Currently, the process's main thread runs `main()` -> `__wine_main()` -> `apple_main_thread()` -> `CFRunLoopRun()`. A run loop source is added that creates the Wine main thread and runs `start_main_thread()`. With this MR, the Wine main thread calls `transform_mac_main_thread()` in `server_init_process()`. That essentially calls `NtCreateThreadEx()`, but instead of creating a new pthread it adds a source to the main thread run loop to run `start_thread()`. This transforms the process main thread into a Wine thread, after that it calls the PE `__wine_mac_run_cfrunloop()` in ntdll. That does a run_mac_cfrunloop Unix call, which just runs `CFRunLoopRun()`. The process main thread is now a Wine thread, but is otherwise still a normal Mac application main thread (including the ability for winemac to transform into a Cocoa application). In theory it might be possible to avoid the initial `CFRunLoopRun()` and have `apple_main_thread()` just spawn the Wine main thread and wait to run `start_thread()`, but the extra `CFRunLoopRun()` on the stack doesn't hurt anything. An exception/crash on the process main thread will be caught like a normal syscall fault, but `__wine_mac_run_cfrunloop()` prints an error and terminates the process. I think this is an improvement over the current behavior, which is a segfault in the signal handler when it tries to access the non-existent TEB that results in the thread hanging and eating 100% CPU. (Arguably it would be even better to unregister the signal handler and re-throw the signal so the user gets a normal macOS crash report with a helpful backtrace, maybe this could be done later.) Besides exception handling, having the main thread be a Wine thread will also allow the full use of Wine logging functions (!9503), and should allow winemac to be simplified (where many AppKit functions must be called from the main thread, and events are delivered to the main thread). -- v4: ntdll: Convert the macOS main thread into a Wine system thread on launch. https://gitlab.winehq.org/wine/wine/-/merge_requests/9579