RESOLVED FIXED302600
Fix main thread initialization when disabling WebThread 
https://bugs.webkit.org/show_bug.cgi?id=302600
Summary Fix main thread initialization when disabling WebThread
David Kilzer (:ddkilzer)
Reported 2025-11-16 15:14:52 PST
Fix main thread initialization when disabling WebThread. When WebThread is enabled, WebCore::initializeMainThreadIfNeeded() (via +[WebView initialize]) returns early because WebCore::StartWebThread() performs "main" thread initialization when the WebThread starts. When WebThread is disabled, WebCore::initializeMainThreadIfNeeded() no longer returns early, which means main thread initialization may now incorrectly occur on a background thread if +[WebView initialize] is called from there. We must avoid this issue in WebCore::initializeMainThreadIfNeeded() by only initializing the main thread if pthread_main_np() is true, else schedule main thread initialization asynchronously on the main thread (a best-effort) using CFRunLoopPerformBlock(). We can't use synchronous main thread initialization (for example, using a semaphore) in case an app is using their own locking mechanism to initialize WebView on a background thread from the main thread.
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Radar WebKit Bug Importer
Comment 1 2025-11-16 15:15:14 PST
<rdar://problem/164844902>
David Kilzer (:ddkilzer)
Comment 2 2025-11-16 15:34:13 PST
Pull request: https://github.com/WebKit/WebKit/pull/54015
EWS
Comment 3 2025-11-16 19:54:50 PST
Committed 303102@main (f89bce6a688c): <https://commits.webkit.org/303102@main> Reviewed commits have been landed. Closing PR #54015 and removing active labels.
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