[enhancement] Enable `clear_cache()` to spread upward in the packet tree

[alxroyer-thales]

Enable clear_cache() to spread upward in the packet tree (underlayer and parent packets),
and not downward as currently (fields and payload).

[alxroyer-thales]

Provides a straight forward implementation for #4706.

Also improves performance in case of large packet trees with many packet fields (see #4705).

[alxroyer-thales]

Successfully implemented in a Scapy wrapper.

I'll provide a PR soon to propose the integration of it directly in Scapy implementation.

[gpotter2]

Hm. Thanks for the suggestion. A bit unsure, but it feels like a pretty decent idea to me. I agree that the current behavior of caching can be hard to grasp.

@p-l- @guedou What do you think?

[alxroyer-thales]

In order to ensure both 1) improve performance, and 2) avoid breaking the API, we could add parameters to clear_cache() to provide both behaviours:

• Clear things downwards (fields and payload) by default,
• Clear things upwards (parent and underlayer) for [bug] Performance issues with nested PacketFields #4705, [bug] Force cache reset as soon as the payload or a packet field is modified #4706 concerns.

[gpotter2]

Thinking about this, I feel like it could be achieved by just doing pkt.firstlayer().clear_cache(). Unsure there's a real use case, considering that building the parent layer already properly rebuilds everything if the child layer has been changed.