[PATCH v2 0/1] MR4683: ntdll: Restore fs32_sel only if pthread TEB was not set already in init_handler.

[Rémi Bernon (＠rbernon)]

Fixes spurious crashes in Steam when downloading games.

The download causes a huge amount of SIGUSR1 signals, and it becomes
very likely that one signal will be received while being inside the
syscall or unix call dispatchers.

When this happens, it can be received within the small range of
instructions where %fs has been restored but we have not yet switched
to the syscall stack, or the other way around in the return path.

The signal handler then was restoring the 32bit %fs while returning to
the syscall dispatcher, then we are entering a syscall with %fs set to
the wrong value.

--
  v2: ntdll: Avoid breaking leave_handler heuristics for fs32_sel restore.

https://gitlab.winehq.org/wine/wine/-/merge_requests/4683