Description

BEEFY voters should only vote on GRANDPA finalized blocks - vote only on canonic chain/fork.

Votes on blocks from different forks should (automatically) lead to stake slashing (more aggressive slashing than GRANDPA equivocations, since this scenario is intentionally dishonest).

We should add client/voter detection of such misbehavior as well as runtime logic for reporting proofs of misbehavior and subsequent slashing consequences.

Proposed solution

Runtime side

• add new pallet::call to pallet_beefy similar to report_equivocation that takes InvalidPayloadProof
• hook pallet_beefy_mmr to pallet_beefy::EquivocationReportSystem to be able to verify InvalidPayloadProof:
  • verify inner GrandpaProof to make sure the current chain fork is finalized at least up to offending vote height,
  • verify offending_vote.mmr_root != on-chain mmr_root (combined with above, it proves what we need)
• report offense to staking so offending vote author gets slashed

Client-side

• add "fisherman" capabilities to voter gossip - on detecting votes for historical forks, it builds the required proof of misbehavior and submits report.

cc @Lederstrumpf, @andresilva