Benchmarking an extrinsic that validates a signature

Question

I have an extrinsic in one of my pallets that looks validates a signature over a message, which is separate from the actual extrinsic being submitted.

The code can be simplified to look like the below

#[pallet::call_index(0)]
pub fn do_something(
  origin: OriginFor<T>,
  msg: SignableMessage,
  user_signer: T::AccountId,
  user_sig: MultiSignature
) -> DispatchResult {
  /* -- snip -- */
  let signer_bytes = Self::account_to_bytes(&user_signer)?;
  let valid = msg.using_encoded(|encoded| {
    let msg = [DOMAIN_SEP_TAG, encoded].concat();
    user_sig.verify(msg.as_ref(), &signer_bytes.into())
  });
  ensure!(valid, <Error<T>>::InvalidSignature);
  /* -- snip -- */
}

The code works fine, but benchmarking it is challenging, because benchmarking setup runs in WASM and as such cannot create the signature. The only way I can think of to get around this is basically to create a signature for all messages I benchmark ahead of time, and write their bytes into a static map, then read that out when doing benchmarks.

Is there a better way to do this? Has anyone else run into the same problem?

For this particular usecase, it does not make sense to use the built-in proxy pallets, or something on-chain.

Answer 1

I think there are a number of options which may allow something like what you've described.

1. Include a function feature-flagged for benchmarks which can generate the signatures. Something like this is done here: https://github.com/paritytech/polkadot/blob/2fe827411438afb323bc8c4334a1d5ec60abfec4/runtime/common/src/crowdloan/mod.rs#L847C9-L855
2. Include a test function which can generate signatures, which you'll then include in your benchmarks plainly. There's something like this here: https://github.com/paritytech/substrate/blob/5cdb054049980eecb449c36a7ccfe4990329c494/frame/babe/src/benchmarking.rs#L80

If I had to guess, option #1 is fewer steps, but I haven't tested an approach like that.