grandpa: report equivocations

[andresilva]

This PR adds equivocation reporting for GRANDPA. Equivocation detection is implemented in finality-grandpa crate and exposed through Environment hooks (prevote_equivocation and precommit_equivocation).

The GRANDPA runtime module is extended with a report_equivocation method that takes an equivocation proof and a session membership proof. This method validates the membership proof using an abstract KeyOwnerProofSystem (historical session module currently) and the equivocation proof (different vote targets, valid signatures, etc.), if both are valid an offence is reported to an abstract ReportOffence type (offences module currently). Additionally, the method submit_report_equivocation_extrinsic creates and submits a signed transaction for reporting an equivocation.

From the node-side, when we want to report an equivocation we need to create the session membership proof. In particular, the membership proof can only be created at a block when the given validator is part of the current session. We figure out the last block at which the current set id is active in a given chain by looking at previously signalled authority changes and create the membership proof at that block. We then call submit_report_equivocation_extrinsic at the best block.

Important

1. In order to report an equivocation we need to send a signed transaction, therefore we need to have some key available for this, taking into account that this is also the account that will be potentially rewarded. I created a new app-specific crypto ("fish") that should be used for all equivocation reporting tasks (this will also be used for BABE later on). Any comments on this?

2. The current implementation for generating membership proofs requires having state available for the block on which the session where the equivocation happened was valid. This is not problematic right now since we require validators to run as archive. A future implementation could leverage offchain workers for generating full membership proof tries when a session ends and storing them locally.

TODO:

• Publish finality-grandpa 0.9.1 (includes fixes for building under no-std)
• Fix execution context of submit_report_equivocation_extrinsic call (needs to be offchain context but it's not easy to use that now outside of actual offchain workers)
• Abstract srml/grandpa reporting logic to trait (so that we can implement it for () to disable reporting, needed to integrate in node-template)
• Merge Add documentation to SubmitSignedTransaction and actually make it work #4200 and drop cherry-picked changes
• Cleanup
• Tests
• Docs

polkadot companion: paritytech/polkadot#1000

[burdges]

We only need the key signing a report transaction to be an account key so that it can pay some fee, right? We've no existing mechanism for actions signed by session keys to change their controller account fees yet? We could maybe permit free reports by the grandpa ed25519 key if the deduplication works well enough, not so universally applicable though. We can chat about this when you like. :)

[rphmeier]

Just took a look at all commits since my last review. LGTM!

[bkchr]

These errors should be unique for the whole runtime.

Now, it could happen that they overlap and will be useless.

[andresilva]

Where should they go then? Or do I just look for the next free u8?

[AurevoirXavier]

This should be in dev-dependencies, pallet-grandpa just used in tests.