Our discourse_public_exceptions middleware is designed to catch bubbled exceptions from lower in the stack, and then use `ApplicationController.rescue_with_handler` to render an appropriate error response.
When the request itself is invalid, we had an escape-hatch to skip re-dispatching the request to ApplicationController. However, it was possible to work around this by 'layering' the errors. For example, if you made a request which resulted in a 404, but **also** had some other invalidity, the escape hatch would not be triggered.
This commit ensures that these kind of 'layered' errors are properly handled, without logging warnings. It also adds detection for invalid JSON bodies and badly-formed multipart requests.
The user-facing behavior is unchanged. This commit simply prevents warnings being logged for invalid requests.
Many security scanners ship invalid mime types, this ensures we return
a very cheap response to the clients and do not log anything.
Previous attempt still re-dispatched the request to get proper error page
but in this specific case we want no error page.
Previously our custom exception handler was unable to handle situations
where an invalid mime type was sent, resulting in a warning log
This ensures we pretend a request is HTML for the purpose of rendering
the error page if an invalid mime type from a scanner is shipped to the app
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
