Background

btcjson.StringOrArray declares its own MarshalJSON method with a value receiver that simply calls json.Marshal(h).
Because the value still satisfies json.Marshaler, json.Marshal re-invokes MarshalJSON, triggering infinite recursion and eventually a stack‐overflow panic at runtime.

This manifests anywhere a StringOrArray is marshalled (e.g. GetNetworkInfoResult.Warnings), crashing downstream apps that depend on btcd/btcjson.

Your environment

• btcd / btcjson version: v0.24.3-0.20250506233109-1eb974aab6ef
• Go version: go1.24.2
• OS: Darwin 23.4.0 arm64 (Apple Silicon macOS 14.4)
• Reproducible on Linux/amd64 as well.

Steps to reproduce

package main

import (
	"encoding/json"
	"github.com/btcsuite/btcd/btcjson"
)

func main() {
	val := btcjson.StringOrArray{"boom"}
	_, _ = json.Marshal(val) // panic: runtime: stack overflow
}

Or run any program that marshals btcjson.GetNetworkInfoResult when the
warnings field is non-nil (see attached stack trace snippet).

Expected behaviour

StringOrArray should serialize to:

• "foo" when length == 1
• ["foo","bar"] when length > 1

without panicking.

Actual behaviour

json.Marshal enters an infinite loop inside StringOrArray.MarshalJSON, eventually crashing with:

runtime: goroutine stack exceeds 1000000000-byte limit
fatal error: stack overflow
...
encoding/json.marshalerEncoder → btcjson.StringOrArray.MarshalJSON → json.Marshal → (repeat)

Proposed fix (PR incoming)

Change the receiver to a pointer (or remove the method entirely) and add unit tests:

func (h *StringOrArray) MarshalJSON() ([]byte, error) {
    return json.Marshal([]string(*h))
}

Tests confirming safe round-trips for single and multiple values are included.