dco: process messages immediately after read

dco: process messages immediately after read

Currently, reading and processing of incoming DCO messages are
decoupled: notifications are read, parsed, and the relevant information
is stored in fields of dco_context_t for later processing (with the only
exception being stats). This approach is problematic on Linux, since
libnl does not allow reading a single netlink message at a time, which
can result in loss of information when multiple notifications are
available.

This change adopts a read -> parse -> process paradigm. On Linux,
processing is now invoked directly from within the parsing callback,
which libnl calls for each received netlink packet. The other interfaces
are adapted accordingly to unify the processing model across all
platforms.

On Linux, however, a DEL_PEER notification from the kernel triggers a
GET_PEER request from userspace, which clutters the netlink
communication logic and can lead to errors or even process exit when
multiple simultaneous DEL_PEER notifications are received. To avoid
this, introduce a lock that prevents requesting stats while we are still
busy parsing other messages.

Reported-by: Stefan Baranoff <stefan.baranoff@trinitycyber.com>
Github: OpenVPN/openvpn#900
Github: OpenVPN/openvpn#918
Github: fixes OpenVPN/openvpn#919

Change-Id: Iefc251cb4483c0b9fb9d6a5207db4445cd884d52
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1403
Message-Id: <20251128112705.12613-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg34785.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>