[Webkit-unassigned] [Bug 201180] New: Cannot send in-scheme CORS requests with WKURLSchemeHandler

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=201180

            Bug ID: 201180
           Summary: Cannot send in-scheme CORS requests with
                    WKURLSchemeHandler
           Product: WebKit
           Version: Safari 12
          Hardware: iPhone / iPad
                OS: iOS 12
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebKit API
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: webkitbugzilla at alastair.is

If I set up a WKURLSchemeHandler and have it load, e.g. test://host-one/index.html, and on that page I make a fetch() request to test://host-two/test.json the request fails before it is able to send, telling me:

> Cross origin requests are only supported for HTTP.

When I dug around the source code a little I found the following comment:

> Cross-origin requests are only allowed for HTTP and registered schemes. We would catch this when checking response headers later, but there is no reason to send a request that's guaranteed to be denied.

https://github.com/WebKit/webkit/blob/fc41ae69af5bce93e59ffdc1c828515199d12c44/Source/WebCore/loader/DocumentThreadableLoader.cpp#L175

A WKURLSchemeHandler is, I presume, a "registered scheme", so it seems as though the intention is for this to work? It would be great to get a confirm that either this is indeed a bug or whether it is intended behaviour, so that we can look for alternative solutions.

I've set up a very simple test XCode project here: https://github.com/alastaircoote/wkwebview-cors-test

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