package main

import (
	"bufio"
	"context"
	"net"
	"os"
	"path/filepath"
	"strings"
	"time"
)

// configureResolver teaches Go's resolver where the nameservers are on Android.
//
// Android has no /etc/resolv.conf. Go's pure-Go resolver falls back to
// localhost, so every lookup fails with "dial tcp [::1]:53: connection
// refused". Termux does ship one, at $PREFIX/etc/resolv.conf, which Go never
// consults. Reading it here uses whatever nameservers are actually configured
// rather than hardcoding any.
//
// A no-op everywhere /etc/resolv.conf exists, which is every other platform we
// build for.
func configureResolver() {
	if _, err := os.Stat("/etc/resolv.conf"); err == nil {
		return
	}
	prefix := os.Getenv("PREFIX")
	if prefix == "" {
		return
	}
	servers := nameservers(filepath.Join(prefix, "etc", "resolv.conf"))
	if len(servers) == 0 {
		return
	}
	net.DefaultResolver = &net.Resolver{
		PreferGo: true,
		Dial: func(ctx context.Context, network, _ string) (net.Conn, error) {
			d := net.Dialer{Timeout: 5 * time.Second}
			var err error
			for _, s := range servers {
				var conn net.Conn
				conn, err = d.DialContext(ctx, network, net.JoinHostPort(s, "53"))
				if err == nil {
					return conn, nil
				}
			}
			return nil, err
		},
	}
}

func nameservers(path string) []string {
	f, err := os.Open(path)
	if err != nil {
		return nil
	}
	defer f.Close()
	var out []string
	sc := bufio.NewScanner(f)
	for sc.Scan() {
		fields := strings.Fields(sc.Text())
		if len(fields) >= 2 && fields[0] == "nameserver" {
			out = append(out, fields[1])
		}
	}
	return out
}