ament_xmllint stuck when no internet

[Kaetera]

Hello,

Context:

• ROS2 Humble
• Ubuntu 22.04.2, on VirtualBox

I have some network issues (among others preventing DDS discovery, but that's another story) leading me to switch from Ethernet to Wifi connection on my VM.
When on Wifi, when I call ament_xmllint from the CLI, it gets stuck for minutes (like 5-10minutes) before outputting a result (whether there is or not an lint error), while when on Ethernet, it completes in few seconds, on the same packages.
Same happens if I just unplug my Ethernet cable.

Expected behavior
ament_xmllint does not depends on network settings or connectivity and completes in few seconds.

Actual behavior
ament_xmllint is stuck for long minutes if I change my network settings.

Step to reproduce

1. Disconnect from the internet, eg. by disabling WiFi and unplugging the Ethernet cable
2. Run ament_xmllint src/my_package from the command line
3. The command should be stuck as described

Additional information
If I ctrl-C when it's stuck, I get his trace:

me@mymachine:/mnt/xxx_shared$ ament_xmllint src/my_package
^C
Traceback (most recent call last):
  File "/opt/ros/humble/bin/ament_xmllint", line 33, in <module>
    sys.exit(load_entry_point('ament-xmllint==0.12.6', 'console_scripts', 'ament_xmllint')())
  File "/opt/ros/humble/lib/python3.10/site-packages/ament_xmllint/main.py", line 104, in main
    subprocess.check_output(
  File "/usr/lib/python3.10/subprocess.py", line 420, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/usr/lib/python3.10/subprocess.py", line 503, in run
    stdout, stderr = process.communicate(input, timeout=timeout)
  File "/usr/lib/python3.10/subprocess.py", line 1139, in communicate
    stdout = self.stdout.read()
KeyboardInterrupt

Looking in the code I see URLs, would it try to download the schema every time and so be stuck on connectivity issues ?