Add @frozen_after_init decorator to make some @dataclass uses safer with V2

[Eric-Arellano]

Problem

In #8423 (comment), we realized the need to be able to not always use frozen=True when using dataclasses for the V2 engine. Specifically, when we need a custom __init__(), we cannot have frozen=True because __init__() must be able to mutate the object's attributes.

However, we still want to keep the benefits of frozen=True as much as possible. After we create the object, we do not want to allow any additional modifications. This is important because the V2 engine uses the __hash__() of an object to determine its node ID - if the object mutates after creation, the ID will become invalid.

Solution

As suggested in #8423 (comment), introduce a new decorator @frozen_after_init that only allows setting attributes in the __init__() and then marks the object as frozen after.

This is done by setting a flag in the __init__() to mark that the object should there-on-out be frozen. Then, we modify __setattr__() to check for that flag to prohibit any mutations other than those in the __init__().

Result

@dataclass(unsafe_hash=True) can now be marked with @frozen_after_init so that we can have the custom __init__()s but not lose the benefits of frozen=True.

This also works on any arbitrary class.

We do not yet enforce that dataclasses must use this attribute (or frozen=True), which is left for #8181.

[cosmicexplorer]

Thank you so much!!!

[benjyw]

NEAT!