ooni-probe-cli/internal/tutorial/netxlite/chapter01/main.go

// -=-=- StartHere -=-=-
//
// # Chapter I: establishing TCP connections
//
// In this chapter we will write together a `main.go` file that
// uses netxlite to establish a new TCP connection.
//
// (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"
	"net"
	"os"
	"time"

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

// ```
//
// ### Main function
//
// ```Go
func main() {
	// ```
	//
	// We use apex/log and configure it to emit debug messages. This
	// setting will allow us to see netxlite emitted logs.
	//
	// ```Go
	log.SetLevel(log.DebugLevel)
	// ```
	//
	// We use the flags package to define command line options and we
	// parse the command line options with `flag.Parse`.
	//
	// ```Go
	address := flag.String("address", "8.8.4.4:443", "Remote endpoint address")
	timeout := flag.Duration("timeout", 60*time.Second, "Timeout")
	flag.Parse()
	// ```
	//
	// We use the standard Go idiom to set a timeout using a context.
	//
	// ```Go
	ctx, cancel := context.WithTimeout(context.Background(), *timeout)
	defer cancel()
	// ```
	//
	// The bulk of the logic has been factored into a `dialTCP` function.
	//
	// ```Go
	conn, err := dialTCP(ctx, *address)
	// ```
	//
	// If there is a failure we invoke a function that prints the
	// error that occurred and then calls `os.Exit(1)`
	//
	// ```Go
	if err != nil {
		fatal(err)
	}
	// ```
	//
	// Otherwise, we're tidy and close the opened connection.
	//
	// ```Go
	conn.Close()
}

// ```
//
// ### Dialing for TCP
//
// We construct a netxlite.Dialer (i.e., a type similar to net.Dialer)
// and we use it to dial the new connection.
//
// Note that the dialer we're constructing here is not attached to
// a resolver. This means that, if `address` contains a domain name
// rather than an IP address, the dial operation will fail.
//
// While it is possible in netxlite to construct a dialer using a
// resolver, here we're focusing on the step-by-step measuring perspective
// where we want to perform each operation independently.
//
// ```Go
func dialTCP(ctx context.Context, address string) (net.Conn, error) {
	d := netxlite.NewDialerWithoutResolver(log.Log)
	return d.DialContext(ctx, "tcp", address)
}

// ```
//
// ### Printing the error
//
// Fundamental netxlite types guarantee that they always return a
// `*netxlite.ErrWrapper` type on error. This type is an `error` and
// we can use `errors.As` to see its content:
//
// - the Failure field is the OONI error string as specified in
// https://github.com/ooni/spec, and is also the string that
// is emitted in case one calls `err.Error`;
//
// - Operation is the name of the operation that failed;
//
// - WrappedErr is the underlying error that occurred and has
// been wrapped by netxlite.
//
// ```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/chapter01
// ```
//
// You will see debug logs describing what is happening along with timing info.
//
// ### Connection timeout
//
// ```bash
// go run -race ./internal/tutorial/netxlite/chapter01 -address 8.8.4.4:1
// ```
//
// should cause a connect timeout error. Try lowering the timout adding, e.g.,
// the `-timeout 5s` flag to the command line.
//
// ### Connection refused
//
// ```bash
// go run -race ./internal/tutorial/netxlite/chapter01 -address '[::1]:1'
// ```
//
// should give you a connection refused error in most cases. (We are quoting
// the `::1` IPv6 address using `[` and `]` here.)
//
// ## Conclusions
//
// We have seen how to use netxlite to establish a TCP connection.