Simple runtime call in Rust

Question

I am building a somewhat generic client for Substrate-based chains in Rust using the subxt crate. To learn about runtime calls, my goal was to call the core.version runtime API directly using the RPC endpoint state_call.

Internally subxt uses a special RPC endpoint state_getRuntimeVersion, but that just gets redirected to the runtime at the specific block anyways:

    /// Fetch the runtime version
    pub async fn runtime_version(
        &self,
        at: Option<T::Hash>,
    ) -> Result<RuntimeVersion, BasicError> {
        let params = rpc_params![at];
        let version = self
            .client
            .request("state_getRuntimeVersion", params)
            .await?;
        Ok(version)
    }

This was my try to make the runtime call directly:

    let version: subxt::rpc::RuntimeVersion = rpc_client
        .rpc()
        .client
        .request("state_call", rpc_params!["core.version", ""])
        .await
        .unwrap();

But it just errs out with Exported method core.version is not found. How can I make this call work?

Answer 1

I think you've found this out, but for others: In chrome you can run polkadot js and then inspect the webpage and switch over to the network tab. From there if you click on ws to filter to the websockets you can see the request and response from the UI for any call whether that's a state query, rpc call, runtime call or submitting an extrinsic.

Here's the request you can see on the wire:

{
  "id":157,
  "jsonrpc":"2.0",
  "method":"state_call",
  "params":["Core_version","0x"]
}

The result is

{
  "jsonrpc":"2.0",
  "result":"0x20706f6c6b61646f743c7061726974792d706f6c6b61646f74000000000e2400000000000038df6acb689907609b0400000037e397fc7c91f5e40100000040fe3ad401f8959a06000000d2bc9897eed08f1503000000f78b278be53f454c02000000af2c0297a23e6d3d0200000049eaaf1b548a0cb00100000091d5df18b0d2cf5801000000ed99c5acb25eedf503000000cbca25e39f14238702000000687ad44ad37f03c201000000ab3c0572291feb8b01000000bc9d89904f5b923f0100000037c8bb1350a9a2a8010000000c00000000",
  "id":157
}

and we can see in polkadot js this decodes to

{
  "specName": "polkadot",
  "implName": "parity-polkadot",
  "authoringVersion": 0,
  "specVersion": 9230,
  "implVersion": 0,
  "apis": [
    [ "0xdf6acb689907609b", 4 ],
    [ "0x37e397fc7c91f5e4", 1 ],
    [ "0x40fe3ad401f8959a", 6 ],
    [ "0xd2bc9897eed08f15", 3 ],
    [ "0xf78b278be53f454c", 2 ],
    [ "0xaf2c0297a23e6d3d", 2 ],
    [ "0x49eaaf1b548a0cb0", 1 ],
    [ "0x91d5df18b0d2cf58", 1 ],
    [ "0xed99c5acb25eedf5", 3 ],
    [ "0xcbca25e39f142387", 2 ],
    [ "0x687ad44ad37f03c2", 1 ],
    [ "0xab3c0572291feb8b", 1 ],
    [ "0xbc9d89904f5b923f", 1 ],
    [ "0x37c8bb1350a9a2a8", 1 ]
  ],
  "transactionVersion": 12,
  "stateVersion": 0
}

polkadot in hex is 70 6f 6c 6b 61 64 6f 74 so that gives you a pretty good steer on the decoding front. The code is here to decode it. The 0x20 prefix will be a Compact<u32> saying what the string length is. But you should be able to call sp_version::RuntimeVersion::decode(&mut &bytes[..]) to decode it all in one go.