Implement MinerEligibleForElection in block validation

[nicola]

We should use the current epoch to perform this actors check.
Miner should not be allowed to mine if not eligible

cc @Stebalien

[anorth]

Wait, what? Should't it be at WinningPoStSectorSetLookback? We've been gathering the election requirements together into one MinerEligibleForElection function on the assumption that it's all using the same lookback. I don't think you meant to say that, e.g., the power meeting consensus minimum check is made at the current epoch.

I think something might have been lost here in the change to consensus fault penalisation. Previously, when a miner was terminated, that check was made at the current epoch. But everything else was at the election lookback. Now it's apparently more complicated.

@nicola could you please lay out all of the election criteria and which epoch they should be evaluated at? The actors will then need to break apart MinerEligibleForElection into two functions, one intended for the state at each of the epochs.

[nicola]

I am currently adding clarity by writing this into this spec draft.

All the checks for MinerEligibleForElection are for the same epoch and this epoch is the epoch of election.

[nicola]

Just to clarify what is written in the spec here:

Lotus should do three checks:

• Eligibility check (consensus fault fee, consensus fault active, debt) using MinerEligibleForElection at current epoch
• Miner validity check (check if miner exists ) at current epoch
  • Note: this is currently done in lotus with minerIsValid
• Minimum power check (check that miner has min miner size) at WinningPoStLookback
  • Note: this is currently done in lotus with MinerNominalPowerMeetsConsensusMinimum

So, in practice, the actions to take in lotus are:

• Make sure that minerIsValid, MinerNominalPowerMeetsConsensusMinimum are called at correct epochs
• Add MinerEligibleForElection check

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Note that this means that eligibility does NOT check for min miner size, the following check should be removed: https://github.com/filecoin-project/specs-actors/blob/master/actors/builtin/miner/miner_state.go#L1104-L1106

This is at least how it currently works, we could try to change this check to be at the same epoch, but this will require analysis and if important we should do it at a later time.

This note is being tracked here: filecoin-project/specs-actors#1102

[anorth]

Actors intends to give you two functions: one to execute at parent state, and one with the lookback state.

[anorth]

The state inspection methods for the election eligibility predicates are now available at https://github.com/filecoin-project/specs-actors/blob/master/actors/states/election.go

[arajasek]

Fixed in #4188