How does one block specific accounts from calling extrinsics for a substrate pallet?

Question

We want the ability to filter certain accounts from calling extrinsics in the democracy pallet. There are more details but basically we can't do this by reserving token balance or we would do that.

One idea was to create a signed extension where the call fails if the AccountId is one of the restricted accounts, which is possible but seems heavy-handed.

We thought, maybe we could set Origin to use a custom EnsureOrigin implementation for pallet_democracy. Unfortunately it's just an associated type, so we can't change it in the pallet_democracy Config.

What is the best way to do this?

Answer 1

I think the best option you have here is using a SignExtension. There, you can filter by who (checking a whitelisted list in storage), for those signed call that match with pallet_democracy::Call::vote, ...

Answer 2

One easy way to do this without needing to change the pallet code too much, as you said, is using signed extensions. Here's one that does a similar thing:

https://github.com/paritytech/substrate/pull/12129/files#diff-103bae13a15813f338cb0bb67e77f3687c2c811fe75b2defd1cdc3d2afd6aa17R487

As for custom origin, you can also do that, but it would require some changes to the pallet code:

// in `trait Config`
type CustomSignedOrigin: EnsureOrigin<Self::AccountId>;

Then, in all of the calls that you want to be restricted:

// remove this
let who = ensure_signed(origin)?;
// add this: 
let who = T::CustomSignedOrigin::ensure_origin(origin)?;

Finally, in the top level runtime, you want an implementation of EnsureOrigin that acts similar to EnsureSigned but filters out certain accounts.

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The main advantage of the second approach is that the overhead will only be felt if the transaction is related to pallet-democracy, whereas in the former approach, the signed extension is checked against all transaction. Although, if you implement it correctly, the first thing that you will do in the signed extension is to check if the transaction's call is from democracy, and if not, you early exist. In my opinion, this is a very negligible overhead.