852128f18 made the screen private preallocated, so it exists on every
screen as soon as the single global key is registered. sync_fd_screen_priv()
therefore never returns NULL again, which breaks the "already initialized"
check in miSyncFdScreenInit() and the guards in miSyncInitFenceFromFD() /
miSyncFDFromFence(), which then call through a zeroed funcs pointer.
Multi-GPU setups crash on this: with glamor on one screen and a non-glamor
driver (NVIDIA blob under PRIME) on the other, XSyncCreateFenceFromFD on
the latter kills the server.
Use a filled-in funcs as the sentinel instead of the private's presence.
Assisted-by: Claude Opus 5 <noreply@anthropic.com>
Signed-off-by: Kiyoshi Spreclerg <kiyoshi_pip@protonmail.com>
3e2fccbb7a