[webkit-changes] [WebKit/WebKit] a9b66f: Mitigate crashes when removing KVO from NSWindow i...

[Wenson Hsieh]

  Branch: refs/heads/main
  Home:   https://github.com/WebKit/WebKit
  Commit: a9b66fad8de2f2a774d86bfd78afa01b77f6df8a
      https://github.com/WebKit/WebKit/commit/a9b66fad8de2f2a774d86bfd78afa01b77f6df8a
  Author: Wenson Hsieh <wenson_hsieh at apple.com>
  Date:   2022-12-12 (Mon, 12 Dec 2022)

  Changed paths:
    M Source/WebKit/UIProcess/mac/WebViewImpl.mm
    M Tools/TestWebKitAPI/Tests/WebKitCocoa/PrepareForMoveToWindow.mm

  Log Message:
  -----------
  Mitigate crashes when removing KVO from NSWindow in -[WKWindowVisibilityObserver stopObserving:]
https://bugs.webkit.org/show_bug.cgi?id=249103
rdar://102360839

Reviewed by Patrick Angle.

After the fix in 256334 at main, Music sometimes crashes when destroying `NSWindow`, when
`WKWindowVisibilityObserver` attempts to remove key-value observers for "contentLayoutRect" and
"titlebarAppearsTransparent" from the window that were not added in the first place.

While I haven't been able to reproduce the crash locally or come up with a test case that (exactly)
replicates the crashing stack during `NSWindow` destruction, it should be possible to avoid it
altogether by guarding KVO registration and unregistration by using an associated object on the
`NSWindow` to indicate when `WKWindowVisibilityObserver` has key-value observers to the window. If
this flag is not set, then we avoid attempting to unregister KVO; similarly, if this flag is set,
then we avoid attempting to re-register KVO.

Test: WKWebView.PrepareForMoveToWindowShouldNotCrashWhenRemovingWindowObservers

* Source/WebKit/UIProcess/mac/WebViewImpl.mm:
(-[WKWindowVisibilityObserver startObserving:]):
(-[WKWindowVisibilityObserver stopObserving:]):

Use the `_impl` pointer as the context key.

* Tools/TestWebKitAPI/Tests/WebKitCocoa/PrepareForMoveToWindow.mm:

Add a (somewhat contrived) API test that exercises the mitigation.

Canonical link: https://commits.webkit.org/257736@main