NetworkManager is single-threaded and uses a mainloop.
However, sometimes we may need multiple threads. For example, we will
need to write sysctl values asynchronously, using the glib thread-pool.
For that to work, we also need to switch the network-namespace of the
thread-pool thread. We want to use NMPNetns for that. Hence it's better
to have NMPNetns thread-safe, instead of coming up with a duplicate
implementation. But NMPNetns may want to log, so we also need nm-logging
thread-safe.
In general, code under "shared/nm-utils" and nm-logging should be usable
from multiple threads. It's simpler to make this code thread-safe than
re-implementing it. Also, it's a bad limitation to be unable to log
from other threads. If there is an error, the best we can often do is to
log about it.
Make nm-logging thread-safe. Actually, we only need to be able to log
from multiple threads. We don't need to setup or configure logging from
multiple threads. This restriction allows us to access logging from the
main-thread without any thread-synchronization (because all changes in
the logging setup are also done from the main-thread).
So, while logging from other threads requires a mutex, logging from the
main-thread is lock-free.
It's not yet used, but it will be. We will need nm_sd_utils_unbase64mem()
to strictly validate WireGuard settings, which contain keys in base64 encoding.
Note that we also need a stub implementation for logging. This will do
nothing for all logging from "libnm-systemd-shared.a". This makes
sense because "libnm.so" as a library should not log directly. Also,
"libnm.so" will only use a small portion of "libnm-systemd-shared.a" which
doesn't log anything. Thus this code is unused and dropped by the linker
with "--gc-sections".
