[RFC PATCH xserver] xwayland: make sure client is not gone in sync callback

[Olivier Fourdan]

> in XWayland, dri3_send_open_reply() is called from a sync callback, so
> there is a possibility that the client might be gone when we get to the
> callback eventually, which leads to a crash in _XSERVTransSendFd() from
> WriteFdToClient() .
> 
> Check if clientGone has been set in the sync callback handler to avoid
> this.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99149
> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1416553
> Signed-off-by: Olivier Fourdan <ofourdan at redhat.com>
> ---
>  This seems to be a fairly rare occurence, but we do have bugs filed both
>  upstream and downstream for this.
>  I don't have any core file unfortunately so this is based solely on
>  the addresses returned by the crash handler, thus the "RFC" on this
>  patch...
> 
>  hw/xwayland/xwayland-glamor.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/xwayland/xwayland-glamor.c b/hw/xwayland/xwayland-glamor.c
> index b3d0aab..65c3c00 100644
> --- a/hw/xwayland/xwayland-glamor.c
> +++ b/hw/xwayland/xwayland-glamor.c
> @@ -435,9 +435,12 @@ static void
>  sync_callback(void *data, struct wl_callback *callback, uint32_t serial)
>  {
>      struct xwl_auth_state *state = data;
> +    ClientPtr client = state->client;
>  
> -    dri3_send_open_reply(state->client, state->fd);
> -    AttendClient(state->client);
> +    if (!client->clientGone) {
> +        dri3_send_open_reply(client, state->fd);
> +        AttendClient(client);
> +    }
>      free(state);
>      wl_callback_destroy(callback);
>  }
> --

The fix has been succesfully tested in bug 100040

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=100040
Tested-by: Mark B <mark.blakeney at bullet-systems.net>