# Django 4.2.11 documentation

# Source code for django.middleware.common

    import re
    from urllib.parse import urlparse

    from django.conf import settings
    from django.core.exceptions import PermissionDenied
    from django.core.mail import mail_managers
    from django.http import HttpResponsePermanentRedirect
    from django.urls import is_valid_path
    from django.utils.deprecation import MiddlewareMixin
    from django.utils.http import escape_leading_slashes

    class CommonMiddleware(MiddlewareMixin):
        """
        "Common" middleware for taking care of some basic operations:

            - Forbid access to User-Agents in settings.DISALLOWED_USER_AGENTS

            - URL rewriting: Based on the APPEND_SLASH and PREPEND_WWW settings,
              append missing slashes and/or prepends missing "www."s.

                - If APPEND_SLASH is set and the initial URL doesn't end with a
                  slash, and it is not found in urlpatterns, form a new URL by
                  appending a slash at the end. If this new URL is found in
                  urlpatterns, return an HTTP redirect to this new URL; otherwise
                  process the initial URL as usual.

              This behavior can be customized by subclassing CommonMiddleware and
              overriding the response_redirect_class attribute.
        """

        response_redirect_class = HttpResponsePermanentRedirect

        def process_request(self, request):
            """
            Check for denied User-Agents and rewrite the URL based on
            settings.APPEND_SLASH and settings.PREPEND_WWW
            """

            # Check for denied User-Agents
            user_agent = request.META.get("HTTP_USER_AGENT")
            if user_agent is not None:
                for user_agent_regex in settings.DISALLOWED_USER_AGENTS:
                    if user_agent_regex.search(user_agent):
                        raise PermissionDenied("Forbidden user agent")

            # Check for a redirect based on settings.PREPEND_WWW
            host = request.get_host()

            if settings.PREPEND_WWW and host and not host.startswith("www."):
                # Check if we also need to append a slash so we can do it all
                # with a single redirect. (This check may be somewhat expensive,
                # so we only do it if we already know we're sending a redirect,
                # or in process_response if we get a 404.)
                if self.should_redirect_with_slash(request):
                    path = self.get_full_path_with_slash(request)
                else:
                    path = request.get_full_path()

                return self.response_redirect_class(f"{request.scheme}://www.{host}{path}")

        def should_redirect_with_slash(self, request):
            """
            Return True if settings.APPEND_SLASH is True and appending a slash to
            the request path turns an invalid path into a valid one.
            """
            if settings.APPEND_SLASH and not request.path_info.endswith("/"):
                urlconf = getattr(request, "urlconf", None)
                if not is_valid_path(request.path_info, urlconf):
                    match = is_valid_path("%s/" % request.path_info, urlconf)
                    if match:
                        view = match.func
                        return getattr(view, "should_append_slash", True)
            return False

        def get_full_path_with_slash(self, request):
            """
            Return the full path of the request with a trailing slash appended.

            Raise a RuntimeError if settings.DEBUG is True and request.method is
            POST, PUT, or PATCH.
            """
            new_path = request.get_full_path(force_append_slash=True)
            # Prevent construction of scheme relative urls.
            new_path = escape_leading_slashes(new_path)
            if settings.DEBUG and request.method in ("POST", "PUT", "PATCH"):
                raise RuntimeError(
                    "You called this URL via %(method)s, but the URL doesn't end "
                    "in a slash and you have APPEND_SLASH set. Django can't "
                    "redirect to the slash URL while maintaining %(method)s data. "
                    "Change your form to point to %(url)s (note the trailing "
                    "slash), or set APPEND_SLASH=False in your Django settings."
                    % {
                        "method": request.method,
                        "url": request.get_host() + new_path,
                    }
                )
            return new_path

        def process_response(self, request, response):
            """
            When the status code of the response is 404, it may redirect to a path
            with an appended slash if should_redirect_with_slash() returns True.
            """
            # If the given URL is "Not Found", then check if we should redirect to
            # a path with a slash appended.
            if response.status_code == 404 and self.should_redirect_with_slash(request):
                return self.response_redirect_class(self.get_full_path_with_slash(request))

            # Add the Content-Length header to non-streaming responses if not
            # already set.
            if not response.streaming and not response.has_header("Content-Length"):
                response.headers["Content-Length"] = str(len(response.content))

            return response

    class BrokenLinkEmailsMiddleware(MiddlewareMixin):
        def process_response(self, request, response):
            """Send broken link emails for relevant 404 NOT FOUND responses."""
            if response.status_code == 404 and not settings.DEBUG:
                domain = request.get_host()
                path = request.get_full_path()
                referer = request.META.get("HTTP_REFERER", "")

                if not self.is_ignorable_request(request, path, domain, referer):
                    ua = request.META.get("HTTP_USER_AGENT", "<none>")
                    ip = request.META.get("REMOTE_ADDR", "<none>")
                    mail_managers(
                        "Broken %slink on %s"
                        % (
                            (
                                "INTERNAL "
                                if self.is_internal_request(domain, referer)
                                else ""
                            ),
                            domain,
                        ),
                        "Referrer: %s\nRequested URL: %s\nUser agent: %s\n"
                        "IP address: %s\n" % (referer, path, ua, ip),
                        fail_silently=True,
                    )
            return response

        def is_internal_request(self, domain, referer):
            """
            Return True if the referring URL is the same domain as the current
            request.
            """
            # Different subdomains are treated as different domains.
            return bool(re.match("^https?://%s/" % re.escape(domain), referer))

        def is_ignorable_request(self, request, uri, domain, referer):
            """
            Return True if the given request *shouldn't* notify the site managers
            according to project settings or in situations outlined by the inline
            comments.
            """
            # The referer is empty.
            if not referer:
                return True

            # APPEND_SLASH is enabled and the referer is equal to the current URL
            # without a trailing slash indicating an internal redirect.
            if settings.APPEND_SLASH and uri.endswith("/") and referer == uri[:-1]:
                return True

            # A '?' in referer is identified as a search engine source.
            if not self.is_internal_request(domain, referer) and "?" in referer:
                return True

            # The referer is equal to the current URL, ignoring the scheme (assumed
            # to be a poorly implemented bot).
            parsed_referer = urlparse(referer)
            if parsed_referer.netloc in ["", domain] and parsed_referer.path == uri:
                return True

            return any(pattern.search(uri) for pattern in settings.IGNORABLE_404_URLS)