A CORS preflight request is an "OPTIONS" request sent to a server prior to a
cross-origin XMLHttpRequest or Fetch request. The server's response determines
which HTTP request methods are allowed and supported, and whether credentials
such as Cookies and HTTP Authentication should be sent with requests.
A CORS preflight request will only be sent if certain conditions are met. For
example, it will be sent for requests that have potentially unsafe HTTP
methods [1] or request headers [2]. See the NeedsPreflight function in
services/network/cors/cors_url_loader.cc for full details.
CORS preflight functionality is implemented in the network service and will not
be triggered if the client handles the request instead of allowing it to proceed
over the network. Since the preflight request itself also runs in the network
service it cannot be intercepted by the client.
[1] https://fetch.spec.whatwg.org/#cors-safelisted-method
[2] https://fetch.spec.whatwg.org/#cors-safelisted-request-header
The request.trusted_params.isolation_info.site_for_cookies value must
match request.site_for_cookies.
This change also adds unit test coverage for cross-origin GET redirects.
This fixes an IsCanonical() DCHECK failure triggered by calling
CanonicalCookie::Create for a non-cookieable URL.
This change also adds unit test coverage for cross-origin cookie
behavior with sub-resource requests (iframe, XHR, Fetch).
Marshall Greenblatt