Node panics upon serializing `u128` in rpc-endpoint

[mustermeiszer]

I am getting this error when querying a custom rpc-endpoint.

Thread 'tokio-runtime-worker' panicked at 'Expected always-serializable type; qed: Error("u128 is not supported", line: 0, column: 0)', src/rpc/pools.rs:12

This is a bug. Please report it at:

    https://github.com/paritytech/cumulus/issues/new

Code

• RPC- API implementation:

  fn pool_value(&self, pool_id: PoolId) -> Result<Balance> {
  let api = self.client.runtime_api();
  let best = self.client.info().best_hash;
  let at = BlockId::hash(best);
  
  api.pool_value(&at, pool_id)
      .map_err(|e| RpcError {
          code: ErrorCode::ServerError(crate::rpc::Error::RuntimeError.into()),
          message: "Unable to query value of pool.".into(),
          data: Some(format!("{:?}", e).into()),
      })?
      .ok_or(RpcError {
          code: ErrorCode::InvalidParams,
          message: "Pool not found.".into(),
          data: Some(format!("PoolId: {:?}", pool_id).into()),
      })
  }

• Runtime-api implementation:

  fn pool_value(pool_id: PoolId) -> Option<Balance> {
      let pool = pallet_pools::Pool::<Runtime>::get(pool_id)?;
      let nav: Balance = pallet_loans::Pallet::<Runtime>::update_nav_of_pool(pool_id)
          .ok()
          .map(|(latest, _)| latest.into())?;
  
      Some(pool.reserve.total.saturating_add(nav))
  }

  where Balance is a type Balance = u128

  Version

• Substrate: polkadot-v0.9.17
• Cumulus: polkadot-v0.9.17
• Polkadot: release-v0.9.17

[mustermeiszer]

Note: Error originates from macro in jsonrpc here https://github.com/paritytech/jsonrpc/blob/0a2702595b8831a9a30dec7dc476157c648c361c/derive/src/to_delegate.rs#L285.

[mustermeiszer]

I also hard-coded the balance as u128 to check whether I mixed the final types in the runtime-api impl. But all compiles fine.

[bkchr]

There are bugs around serde-json and u128 serialized as a serde_json::Value or something. I don't remember all the details, sorry! But now as we have jsonrpsee, this is maybe solved? Maybe @niklasad1 can help here.

[niklasad1]

I think this particular usecase should work in jsonrpsee as long plain u128/i128 are used and not in a type with serde flatten/untagged attributes. This will only work in the servers, the clients still have this issue.

with serde flatten/untagged it is pretty much the same story you can find more information in https://github.com/paritytech/jsonrpsee/issues/480

but the jsonrpc crate supports arbitrary precision which jsonrpsee doesn't but nothing that prevents folks from enabling that feature on serde_json themselves but then you will run in weird bugs with tagged enums (such as Id::Number(u64)) will not work.

[xlc]

It is known issue u128 doesn't play nicely with RPC. See my fix in Substrate https://github.com/paritytech/substrate/pull/4898