I'm going to commit this change tomorrow, February 7.
It's always possible to change the implementation later if so desired.

if_foreach_ifp ?

Do you have by chance any plans on merging accessors to 13/ to ease other merges?

Fix python code formatting.

Works on -HEAD for me.
I'm going to commit it tomorrow, on Jan 30 if there are no objections.

Ack, that's a good one!
My initial approach was to perform zero in the -> init() functions (e.g. nlmsg_get_ns_<mbuf|lbuf|buf>), preferably via explicit field initialisation. Apparently the latter one has failed.
ATM I'm leaning towards doing the explicit bzero() in all init functions, but need to think a bit more about it.
To unblock the fix, let's require the explicit writer initialisation for now.

In D37923#861728, @jlduran_gmail.com wrote:

In D37923#861709, @asomers wrote:

It also means that an unprivileged user won't be able to simply type "kyua test ..." and expect the unprivileged test cases to work.

I'll retry everything tomorrow, but in this regard, things are working as expected.
Unprivileged users:

• Can run unprivileged test cases
• root test cases are skipped
• VNET test cases are skipped entirely (because an unprivileged user cannot create jails)

What is not working for me, is:

class TestVnetSimple(SingleVnetTestTemplate):
    @pytest.mark.parametrize(
        "user_tuple",
        [
            pytest.param(
                ["kldload if_epair", "module already loaded or in kernel"],
                marks=pytest.mark.require_user("root"),
                id="root",
            ),
            pytest.param(
                ["kldload if_epair", "Operation not permitted"],
                marks=pytest.mark.require_user("unprivileged"),
                id="unprivileged",
            ),
        ],
    )
    def test_parametrize_require_user(self, user_tuple):
        command, output = user_tuple
        assert output in subprocess.getoutput(command)

Specifically the test_parametrize_require_user[root] parameter. It gets an Operation not permitted error, as if unprivileged.
If you s/SingleVnetTestTemplate/BaseTest everything works as expected.

Thank you for testing & catching the issue!
I'll check this case and update the diff.

4. In the meantime the kernel delayed object reclamation model issue is fixed, maybe adapt 80fc25025ffcb0d369fc0b6d4d272ad6fd3f53c3 ?

Committed in 20ea7f26e413 .

3. An atf_get_srcdir-equivalent to eventually read files relative to the source directory, however I think the idea is to disassociate from ATF later on.

This one slipped through the cracks, thanks for reminding! Will add in a day or two.

I thought of passing it via wrapper initially, but ended up with a bit more natural way - getting it from pytest itself.
I committed an example here.

Thanks for cleaning that up!
I have the new manpages as WIP, but currently focused on the netlink. Will probably come up with something within a month or so.