package main

import (
	"bufio"
	"context"
	"fmt"
	"io"
	"log"
	"os"
	"os/exec"
	"time"
)

func main() {
	if err := run(); err != nil {
		log.Fatal(err)
	}
}

func run() error {
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()
	cmd := exec.CommandContext(ctx, "go", "run", "child/main.go")
	stdout, err := cmd.StdoutPipe()
	if err != nil {
		return err
	}
	defer stdout.Close()
	cmd.Stderr = os.Stderr
	cmd.Env = os.Environ()
	if err := cmd.Start(); err != nil {
		return err
	}
	cr, err := getCancelableReader(stdout)
	if err != nil {
		return err
	}
	if err := scan(cr); err != nil {
		return err
	}
	return cmd.Wait()
}

type cancelableReader interface {
	io.Reader
	SetReadDeadline(time.Time) error
}

func getCancelableReader(r io.Reader) (cancelableReader, error) {
	if cr := r.(cancelableReader); cr != nil {
		return cr, nil
	}
	return nil, fmt.Errorf("not a cancelable reader")
}

func scan(cr cancelableReader) error {
	scanner := bufio.NewScanner(cr)
	for scanner.Scan() {
		if err := cr.SetReadDeadline(time.Now().Add(2 * time.Second)); err != nil {
			return err
		}
		fmt.Println("scanned", scanner.Text())
	}
	return scanner.Err()
}