Implementing advmss ip route option

[barvius]

Hello everybody. I was missing the advmss functionality for static routes, and I decided to implement it. This option has been tested successfully in systemd-networkd, but has not tested in network manager.

Param name for ip route: advmss
Param name for systemd networkd: TCPAdvertisedMaximumSegmentSize
Param name for network manager: advmss

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[slyon]

Thank you very much for your contribution to Netplan, this PR is looking great! You covered all the backends and added nice testing around it. Kudos!

Could you please move the integration test (in routinging.py) up into the _CommonTests class? So we can check it actually works on NetworkManager, in addition to test_route_advmss (__main__.TestNetworkd) ... eth42 .............ok, e.g. next to test_per_route_advertised_receive_window.

Also, please add the new setting to the fuzzer config in tests/config_fuzzer/schemas/common.js:

                "advertised-mss": {
                    type: "integer",
                    minimum: 0
                }

Besides that, I left a few small inline comments. Also, let's wait for some comments from @vorlonofportland and @daniloegea.

[slyon]

Thank you very much for adding proper integration tests!

The PR overall LGTM!

But: CI is still failing the NetworkManager tests... this seems to be because the advmss route_option (https://networkmanager.dev/docs/api/latest/nm-settings-nmcli.html) is only supported since NetworkManager 1.39.8 (1.40.0 stable) (NetworkManager/NetworkManager@90e7afc), which is not available in Ubuntu Jammy, that is still used in our CI system currently.

I see this output on Jammy, while it works fine on Noble:

# nmcli connection modify netplan-eth0 +ipv4.routes "192.168.122.0/24 10.10.10.1 advmss=1400"
Error: failed to modify ipv4.routes: invalid option 'advmss=1400': unknown attribute 'advmss'.

So we might need to block this PR on #465, or alternatively, mark the corresponding NetworkManager tests to be skipped on NetworkManager < 1.39.8, e.g. something along those lines:

import gi

gi.require_version("NM", "1.0")
from gi.repository import NM

print("version:", NM.Client.new(None).get_version())

And then @unittest.skipif() backend is NetworkManager and version is too small.

[slyon]

I'm not sure about this one. But considering this is part of a private data structure _NetdefRouteIterator, accessing internal data structures without a getter should be acceptable.

[slyon]

I implemented the test skipping, rebased and squashed your commits. This should now be ready for merging!

Thanks a lot for your contribution to Netplan :)