Add CANCEL_ROLE to TimelockController

🧐 Motivation In the forthcoming Threshold Network DAO, there’s a council with veto power over proposals. The only way to allow this with current implementation of TimelockController would be to grant the council the PROPOSER_ROLE, but this council shouldn’t be able to make proposals, according to our governance design. We need something like a CANCEL_ROLE that’s different from PROPOSER_ROLE and EXECUTION_ROLE. This feature could also be a solution for the “Proposer fight” problem that’s discussed in the documentation.

Discussion I wanted to do this by creating a contract that inherits from TimelockController, but since its variables and functions are marked as private it’s being difficult. In particular, we need to further restrict the cancel operation to a different role than proposers, but the modifier in cancel() forces us to override it and re-implement it, and we encounter the problem of _timestamps being private.

Anyway, if you find this interesting, I’d love to help, since I’m working on that anyway.

[cygnusv]

In the Threshold DAO, it was decided that a council multisig has veto power in case there’s a dangerous proposal (see https://forum.threshold.network/t/threshold-network-dao-proposal-v2/57)

[cygnusv]

Thanks @frangio and @Amxx !

After playing around with the idea of a cancel proxy I realized that we actually want the cancel functionality to happen at the Governor level, and since it’s already a proposer in the timelock, it can cancel. We then just add an access control check in Governor.cancel()