Method in the Gateway to adopt and return the last accumulated validator set

[aakoshh]

As the Fendermint interpreter, during the checkpointing period I want to hand cross messages to the Gateway and expect it to accumulate validator set changes without adopting them immediately.

At the end of the checkpointing period, I want to call a method on the Gateway which returns to me the next validator set to adopt in the upcoming blocks, that is, the last finalized validator set in the parent, including its Configuration Number and the power table. I want the contract to remember and adopt this as the current validator set from now on.

[dnkolegov]

Implemented here - https://github.com/consensus-shipyard/ipc-solidity-actors/tree/membership

You can use

• newMembership(uint64 n, FvmAddress[] memory validators, uint256[] memory weights) to adopt a new membership (validator set) explicitly under SYSTEM actor privileges if needed
• commitParentFinality(ParentFinality calldata finality, uint64 n, FvmAddress[] calldata validators, uint256[] calldata weights to adopt a new membership along with a parent finality message under SYSTEM actor
• getLastMembership to return the last adopted membership