EPMB: Improve `Emergency` and `manage` scenarios

[kianenigma]

See:

polkadot-sdk/substrate/frame/election-provider-multi-block/src/lib.rs

Lines 605 to 658 in 18f6251

	/// Manage this pallet.
	///
	/// The origin of this call must be [`Config::AdminOrigin`].
	///
	/// See [`AdminOperation`] for various operations that are possible.
	#[pallet::weight(T::WeightInfo::manage())]
	#[pallet::call_index(0)]
	pub fn manage(origin: OriginFor<T>, op: AdminOperation<T>) -> DispatchResultWithPostInfo {
	use crate::verifier::Verifier;
	use sp_npos_elections::EvaluateSupport;
	let _ = T::AdminOrigin::ensure_origin(origin);
	match op {
	AdminOperation::EmergencyFallback => {
	ensure!(Self::current_phase() == Phase::Emergency, Error::<T>::UnexpectedPhase);
	// note: for now we run this on the msp, but we can make it configurable if need
	// be.
	let voters = Snapshot::<T>::voters(Self::msp()).ok_or(Error::<T>::Snapshot)?;
	let targets = Snapshot::<T>::targets().ok_or(Error::<T>::Snapshot)?;
	let desired_targets =
	Snapshot::<T>::desired_targets().ok_or(Error::<T>::Snapshot)?;
	let fallback = T::Fallback::instant_elect(
	voters.into_inner(),
	targets.into_inner(),
	desired_targets,
	)
	.map_err(|e| {
	log!(warn, "Fallback failed: {:?}", e);
	Error::<T>::Fallback
	})?;
	let score = fallback.evaluate();
	T::Verifier::force_set_single_page_valid(fallback, 0, score);
	Ok(().into())
	},
	AdminOperation::EmergencySetSolution(supports, score) => {
	ensure!(Self::current_phase() == Phase::Emergency, Error::<T>::UnexpectedPhase);
	// TODO: hardcoding zero here doesn't make a lot of sense
	T::Verifier::force_set_single_page_valid(*supports, 0, score);
	Ok(().into())
	},
	AdminOperation::ForceSetPhase(phase) => {
	Self::phase_transition(phase);
	Ok(().into())
	},
	AdminOperation::ForceRotateRound => {
	Self::rotate_round();
	Ok(().into())
	},
	AdminOperation::SetMinUntrustedScore(score) => {
	T::Verifier::set_minimum_score(score);
	Ok(().into())
	},
	}
	}

Some initial code is here, but it is not very well thought out.

The main scenario that we should be able to support is:

• Election is not succeeding for whatever reason.
• Governance wants to set a manual solution, and make it go through.
• They would have to:
  • ForceSetPhase(Emergency)
  • EmergencySetSolution for multiple pages (not possible now)
  • ForceSetPhase(Done) (signaling staking to be complete)
• This scenario should be well tested.
• If the rest of the scenarios remain, they should also be fully tested and their side effects understood. e.g. Fix origin check in EPMB's manage extrinsic #10248 (comment)

[kdonthi]

I can also take this one

[kianenigma]

@kdonthi won't advice taking this one, as there is ongoing work on the pallet and discussion. See #10248