[Webkit-unassigned] [Bug 254777] New: REGRESSION: PostMessage with transfer object is broken between contexts

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=254777

            Bug ID: 254777
           Summary: REGRESSION: PostMessage with transfer object is broken
                    between contexts
           Product: WebKit
           Version: Safari 16
          Hardware: All
                OS: All
            Status: NEW
          Severity: Major
          Priority: P2
         Component: WebCore Misc.
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: tooker at gmail.com

Sending a transferable object like a MessagePort to communicate between different contexts no longer works properly.  When I use the transfer argument to postMessage the event handler gets an event with a null data property.

Steps to reproduce:

1. Call postMessage({foo: 'bar', port}, self.origin, [port]) from page script.
2. Add window.message event handler to a different context such as an
   extension's content script context and evaluate the event callback arg.
3. Observe the ev.data is null instead of {foo: 'bar', port: port}

Expected behavior:
For the message handler to get an event object with a non-null data property filed with the contents sent from the postMessage caller.

I first noticed this on the Orion Safari fork and filed a bug with them, https://orionfeedback.org/d/4730-postmessage-with-transfer-object-is-broken, but after updating to Safari 16.4 on the macOS 13.3 I see the same bug there, which leads me to believe this is a core WebKit regression.  This is directly affecting my Safari extension which no longer works on anyone's 16.4 based Safari browsers.  The same code works fine on all Chromium and Firefox.

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