// -=-=- StartHere -=-=-
//
// # Chapter I: Using the "stdlib" DNS resolver
//
// In this chapter we will write together a `main.go` file that
// uses the "stdlib" DNS resolver to lookup domain names.
//
// The "stdlib" resolver is `getaddrinfo` on Unix. If we're compiled with
// `CGO_ENABLED=1`, we use the `getaddrinfo` stdlib call directly. Otherwise,
// we use the `net.Resolver` resolver, which may or may not use
// `getaddrinfo` (or equivalent stdlib calls) under the hood.
//
// (This file is auto-generated from the corresponding source file,
// so make sure you don't edit it manually.)
//
// ## The main.go file
//
// We define `main.go` file using `package main`.
//
// ```Go
package main

import (
	"context"
	"errors"
	"flag"
	"os"
	"time"

	"github.com/apex/log"
	"github.com/ooni/probe-cli/v3/internal/netxlite"
)

func main() {
	// ```
	//
	// The beginning of the program is equal to the previous chapters,
	// so there is not much to say about it.
	//
	// ```Go
	log.SetLevel(log.DebugLevel)
	hostname := flag.String("hostname", "dns.google", "Hostname to resolve")
	timeout := flag.Duration("timeout", 60*time.Second, "Timeout")
	flag.Parse()
	ctx, cancel := context.WithTimeout(context.Background(), *timeout)
	defer cancel()
	// ```
	//
	// We create a new resolver using the standard library to perform
	// domain name resolutions. Unless you're cross compiling, this
	// resolver will call the system resolver using a C API. On Unix
	// the called C API is `getaddrinfo`.
	//
	// The returned resolver implements an interface that is very
	// close to the API of the `net.Resolver` struct.
	//
	// ```Go
	reso := netxlite.NewStdlibResolver(log.Log)
	// ```
	//
	// We call `LookupHost` to map the hostname to IP addrs. The returned
	// value is either a list of addrs or an error.
	//
	// ```Go
	addrs, err := reso.LookupHost(ctx, *hostname)
	if err != nil {
		fatal(err)
	}
	log.Infof("resolver addrs: %+v", addrs)
}

// ```
//
// This function is exactly like it was in previous chapters.
//
// ```Go
func fatal(err error) {
	var ew *netxlite.ErrWrapper
	if !errors.As(err, &ew) {
		log.Fatal("cannot get ErrWrapper")
	}
	log.Warnf("error string    : %s", err.Error())
	log.Warnf("OONI failure    : %s", ew.Failure)
	log.Warnf("failed operation: %s", ew.Operation)
	log.Warnf("underlying error: %+v", ew.WrappedErr)
	os.Exit(1)
}

// ```
//
// ## Running the code
//
// ### Vanilla run
//
// You can now run this code as follows:
//
// ```bash
// go run -race ./internal/tutorial/netxlite/chapter05
// ```
//
// You will see debug logs describing what is happening along with timing info.
//
// ### NXDOMAIN error
//
// ```bash
// go run -race ./internal/tutorial/netxlite/chapter05 -hostname antani.ooni.io
// ```
//
// should cause a `dns_nxdomain_error`, because the domain does not exist.
//
// ### Timeout
//
// ```bash
// go run -race ./internal/tutorial/netxlite/chapter05 -timeout 10us
// ```
//
// should cause a timeout error, because the timeout is ridicolously small.
//
// ## Conclusions
//
// We have seen how to use the "stdlib" DNS resolver.