rpc.Client.CallContext incorrectly unmarshalling null results

[JasonYuan869]

System information

Geth version: v1.10.26 (but also in master branch)

Expected behaviour

When {"result": null} is returned from a JSON RPC call in rpc.CallContext, it should unmarshal into a json.RawMessage as json.RawMessage("null").

Actual behaviour

It actually gets unmarshalled as nil.

Steps to reproduce the behaviour

1. Create a local blockchain private network and expose it with --http or --ws
2. Set RPC_ENDPOINT environment variable to the URL of the network
3. The following code attempts to retrieve a transaction receipt for an invalid hash, which should set result to json.RawMessage("null")

package main

import (
	"encoding/json"
	"fmt"
	"os"

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

func main() {
	client, err := rpc.Dial(os.Getenv("RPC_ENDPOINT"))
	if err != nil {
		panic(err)
	}
	defer client.Close()

	var result json.RawMessage
	invalidHash := "0x1234567890123456789012345678901234567890123456789012345678901234"

	err = client.Call(&result, "eth_getTransactionReceipt", invalidHash)
	if err != nil {
		panic(err)
	}

	if result == nil {
		panic("result should not be nil")
	} else {
		fmt.Printf("result: %s", result)
	}
}

3. The go code panics since result is nil
4. Observe that running curl against the endpoint with the same RPC call properly returns the JSON field "result": null.

Fix

The error seems to be caused by a redundant reference operator in rpc.Client.CallContext(), when the JSON unmarshaller is called.

go-ethereum/rpc/client.go

Line 348 in dbd6c13

return json.Unmarshal(resp.Result, &result)

Line 321 already checks that result is a pointer, so there shouldn’t be any need to reference it again.

go-ethereum/rpc/client.go

Lines 321 to 323 in dbd6c13

	if result != nil && reflect.TypeOf(result).Kind() != reflect.Ptr {
	return fmt.Errorf("call result parameter must be pointer or nil interface: %v", result)
	}

Upon further investigation, it seems that nested pointers will be set to nil specifically when decoding null values (https://go.dev/src/encoding/json/decode.go#L424).

[JasonYuan869]

Closed in #26723