Correct and clarify "6.3.1 Updating targets metadata" for delegated targets roles #214
Description
Step three of 6.3.1 states:
Sign the updated targets metadata with at least a THRESHOLD of keys for the associated targets role (either the top-level targets role, or a delegated targets role).
For delegated targets roles, keys are associated with delegations (edges), not roles (nodes) in the delegation graph. The spec glosses over a lot of detail here. I think the intended meaning is that for each incoming delegation to a role, the metadata is signed for a threshold of the keys for that specific delegation edge. There is not necessarily just one threshold or one set of keys.
I see two choices: we could fix the spec for all these edge cases for strange delegation graphs (e.g. #177), or adjust the spec to explicitly state that delegation graphs must be trees (i.e. each targets role must have only one incoming delegation). I haven't heard of anyone using non-tree delegation graphs in practice, and supporting these use cases makes code more complex, and therefore probably less secure.