ngclient updater: API functions argument validation

Description of issue or feature request: In the tuf/ngclient/updater.py module we are providing a couple API functions with arguments: Updater.__init__, get_one_valid_targetinfo, updated_targets and download_targets. It seems to me it’s a good idea to decide which arguments in those functions require validation and if there should be any what it should be.

Current behavior: No validation is done for:

• repository_dir in Updater.__init__()
• target_path in Updater.get_targetinfo()
• maybe targetinfo in Updater.download_target()
• maybe target_base_url in Updater.download_target()

Expected behavior: Decide if we want to add validation and if there is any what it should be:

• repository_dir in Updater.__init__()
• target_path in Updater.get_targetinfo()
• targetinfo in Updater.download_target()
• target_base_url in Updater.download_target()

[jku]

WRT validating TargetFile arguments, I guess the potential check would be “is this really a TargetFile instance?” but considering that A) we support static type checking (well almost anyway: #1633) and B) there is never any need to modify the targetfile in application code C) it will fail pretty gracefully anyway, I don’t think that check is needed…

We can validate that repository_dir does exist and throw an error otherwise

I think this currently ends up complaining that repository_dir/root.json does not exist – which seems like an accurate description of the problem: the updater isn’t interested in just the directory, it needs a directory with a root.json.

So potentially we don’t need to do anything here: I’m fine with that

[jku]

Yep, I think we agree here, let’s close