302600 2025-11-16 15:14:52 -0800 Fix main thread initialization when disabling WebThread 2025-11-16 19:54:52 -0800 1 1 1 Unclassified WebKit WebCore Misc. WebKit Nightly Build Unspecified Unspecified RESOLVED FIXED InRadar P2 Normal --- 176812 1 ddkilzer ddkilzer webkit-bug-importer oldest_to_newest 2159121 0 ddkilzer 2025-11-16 15:14:52 -0800 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. 2159122 1 webkit-bug-importer 2025-11-16 15:15:14 -0800 <rdar://problem/164844902> 2159128 2 ddkilzer 2025-11-16 15:34:13 -0800 Pull request: https://github.com/WebKit/WebKit/pull/54015 2159165 3 ews-feeder 2025-11-16 19:54:50 -0800 Committed 303102@main (f89bce6a688c): <https://commits.webkit.org/303102@main> Reviewed commits have been landed. Closing PR #54015 and removing active labels.