Fix double firing of `check_request_enabled`

[adamchainz]

The check_request_enabled signal can fire twice during process_response, but really we should only do it once. Handlers could be expensive, e.g. doing database queries, and there's no reason they should be expected to cache their results.

Reported by @MarkusH.

[romantomjak]

Correct me if I'm wrong, but I think it already executes just once.

Before process_response Django runs process_request which sets therequest._cors_enabled and that makes these lines redundant, because it will always be set and the "second" call to self.is_enabled(request) will not be made. Or am I missing something blatantly obvious?

[jaap3]

The signal fires each time CorsMiddleware.check_signal is called:

django-cors-headers/src/corsheaders/middleware.py

Lines 183 to 185 in eb278fc

	def check_signal(self, request: HttpRequest) -> bool:
	signal_responses = check_request_enabled.send(sender=None, request=request)
	return any(return_value for function, return_value in signal_responses)

The CorsMiddleware.is_enabled method calls self.check_signal:

django-cors-headers/src/corsheaders/middleware.py

Lines 178 to 181 in eb278fc

	def is_enabled(self, request: HttpRequest) -> bool:
	return bool(
	re.match(conf.CORS_URLS_REGEX, request.path_info)
	) or self.check_signal(request)

CorsMiddleware.process_request calls self.is_enabled and caches the result on the request object as request._cors_enabled:

django-cors-headers/src/corsheaders/middleware.py

Line 87 in eb278fc

request._cors_enabled = self.is_enabled(request)

The CorsMiddleware.process_response may have another call to self.is_enabled, but only if request._cors_enabled is not set (or set to None) (However I believe @romantomjak analysis is correct and that call can never be made since request._cors_enabled is always set to some boolean value at that point):

django-cors-headers/src/corsheaders/middleware.py

Lines 121 to 123 in eb278fc

	enabled = getattr(request, "_cors_enabled", None)
	if enabled is None:
	enabled = self.is_enabled(request)

Finally there's a second call to self.check_signal in process_response:

django-cors-headers/src/corsheaders/middleware.py

Lines 140 to 144 in eb278fc

	if (
	not conf.CORS_ALLOW_ALL_ORIGINS
	and not self.origin_found_in_white_lists(origin, url)
	and not self.check_signal(request)
	):

So technically check_request_enabled could be called 3 times during the course of a request/response cycle. In reality it can be called twice.

For the double firing to happen CORS_URLS_REGEX needs to be configured to a non-permissive value (i.e. not the default), CORS_ALLOW_ALL_ORIGINS needs to be set to False (the default) and the origin should not be included in either CORS_ALLOWED_ORIGINS or match any of the CORS_ALLOWED_ORIGIN_REGEXES.