Re: [PATCH v22 0/8] MR3103: server: Create and use a desktop shared mapping for GetCursorPos.

[Jinoh Kang (＠iamahuman)]

Jinoh Kang (@iamahuman) commented about dlls/win32u/input.c:
> - SERVER_START_REQ( set_cursor )
> - while (get_shared_desktop( &lock ))
> {
> -
>    if ((ret = !wine_server_call( req )))
> -
>    SHARED_READ_BEGIN( &lock.shared->desktop, desktop_shm_t )
>    {
> -
>        pt->x = reply->new_x;
> -
>        pt->y = reply->new_y;
> -
>        last_change = reply->last_change;
> -
>        if ((valid = lock.id == shared->obj.id))
> -
>        {
> -
>            pt->x = shared->cursor.x;
> -
>            pt->y = shared->cursor.y;
> -
>            last_change = shared->cursor.last_change;
> -
>            ret = TRUE;

If the current shared-read iteration fails due to destroyed object, `ret = TRUE` will live on even if the next iteration sets `valid = FALSE`. This is kept as-is if `get_shared_desktop()` returned NULL.

In general, the number of shared read retries shouldn't affect the outcome of the loop. This means that any variables *conditionally*[^1] written by the read critical section should be considered unknown / garbage.[^2]

[^1]: `valid` is unconditionally written.
[^2]: Rule of thumb: a read section should *always* write to a variable or not at all. If a variable is ever written, it should be read *only after* it has been written in the current iteration.