_ip: str,
    requester_for_logs: str,
    client_name: str,
    client_version: Optional[str] = None,
    status_code: int = 200,
    error_content: Optional[bytes] = None,
) -> None:
    time_delta = -1
    # A time duration of -1 means the StartLogRequests middleware
    # didn't run for some reason
    optional_orig_delta = ""
    if "time_started" in log_data:
        time_delta = time.time() - log_data["time_started"]
    if "time_stopped" in log_data:
        orig_time_delta = time_delta
        time_delta = (log_data["time_stopped"] - log_data["time_started"]) + (
            time.time() - log_data["time_restarted"]
        )
        optional_orig_delta = f" (lp: {format_timedelta(orig_time_delta)})"
    remote_cache_output = ""
    if "remote_cache_time_start" in log_data:
        remote_cache_time_delta = get_remote_cache_time() - log_data["remote_cache_time_start"]
        remote_cache_count_delta = (
            get_remote_cache_requests() - log_data["remote_cache_requests_start"]
        )
        if "remote_cache_requests_stopped" in log_data:
            # (now - restarted) + (stopped - start) = (now - start) + (stopped - restarted)
            remote_cache_time_delta += (
                log_data["remote_cache_time_stopped"] - log_data["remote_cache_time_restarted"]
            )
            remote_cache_count_delta += (
                log_data["remote_cache_requests_stopped"]
                - log_data["remote_cache_requests_restarted"]
            )

        if remote_cache_time_delta > 0.005:
            remote_cache_output = (
                f" (mem: {format_timedelta(remote_cache_time_delta)}/{remote_cache_count_delta})"
            )

    startup_output = ""
    if "startup_time_delta" in log_data and log_data["startup_time_delta"] > 0.005:
        startup_output = " (+start: {})".format(format_timedelta(log_data["startup_time_delta"]))

    markdown_output = ""
    if "markdown_time_start" in log_data:
        markdown_time_delta = get_markdown_time() - log_data["markdown_time_start"]
        markdown_count_delta = get_markdown_requests() - log_data["markdown_requests_start"]
        if "markdown_requests_stopped" in log_data:
            # (now - restarted) + (stopped - start) = (now - start) + (stopped - restarted)
            markdown_time_delta += (
                log_data["markdown_time_stopped"] - log_data["markdown_time_restarted"]
            )
            markdown_count_delta += (
                log_data["markdown_requests_stopped"] - log_data["markdown_requests_restarted"]
            )

        if markdown_time_delta > 0.005:
            markdown_output = (
                f" (md: {format_timedelta(markdown_time_delta)}/{markdown_count_delta})"
            )

    # Get the amount of time spent doing database queries
    db_time_output = ""
    queries = connection.connection.queries if connection.connection is not None else []
    if len(queries) > 0:
        query_time = sum(float(query.get("time", 0)) for query in queries)
        db_time_output = f" (db: {format_timedelta(query_time)}/{len(queries)}q)"

    if "extra" in log_data:
        extra_request_data = " {}".format(log_data["extra"])
    else:
        extra_request_data = ""
    if client_version is None:
        logger_client = f"({requester_for_logs} via {client_name})"
    else:
        logger_client = f"({requester_for_logs} via {client_name}/{client_version})"
    logger_timing = f"{format_timedelta(time_delta):>5}{optional_orig_delta}{remote_cache_output}{markdown_output}{db_time_output}{startup_output} {path}"
    logger_line = f"{remote_ip:<15} {method:<7} {status_code:3} {logger_timing}{extra_request_data} {logger_client}"
    if status_code in [200, 304] and method == "GET" and path.startswith("/static"):
        logger.debug(logger_line)
    else:
        logger.info(logger_line)

    if is_slow_query(time_delta, path):
        slow_query_logger.info(logger_line)

    if settings.PROFILE_ALL_REQUESTS:
        log_data["prof"].disable()
        with tempfile.NamedTemporaryFile(
            prefix="profile.data.{}.{}.".format(path.split("/")[-1], int(time_delta * 1000)),
            delete=False,
        ) as stats_file:
            log_data["prof"].dump_stats(stats_file.name)

    # Log some additional data whenever we return certain 40x errors
    if 400 <= status_code < 500 and status_code not in [401, 404, 405]:
        error_data = repr(error_content)
        if len(error_data) > 200:
            error_data = "[content more than 200 characters]"
        logger.info("status=%3d, data=%s, uid=%s", status_code, error_data, requester_for_logs)


# We take advantage of `typed_endpoint` being called multiple times
# when processing a request in order to process any `client` parameter that
# may have been sent in the request content.
@typed_endpoint
def parse_client(
    request: HttpRequest,
    # As `client` is a common element to all API endpoints, we choose
    # not to document on every endpoint's individual parameters.
    *,
    req_client: Annotated[
        Optional[str], ApiParamConfig("client", documentation_status=INTENTIONALLY_UNDOCUMENTED)
    ] = None,
) -> Tuple[str, Optional[str]]:
    # If the API request specified a client in the request content,
    # that has priority. Otherwise, extract the client from the
    # USER_AGENT.
    if req_client is not None:
        return req_client, None
    if "User-Agent" in request.headers:
        user_agent: Optional[Dict[str, str]] = parse_user_agent(request.headers["User-Agent"])
    else:
        user_agent = None
    if user_agent is None:
        # In the future, we will require setting USER_AGENT, but for
        # now we just want to tag these requests so we can review them
        # in logs and figure out the extent of the problem
        return "Unspecified", None

    client_name = user_agent["name"]
    if client_name.startswith("Zulip"):
        return client_name, user_agent.get("version")

    # We could show browser versions in logs, and it'd probably be a
    # good idea, but the current parsing will just get you Mozilla/5.0.
    #
    # Fixing this probably means using a third-party library, and
    # making sure it's fast enough that we're happy to do it even on
    # hot-path cases.
    return client_name, None


class LogRequests(MiddlewareMixin):
    # We primarily are doing logging using the process_view hook, but
    # for some views, process_view isn't run, so we call the start
    # method here too
    def process_request(self, request: HttpRequest) -> None:
        maybe_tracemalloc_listen()
        request_notes = RequestNotes.get_notes(request)

        if request_notes.log_data is not None:
            # Sanity check to ensure this is being called from the
            # Tornado code path that returns responses asynchronously.
            assert request_notes.saved_response is not None

            # Avoid re-initializing request_notes.log_data if it's already there.
            return

        try:
            request_notes.client_name, request_notes.client_version = parse_client(request)
        except JsonableError as e:
            logging.exception(e)
            request_notes.client_name = "Unparsable"
            request_notes.client_version = None

        set_tag("client", request_notes.client_name)

        request_notes.log_data = {}
        record_request_start_data(request_notes.log_data)

    def process_view(
        self,
        request: HttpRequest,
        view_func: Callable[Concatenate[HttpRequest, ParamT], HttpResponseBase],
        args: List[object],
        kwargs: Dict[str, Any],
    ) -> None:
        request_notes = RequestNotes.get_notes(request)
        if request_notes.saved_response is not None:
            # The below logging adjustments are unnecessary (because
            # we've already imported everything) and incorrect
            # (because they'll overwrite data from pre-long-poll
            # request processing) when returning a saved response.
            return

        # process_request was already run; we save the initialization
        # time (i.e. the time between receiving the request and
        # figuring out which view function to call, which is primarily
        # importing modules on the first start)
        assert request_notes.log_data is not None
        request_notes.log_data["startup_time_delta"] = (
            time.time() - request_notes.log_data["time_started"]
        )
        # And then completely reset our tracking to only cover work
        # done as part of this request
        record_request_start_data(request_notes.log_data)

    def process_response(
        self, request: HttpRequest, response: HttpResponseBase
    ) -> HttpResponseBase:
        if isinstance(response, AsynchronousResponse):
            # This special AsynchronousResponse sentinel is
            # discarded after going through this code path as Tornado
            # intends to block, so we stop here to avoid unnecessary work.
            return response

        remote_ip = request.META["REMOTE_ADDR"]

        # Get the requester's identifier and client, if available.
        request_notes = RequestNotes.get_notes(request)
        requester_for_logs = request_notes.requester_for_logs
        if requester_for_logs is None:
            if request_notes.remote_server is not None:
                requester_for_logs = request_notes.remote_server.format_requester_for_logs()
            elif request.user.is_authenticated:
                requester_for_logs = request.user.format_requester_for_logs()
            else:
                requester_for_logs = "unauth@{}".format(get_subdomain(request) or "root")

        content = response.content if isinstance(response, HttpResponse) else None

        assert request_notes.client_name is not None and request_notes.log_data is not None
        assert request.method is not None
        write_log_line(
            request_notes.log_data,
            request.path,
            request.method,
            remote_ip,
            requester_for_logs,
            request_notes.client_name,
            client_version=request_notes.client_version,
            status_code=response.status_code,
            error_content=content,
        )
        return response


class JsonErrorHandler(MiddlewareMixin):
    def process_exception(
        self, request: HttpRequest, exception: Exception
    ) -> Optional[HttpResponse]:
        if isinstance(exception, MissingAuthenticationError):
            if "text/html" in request.headers.get("Accept", ""):
                # If this looks like a request from a top-level page in a
                # browser, send the user to the login page.
                #
                # TODO: The next part is a bit questionable; it will
                # execute the likely intent for intentionally visiting
                # an API endpoint without authentication in a browser,
                # but that's an unlikely to be done intentionally often.
                return HttpResponseRedirect(
                    f"{settings.HOME_NOT_LOGGED_IN}?{urlencode({'next': request.path})}"
                )
            if request.path.startswith("/api"):
                # For API routes, ask for HTTP basic auth (email:apiKey).
                return json_unauthorized()
            else:
                # For /json routes, ask for session authentication.
                return json_unauthorized(www_authenticate="session")

        if isinstance(exception, JsonableError):
            response = json_response_from_error(exception)
            if response.status_code < 500 or isinstance(exception, WebhookError):
                # Webhook errors are handled in
                # authenticated_rest_api_view / webhook_view, so we
                # just return the response without logging further.
                return response
        elif RequestNotes.get_notes(request).error_format == "JSON" and not settings.TEST_SUITE:
            response = json_response(res_type="error", msg=_("Internal server error"), status=500)
        else:
            return None

        # Send the same signal that Django sends for an unhandled exception.
        # This is received by Sentry to log exceptions, and also by the Django
        # test HTTP client to show better error messages.
        try:
            raise exception  # Ensure correct sys.exc_info().
        except BaseException:
            signals.got_request_exception.send(sender=None, request=request)

        # Here we use Django's log_response the way Django uses
        # it normally to log error responses. However, we make the small
        # modification of including the traceback to make the log message
        # more helpful. log_response takes care of knowing not to duplicate
        # the logging, so Django won't generate a second log message.
        log_response(
            "%s: %s",
            response.reason_phrase,
            request.path,
            response=response,
            request=request,
            exception=exception,
        )
        return response


class TagRequests(MiddlewareMixin):
    def process_view(
        self,
        request: HttpRequest,
        view_func: Callable[Concatenate[HttpRequest, ParamT], HttpResponseBase],
        args: List[object],
        kwargs: Dict[str, Any],
    ) -> None:
        self.process_request(request)

    def process_request(self, request: HttpRequest) -> None:
        if request.path.startswith("/api/") or request.path.startswith("/json/"):
            RequestNotes.get_notes(request).error_format = "JSON"
        else:
            RequestNotes.get_notes(request).error_format = "HTML"


class CsrfFailureError(JsonableError):
    http_status_code = 403
    code = ErrorCode.CSRF_FAILED
    data_fields = ["reason"]

    def __init__(self, reason: str) -> None:
        self.reason: str = reason

    @staticmethod
    @override
    def msg_format() -> str:
        return _("CSRF error: {reason}")


def csrf_failure(request: HttpRequest, reason: str = "") -> HttpResponse:
    if RequestNotes.get_notes(request).error_format == "JSON":
        return json_response_from_error(CsrfFailureError(reason))
    else:
        return render(request, "4xx.html", context={"csrf_failure": True}, status=403)


class LocaleMiddleware(DjangoLocaleMiddleware):
    @override
    def process_response(
        self, request: HttpRequest, response: HttpResponseBase
    ) -> HttpResponseBase:
        # This is the same as the default LocaleMiddleware, minus the
        # logic that redirects 404's that lack a prefixed language in
        # the path into having a language.  See
        # https://code.djangoproject.com/ticket/32005
        language = translation.get_language()
        language_from_path = translation.get_language_from_path(request.path_info)
        urlconf = getattr(request, "urlconf", settings.ROOT_URLCONF)
        i18n_patterns_used, _ = is_language_prefix_patterns_used(urlconf)
        if not (i18n_patterns_used and language_from_path):
            patch_vary_headers(response, ("Accept-Language",))
        assert language is not None
        response.setdefault("Content-Language", language)

        # An additional responsibility of our override of this middleware is to save the user's language
        # preference in a cookie. That determination is made by code handling the request
        # and saved in the set_language flag so that it can be used here.
        set_language = RequestNotes.get_notes(request).set_language
        if set_language is not None:
            response.set_cookie(
                settings.LANGUAGE_COOKIE_NAME,
                set_language,
                max_age=settings.LANGUAGE_COOKIE_AGE,
                path=settings.LANGUAGE_COOKIE_PATH,
                domain=settings.LANGUAGE_COOKIE_DOMAIN,
                secure=settings.LANGUAGE_COOKIE_SECURE,
                httponly=settings.LANGUAGE_COOKIE_HTTPONLY,
                samesite=settings.LANGUAGE_COOKIE_SAMESITE,
            )

        return response


class RateLimitMiddleware(MiddlewareMixin):
    def set_response_headers(
        self, response: HttpResponseBase, rate_limit_results: List[RateLimitResult]
    ) -> None:
        # The limit on the action that was requested is the minimum of the limits that get applied:
        limit = min(result.entity.max_api_calls() for result in rate_limit_results)
        response["X-RateLimit-Limit"] = str(limit)
        # Same principle applies to remaining API calls:
        remaining_api_calls = min(result.remaining for result in rate_limit_results)
        response["X-RateLimit-Remaining"] = str(remaining_api_calls)

        # The full reset time is the maximum of the reset times for the limits that get applied:
        reset_time = time.time() + max(result.secs_to_freedom for result in rate_limit_results)
        response["X-RateLimit-Reset"] = str(int(reset_time))

    def process_response(
        self, request: HttpRequest, response: HttpResponseBase
    ) -> HttpResponseBase:
        if not settings.RATE_LIMITING:
            return response

        # Add X-RateLimit-*** headers
        ratelimits_applied = RequestNotes.get_notes(request).ratelimits_applied
        if len(ratelimits_applied) > 0:
            self.set_response_headers(response, ratelimits_applied)

        return response


class FlushDisplayRecipientCache(MiddlewareMixin):
    def process_response(
        self, request: HttpRequest, response: HttpResponseBase
    ) -> HttpResponseBase:
        # We flush the per-request caches after every request, so they
        # are not shared at all between requests.
        flush_per_request_caches()
        return response


class HostDomainMiddleware(MiddlewareMixin):
    def process_request(self, request: HttpRequest) -> Optional[HttpResponse]:
        # Match against ALLOWED_HOSTS, which is rather permissive;
        # failure will raise DisallowedHost, which is a 400.
        request.get_host()

        # This check is important to avoid doing the extra work of
        # `get_realm` (which does a database query that could be
        # problematic for Tornado).  Also the error page below is only
        # appropriate for a page visited in a browser, not the API.
        #
        # API authentication will end up checking for an invalid
        # realm, and throw a JSON-format error if appropriate.
        if request.path.startswith(("/static/", "/api/", "/json/")) or request.path == "/health":
            return None

        subdomain = get_subdomain(request)
        if subdomain in [
            settings.SOCIAL_AUTH_SUBDOMAIN,
            settings.SELF_HOSTING_MANAGEMENT_SUBDOMAIN,
        ]:
            # Realms are not supposed to exist on these subdomains.
            return None

        request_notes = RequestNotes.get_notes(request)
        try:
            request_notes.realm = get_realm(subdomain)
            request_notes.has_fetched_realm = True
        except Realm.DoesNotExist:
            if subdomain == Realm.SUBDOMAIN_FOR_ROOT_DOMAIN:
                # The root domain is used for creating new
                # organizations even if it does not host a realm.
                return None

            return render(request, "zerver/invalid_realm.html", status=404)

        set_tag("realm", request_notes.realm.string_id)

        # Check that we're not using the non-canonical form of a REALM_HOSTS subdomain
        if subdomain in settings.REALM_HOSTS:
            host = request.get_host().lower()
            formal_host = request_notes.realm.host
            if host != formal_host and not host.startswith(formal_host + ":"):
                return HttpResponseRedirect(
                    urljoin(request_notes.realm.uri, request.get_full_path())
                )
        return None


class SetRemoteAddrFromRealIpHeader(MiddlewareMixin):
    """Middleware that sets REMOTE_ADDR based on the X-Real-Ip header.

    This middleware is similar to Django's old
    SetRemoteAddrFromForwardedFor middleware.  We use X-Real-Ip, and
    not X-Forwarded-For, because the latter is a list of proxies, some
    number of which are trusted by us, and some of which could be
    arbitrarily set by the user.  nginx has already parsed which are
    which, and has set X-Real-Ip to the first one, going right to
    left, which is untrusted.

    Since we are always deployed behind nginx, we can trust the
    X-Real-Ip which is so set.  In development, we fall back to the
    REMOTE_ADDR supplied by the server.

    """

    def process_request(self, request: HttpRequest) -> None:
        try:
            real_ip = request.headers["X-Real-IP"]
        except KeyError:
            pass
        else:
            request.META["REMOTE_ADDR"] = real_ip


class ProxyMisconfigurationError(JsonableError):
    http_status_code = 500
    data_fields = ["proxy_reason"]

    def __init__(self, proxy_reason: str) -> None:
        self.proxy_reason = proxy_reason

    @staticmethod
    @override
    def msg_format() -> str:
        return _("Reverse proxy misconfiguration: {proxy_reason}")


class DetectProxyMisconfiguration(MiddlewareMixin):
    def process_view(
        self,
        request: HttpRequest,
        view_func: Callable[Concatenate[HttpRequest, ParamT], HttpResponseBase],
        args: List[object],
        kwargs: Dict[str, Any],
    ) -> None:
        proxy_state_header = request.headers.get("X-Proxy-Misconfiguration", "")
        # Our nginx configuration sets this header if:
        #  - there is an X-Forwarded-For set but no proxies configured in Zulip
        #  - proxies are configured but the request did not come from them
        #  - proxies are configured and the request came through them,
        #    but there was no X-Forwarded-Proto header
        #
        # Note that the first two may be false-positives.  We only
        # display the error if the request also came in over HTTP (and
        # a trusted proxy didn't say they get it over HTTPS), which
        # should be impossible because Zulip only supports external
        # https:// URLs in production.  nginx configuration ensures
        # that request.is_secure() is only true if our nginx is
        # serving the request over HTTPS, or it came from a trusted
        # proxy which reports that it is doing so.  This will result
        # in false negatives if Zulip's nginx is serving responses
        # over HTTPS to a proxy whose IP is not configured, or
        # misconfigured, but we cannot distinguish this from a random
        # client which is providing proxy headers to a correctly
        # configured Zulip.
        #
        # There are a couple complications to the above logic --
        # first, we do expect that requests not through the proxy may
        # happen from localhost over HTTP (e.g. the email gateway).
        # Second, we also expect that the proxy itself may make
        # healthcheck requests, which will not have an
        # X-Forwarded-Proto or X-Forwarded-For.  We handle the latter
        # case in the nginx config (as it involves CIDRs and proxy
        # ranges) and the former case here.
        if (
            proxy_state_header != ""
            and not request.is_secure()
            and request.META["REMOTE_ADDR"] not in ("127.0.0.1", "::1")
        ):
            raise ProxyMisconfigurationError(proxy_state_header)


def validate_scim_bearer_token(request: HttpRequest) -> bool:
    """
    This function verifies the request is allowed to make SCIM requests on this subdomain,
    by checking the provided bearer token and ensuring it matches a scim client configured
    for this subdomain in settings.SCIM_CONFIG.
    Returns True if successful.
    """

    subdomain = get_subdomain(request)
    scim_config_dict = settings.SCIM_CONFIG.get(subdomain)
    if not scim_config_dict:
        return False

    valid_bearer_token = scim_config_dict.get("bearer_token")
    scim_client_name = scim_config_dict.get("scim_client_name")
    # We really don't want a misconfiguration where this is unset,
    # allowing free access to the SCIM API:
    assert valid_bearer_token
    assert scim_client_name

    authorization = request.headers.get("Authorization")
    if authorization is None or not constant_time_compare(
        authorization, f"Bearer {valid_bearer_token}"
    ):
        return False

    request_notes = RequestNotes.get_notes(request)
    assert request_notes.realm is not None
    request_notes.requester_for_logs = (
        f"scim-client:{scim_client_name}:realm:{request_notes.realm.id}"
    )

    return True


class ZulipSCIMAuthCheckMiddleware(SCIMAuthCheckMiddleware):
    """
    Overridden version of middleware implemented in django-scim2
    (https://github.com/15five/django-scim2/blob/master/src/django_scim/middleware.py)
    to also handle authenticating the client.

    This doesn't actually function as a regular middleware class that's registered in
    settings.MIDDLEWARE, but rather is called inside django-scim2 logic to authenticate
    the request when accessing SCIM endpoints.
    """

    def process_request(self, request: HttpRequest) -> Optional[HttpResponse]:
        # Defensive assertion to ensure this can't accidentally get called on a request
        # to a non-SCIM endpoint.
        assert request.path.startswith(self.reverse_url)

        # This determines whether this is a SCIM request based on the request's path
        # and if it is, logs request information, including the body, as well as the response
        # for debugging purposes to the `django_scim.middleware` logger, at DEBUG level.
        # We keep those logs in /var/log/zulip/scim.log
        if self.should_log_request(request):
            self.log_request(request)

        if not validate_scim_bearer_token(request):
            # In case of failed authentication, a response should be returned to
            # prevent going further down the codepath (to the SCIM endpoint), since
            # this aspect works like regular middleware.
            response = HttpResponse(status=401)
            response["WWW-Authenticate"] = scim_settings.WWW_AUTHENTICATE_HEADER
            return response

        return None