bug#45821: Emacs UDP support on Windows

[Robert Pluim <rpluim@gmail.com>]

>>>>> On Mon, 02 Jan 2023 14:41:00 +0200, Eli Zaretskii <eliz@gnu.org> said:

    >> Cc: "45821@debbugs.gnu.org" <45821@debbugs.gnu.org>
    >> From: Robert Pluim <rpluim@gmail.com>
    >> Date: Mon, 02 Jan 2023 11:22:03 +0100
    >> 
    Alex> ❌ Indeed, TLS is broken -> Eww to
    >> https://www.gnu.org<https://www.gnu.org/> fails to load the page (
    >> see attached image – Emacs instance on the left, compiled with UDP
    >> patch, didn’t load gnu.org while on the right side- default Emacs
    >> build for 28.1 opens it without any issues)
    >> 
    >> Yep. Last time I looked at this, the TLS handshaking fails to complete
    >> (see src/process.c around line 5329 and the checking against
    >> GNUTLS_EMACS_HANDSHAKES_LIMIT) which means weʼre continually retrying
    >> the handshake without giving the remote end a chance to send us
    >> anything. Which I think means that our state machine for TLS
    >> negotiation is subtly incorrect, but only on MS-Windows.

    Eli> On MS-Windows, there's another state machine involved, the one
    Eli> vis-a-vis the reader thread we start to read the stuff from the
    Eli> network connection.  See reader_thread and sys_select in w32proc.c and
    Eli> sys_write, sys_read, _sys_read_ahead, _sys_wait_accept, and
    Eli> _sys_wait_connect in w32.c.

Hmm, in that case I then suspect that `sys_select' is indicating that
the socket it connected even when it isnʼt. I took a look, but nothing
looks obviously wrong (and itʼs not something I can currently test).

Robert
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