How to correctly send RPC call using Golang to get smart-contract owner?

Question

Update 12 November 2018

Someone shared a very close solution to what I was looking for here.

Update 10 November 2018

Since I'm not able to achieve this using the approach in this question, I created my own library to do the same thing (link). It doesn't rely on go-ethereum package but use the normal net/http package to do JSON RPC request.

I still love to know what I did wrong in my approach below.

---

Definitions:

• owner = public variable in contract with address type
• contract = smart-contract that has owner

This is the curl request to get the owner of a contract. I managed to get the owner.

curl localhost:8545 -X POST \
--header 'Content-type: application/json' \
--data '{"jsonrpc":"2.0", "method":"eth_call", "params":[{"to": "0x_MY_CONTRACT_ADDRESS", "data": "0x8da5cb5b"}, "latest"], "id":1}'

{"jsonrpc":"2.0","id":1,"result":"0x000000000000000000000000_OWNER"}

But when I try to replicate it in Golang (code below), I got json: cannot unmarshal string into Go value of type main.response error.

package main

import (
    "fmt"
    "log"
    "os"

    "github.com/ethereum/go-ethereum/rpc"
)

func main() {
    client, err := rpc.DialHTTP(os.Getenv("RPC_SERVER"))
    if err != nil {
        log.Fatal(err)
    }
    defer client.Close()

    type request struct {
        To   string `json:"to"`
        Data string `json:"data"`
    }

    type response struct {
        Result string
    }

    req := request{"0x_MY_CONTRACT_ADDRESS", "0x8da5cb5b"}
    var resp response
    if err := client.Call(&resp, "eth_call", req, "latest"); err != nil {
        log.Fatal(err)
    }

    fmt.Printf("%v\n", resp)
}

What did I miss here?

Expected result:

Address in string format. E.g. 0x3ab17372b25154400738C04B04f755321bB5a94b

P/S — I'm aware of abigen and I know it's better and easier to do this using abigen. But I'm trying to solve this specific issue without using abigen method.

Answer 1

Instead of

type response struct {
    Result string
}

just pass the result string

var result string
if err := client.Call(&result, "eth_call", req, "latest")

and parse the address

owner := common.HexToAddress(result)
fmt.Printf("%s\n", owner.Hex())

Full code

package main

import (
    "fmt"
    "log"

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/rpc"
)

func main() {
    client, err := rpc.DialHTTP("https://mainnet.infura.io")
    if err != nil {
        log.Fatal(err)
    }
    defer client.Close()

    type request struct {
        To   string `json:"to"`
        Data string `json:"data"`
    }

    var result string

    req := request{"0xcc13fc627effd6e35d2d2706ea3c4d7396c610ea", "0x8da5cb5b"}
    if err := client.Call(&result, "eth_call", req, "latest"); err != nil {
        log.Fatal(err)
    }

    owner := common.HexToAddress(result)
    fmt.Printf("%s\n", owner.Hex()) // 0x281017b4E914b79371d62518b17693B36c7a221e
}