Metadata API: prevent Delegation role names to be one of top level metadata roles

[MVrachev]

Description of issue or feature request:

Current behavior:
Delegation role names are not restricted in any way in the spec, but they are targets metadata role names.
That could lead to a problem if delegation role names are one of root, timestamp, snapshot or targets.

Expected behavior:
Make sure that delegation role names aren't one of the top-level metadata roles.

[joshuagl]

I wonder if this kind of logic and role name sanitisation more generally belongs in metadata API? Perhaps it should be part of the (as yet undesigned) repository interface?

[jku]

same check does make sense in a client: we would not want to accidentally overwrite our local root.json because a targets decides to delegate to a "root" role version 9000.

I don't think there are practical risks of that -- the client would have to succeed in downloading and verifying the file as a targets metadata before it would be written to local disk but this still seems like a reasonable check for a client to make

[jku]

the legacy client also prevents empty string as rolename -- this seems like a reasonable take for ngclient too

[joshuagl]

@MVrachev is this issue covered by #1630?

[MVrachev]

No, as the validation itself happens in the delegator class who have the full list of roles or in the case of #1630 that's Root.
Here the validation should happen in Targets the same way it's done in #1630.

[MVrachev]

When solving this issue please add tests in test_metadata_serialization.py the same way it's done in #1630 specifically see incorect_roots.

[kairoaraujo]

I am working on this issue.