thread safety of fhandler_base_overlapped

[David McFarland]

I'm seeing a problem where occasionally write() returns EAGAIN on a
blocking pipe. I believe this happens when multiple threads are writing
concurrently to the pipe, and the overlapped structure is modified by
one thread while another one is between calls to WFMO and
GetOverlappedResult.

I found this change:

=====
commit 2a4b1de773e6159ea8197b6fc3f7e4845479b476
Author: Corinna Vinschen <corinna@vinschen.de>
Date:   Mon Mar 23 21:06:00 2020 +0100

    Cygwin: serial: use per call OVERLAPPED structs

    Sharing the OVERLAPPED struct and event object in there between
    read and select calls in the fhandler might have been a nice
    optimization way back when, but it is a dangerous, not thread-safe
    approach.  Fix this by creating per-fhandler, per-call OVERLAPPED
    structs and event objects on demand.

    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
=====

which seems to fix a similar problem in fhandler_serial.

Is is possible that this needs to be done for fhandler_base_overlapped
as well? I'll try using per-call buffers to see if it fixes my problem.