// Author: NiuJiuRu
// Email: niujiuru@qq.com

package shell

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"io"
	"os/exec"
	"syscall"
	"time"

	"github.com/mattn/go-shellwords"
)

const (
	defaultTimeout    = 5 * time.Second
	gracePeriod       = 2 * time.Second
	forceKillWait     = 2 * time.Second
	exitTimeoutCode   = 124
	maxOutputSize     = 1 << 20 // 最大 1 MB, 限制输出大小
	checkProcessDelay = 50 * time.Millisecond
)

var (
	ErrInvalidCommand      = errors.New("invalid command")
	ErrExecutorLostControl = errors.New("executor lost control of process")
)

type ExecuteParams struct {
	Cmd     string `json:"cmd"`               // 命令
	Timeout int    `json:"timeout,omitempty"` // 超时(秒)
}

type ExecuteResult struct {
	Stdout   string `json:"stdout"`    ///////// 标准输出
	Stderr   string `json:"stderr"`    ///////// 错误输出
	ExitCode int    `json:"exit_code"` ///////// 退出状态码: 0表示成功, 非0表示失败
}

type limitedBuffer struct {
	buf   *bytes.Buffer
	limit int
}

func (l *limitedBuffer) Write(p []byte) (int, error) {
	remain := l.limit - l.buf.Len()
	if remain <= 0 {
		return len(p), nil
	}
	if len(p) > remain {
		p = p[:remain]
	}
	return l.buf.Write(p)
}

type processGroup struct {
	cmd  *exec.Cmd
	pgid int
}

// 进程组是否存在
func (pg *processGroup) isProcessGroupAlive() bool {
	err := syscall.Kill(-pg.pgid, syscall.Signal(0))
	if err == nil {
		return true
	}

	if errno, ok := err.(syscall.Errno); ok && errno == syscall.ESRCH {
		return false
	}

	return true
}

// 等待进程组终止
func (pg *processGroup) waitForTermination(timeout time.Duration) bool {
	deadline := time.Now().Add(timeout)

	for time.Now().Before(deadline) {
		if !pg.isProcessGroupAlive() {
			return true
		}
		time.Sleep(checkProcessDelay)
	}

	return !pg.isProcessGroupAlive()
}

// 终止整个进程组
func (pg *processGroup) terminate() error {
	if pg.cmd.Process == nil || pg.pgid <= 0 || !pg.isProcessGroupAlive() {
		return nil
	}

	// 第一阶段: 尝试优雅终止, SIGTERM
	if err := syscall.Kill(-pg.pgid, syscall.SIGTERM); err != nil { // 如果发送信号失败，可能进程已经不存在
		if errno, ok := err.(syscall.Errno); ok && errno == syscall.ESRCH {
			return nil
		}
	}

	if pg.waitForTermination(gracePeriod) {
		return nil
	}

	// 第二阶段: 最后强制终止, SIGKILL
	if err := syscall.Kill(-pg.pgid, syscall.SIGKILL); err != nil { // 如果发送信号失败，可能进程已经不存在
		if errno, ok := err.(syscall.Errno); ok && errno == syscall.ESRCH {
			return nil
		}
		return fmt.Errorf("failed to send SIGKILL to process group: %w", err)
	}

	if pg.waitForTermination(forceKillWait) {
		return nil
	}

	return fmt.Errorf("%w: process group %d still alive after force kill",
		ErrExecutorLostControl, pg.pgid)
}

func Execute(p ExecuteParams) (*ExecuteResult, error) {
	swp := shellwords.NewParser()
	swp.ParseEnv = true      // 展开 "环境变量"
	swp.ParseBacktick = true // 展开 `...`命令

	argv, err := swp.Parse(p.Cmd)
	if err != nil || len(argv) == 0 {
		return nil, ErrInvalidCommand
	}

	timeout := time.Duration(p.Timeout) * time.Second
	if timeout <= 0 {
		timeout = defaultTimeout
	}

	ctx, cancel := context.WithTimeout(context.Background(), timeout)
	defer cancel()

	cmd := exec.Command(argv[0], argv[1:]...)

	cmd.SysProcAttr = &syscall.SysProcAttr{ // 新的进程组
		Setpgid:   true,
		Pdeathsig: syscall.SIGKILL,
	}

	var stdout, stderr bytes.Buffer
	cmd.Stdout = io.Writer(&limitedBuffer{buf: &stdout, limit: maxOutputSize})
	cmd.Stderr = io.Writer(&limitedBuffer{buf: &stderr, limit: maxOutputSize})

	if err := cmd.Start(); err != nil {
		return nil, err
	}

	processInfo := &processGroup{
		cmd:  cmd,
		pgid: cmd.Process.Pid, // 进程组ID就是主进程的PID
	}

	done := make(chan error, 1)
	go func() {
		done <- cmd.Wait()
	}()

	exitCode := 0
	var finalErr error

	select {
	case err := <-done: // 命令已结束, 输出结果
		if err != nil {
			if ee, ok := err.(*exec.ExitError); ok {
				exitCode = ee.ExitCode()
			} else {
				return nil, err
			}
		}
	case <-ctx.Done(): /// 超时, kill整个进程组
		exitCode = exitTimeoutCode

		if err := processInfo.terminate(); err != nil {
			finalErr = err
			break
		}

		select {
		case <-done:
		case <-time.After(forceKillWait):
			finalErr = ErrExecutorLostControl
		}
	}

	return &ExecuteResult{
		Stdout:   stdout.String(),
		Stderr:   stderr.String(),
		ExitCode: exitCode,
	}, finalErr
}