Limit get_tree to a predefined set of types

[adrinjalali]

Right now get_tree is called in __init__ of many nodes, where they know what kind of child they expect. This can be a dict or a list for instance. In these cases, it'd be better for get_tree to accept an argument to limit the types of nodes it can return/construct.

For instance, instead of

"kwds": get_tree(state["content"]["kwds"], load_context),

we could have

"kwds": get_tree(state["content"]["kwds"], load_context, allowed_types=[DictNode]),

This would reduce the attack surface when loading a file.

[BenjaminBossan]

This would work for some but not all cases, right? E.g. a TupleNode can't know the type of its content.

For all other cases, it probably wouldn't hurt. However, I don't see yet how this could be used as an attack, since at this stage we're not actually instantiating any objects (which doesn't mean we shouldn't still do it).

A minor problem is that this could lead to circular imports, but I haven't checked if it actually occurs.

[adrinjalali]

This would work for some but not all cases, right? E.g. a TupleNode can't know the type of its content.

Correct, collections can load anything.

As for attacks, I can't think of a specific attack which would use this, but it would reduce the attack surface.