slashing interface

[niklasad1]

What does this PR:

It tries to introduce a few traits (Misconduct, OnEndEra, OnSlashing) to introduce a generic interface to slash misbehavior(s)

Roughly those have the following functionality:

• Misconduct: Trait that defines a type that handles severity level
• OnEndEra: Misconduct that should be slashed in the end of era instead of rolling
• OnSlashing: A trait to handles balance and concrete slashing, i.e, need to both call T::Currency::free_balance() and T::Currency::slash() based on severity_level estimated by the
  Assumed to implemented on top of a module that handles Currency a.ka BalanceOf<T>
  Misconduct

Example use case

pub struct StakingSlasher<T>(T);

impl<T: Trait> OnSlashing<T::AccountId, Exposure, Severity> for StakingSlasher<T>
where
	Severity: Into<BalanceOf<T>>,
	Exposure: Into<BalanceOf<T>>,
{....}

/// Misconduct type
pub struct MyMisconduct<T>(T);

impl<T: Trait> Misconduct for MyMisconduct<T> {
   type Severity = u64;

    fn as_misconduct_level(&self, _severity: Fraction<Self::Severity>) -> u8 {...}

    fn on_misconduct(&mut self, _misbehaved: &[Self::AccountId]) -> Fraction<Self::Severity> {..}
}

let misconduct = MyMisconduct(SomeModule);
let misbehaved = vec![FakeSlashRecipient::default()];
// will probably require type parameters..
let misconduct_level = rolling_data(&misbehaved, &mut misconduct);
// use severity_level to determine whether to kick out the validator or not

TODO (before merge)

1. Use checked_operations on Fraction
2. Add documentation (will not do before we have some sort of consensus)
3. bump impl version
4. How to deal with I'm online messages #2719

A possible solution to #2719 could be to change
pub fn end_of_era(end_of_era: &E) -> u8
// don't slash ignored based on some proof
pub fn end_of_era(end_of_era: &E, ignore: &[AccountId]) -> u8

Follow-up

Implement the concrete misconduct types

References

https://research.web3.foundation/en/latest/polkadot/slashing/

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[rphmeier]

docs?

[niklasad1]

Closing, has issues