从源码解析Go exec timeout 实现机制

 背景

环境：golang 1.9，drawn
测试使用golang exec 执行命令，并配置过期时间，测试脚本如下。
现象：执行脚本后，到超时时间后并为超时退出，反而阻塞住了

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func TestExecWithTimeout(t *testing.T) { ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second) defer cancel() start := time.Now() cmd := exec.CommandContext(ctx, "sh", "-c", "echo start && sleep 10 && echo end.") out, err := cmd.CombinedOutput() fmt.Printf("error: [%v]\n", err) fmt.Printf("out: [%v]\n", string(out)) if ctx.Err() == context.DeadlineExceeded { fmt.Printf("ctx.Err: [%v]\n", ctx.Err()) } fmt.Printf("process end: %v", time.Since(start)) }

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error: [signal: killed] out: [start ] ctx.Err: [context deadline exceeded] process end: 10.010193583sPASS

在用ps查看进程状态，发现，超时时间到达后，sh进程就被kill了，而由ssh启动的sleep 10命令还在进行中，且该进程被1号进程接管，变成了僵尸进程。当我们手动执行kill命令后，程序退出。

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~/workpro//mkskit ❯ ps -ef | grep "sleep" 501 85600 85597 0 3:34下午 ?? 0:00.00 sh -c echo start && sleep 20 && echo end. 501 85601 85600 0 3:34下午 ?? 0:00.00 sleep 20 501 85608 17687 0 3:34下午 ttys004 0:00.00 grep --color=auto --exclude-dir=.bzr --exclude-dir=CVS --exclude-dir=.git --exclude-dir=.hg --exclude-dir=.svn --exclude-dir=.idea --exclude-dir=.tox sleep ~/workpro//mkskit ❯ ps -ef | grep "sleep" 501 85601 1 0 3:34下午 ?? 0:00.00 sleep 20 501 85652 17687 0 3:35下午 ttys004 0:00.00 grep --color=auto --exclude-dir=.bzr --exclude-dir=CVS --exclude-dir=.git --exclude-dir=.hg --exclude-dir=.svn --exclude-dir=.idea --exclude-dir=.tox sleep

2. 原因分析

从现象推测，exec 创建了两个进程，超时后 sh进程退出，但是sleep进程还在变成了僵尸进程，sleep进程还未退出，导致整改go主进程阻塞。

2.1 源码-分析#

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// CombinedOutput runs the command and returns its combined standard // output and standard error. func (c *Cmd) CombinedOutput() ([]byte, error) { if c.Stdout != nil { return nil, errors.New("exec: Stdout already set") } if c.Stderr != nil { return nil, errors.New("exec: Stderr already set") } var b bytes.Buffer c.Stdout = &b c.Stderr = &b err := c.Run() return b.Bytes(), err } // 执行命令，并等待命令执行完成 func (c *Cmd) Run() error { if err := c.Start(); err != nil { return err } return c.Wait() }

CombinedOutput 方法会初始化一个字节缓冲区，并标准输出和标准错误导向该缓冲区。然后启动执行命令。
Run方法中调用Start和Wait方法：

• Start方法用于启动子进程，启动后立即返回
• Wait方法则阻塞，等待子进程结束并回收资源

2.1.1 Wait 方法#

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// Wait waits for the command to exit and waits for any copying to // stdin or copying from stdout or stderr to complete. // // The command must have been started by Start. // // The returned error is nil if the command runs, has no problems // copying stdin, stdout, and stderr, and exits with a zero exit // status. // // If the command fails to run or doesn't complete successfully, the // error is of type *ExitError. Other error types may be // returned for I/O problems. // // If any of c.Stdin, c.Stdout or c.Stderr are not an *os.File, Wait also waits // for the respective I/O loop copying to or from the process to complete. // // Wait releases any resources associated with the Cmd. func (c *Cmd) Wait() error { if c.Process == nil { return errors.New("exec: not started") } if c.ProcessState != nil { return errors.New("exec: Wait was already called") } state, err := c.Process.Wait() if err == nil && !state.Success() { err = &ExitError{ProcessState: state} } c.ProcessState = state // Wait for the pipe-copying goroutines to complete. var copyError error for range c.goroutine { if err := <-c.goroutineErrs; err != nil && copyError == nil { copyError = err } } c.goroutine = nil // Allow the goroutines' closures to be GC'd. if c.ctxErr != nil { interruptErr := <-c.ctxErr // If c.Process.Wait returned an error, prefer that. // Otherwise, report any error from the interrupt goroutine. if interruptErr != nil && err == nil { err = interruptErr } } // Report errors from the copying goroutines only if the program otherwise // exited normally on its own. Otherwise, the copying error may be due to the // abnormal termination. if err == nil { err = copyError } c.closeDescriptors(c.closeAfterWait) c.closeAfterWait = nil return err }

根据debug可知，阻塞发生在第34行, err := <-c.goroutineErrs;这句，从goroutineErrs中读取错误信息并返回第一次错误给调用者。而 <-ch从通道中获取数据阻塞的原因只有发送发未准备好，那么goroutineErr对应的发送方是谁呢？

2.1.2 Start 方法#

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// Start starts the specified command but does not wait for it to complete. // // If Start returns successfully, the c.Process field will be set. // // After a successful call to Start the Wait method must be called in // order to release associated system resources. func (c *Cmd) Start() error { if c.Path == "" && c.Err == nil && c.lookPathErr == nil { c.Err = errors.New("exec: no command") } if c.Err != nil || c.lookPathErr != nil { c.closeDescriptors(c.closeAfterStart) c.closeDescriptors(c.closeAfterWait) if c.lookPathErr != nil { return c.lookPathErr } return c.Err } if runtime.GOOS == "windows" { lp, err := lookExtensions(c.Path, c.Dir) if err != nil { c.closeDescriptors(c.closeAfterStart) c.closeDescriptors(c.closeAfterWait) return err } c.Path = lp } if c.Process != nil { return errors.New("exec: already started") } if c.ctx != nil { select { case <-c.ctx.Done(): c.closeDescriptors(c.closeAfterStart) c.closeDescriptors(c.closeAfterWait) return c.ctx.Err() default: } } c.childFiles = make([]*os.File, 0, 3+len(c.ExtraFiles)) type F func(*Cmd) (*os.File, error) for _, setupFd := range []F{(*Cmd).stdin, (*Cmd).stdout, (*Cmd).stderr} { fd, err := setupFd(c) if err != nil { c.closeDescriptors(c.closeAfterStart) c.closeDescriptors(c.closeAfterWait) return err } c.childFiles = append(c.childFiles, fd) } c.childFiles = append(c.childFiles, c.ExtraFiles...) env, err := c.environ() if err != nil { return err } c.Process, err = os.StartProcess(c.Path, c.argv(), &os.ProcAttr{ Dir: c.Dir, Files: c.childFiles, Env: env, Sys: c.SysProcAttr, }) if err != nil { c.closeDescriptors(c.closeAfterStart) c.closeDescriptors(c.closeAfterWait) return err } c.closeDescriptors(c.closeAfterStart) // Don't allocate the goroutineErrs channel unless there are goroutines to fire. if len(c.goroutine) > 0 { errc := make(chan error, len(c.goroutine)) c.goroutineErrs = errc for _, fn := range c.goroutine { go func(fn func() error) { errc <- fn() }(fn) } } c.ctxErr = c.watchCtx() return nil }

从第74-82行可以看到，创建了一个len(c.goroutine)的channel， 然后启动goroutine执行c.goroutine中的方法并将错误写入errc，由wait的现象可知，应该是这个fn()调用阻塞了。
继续追踪，c.goroutine是在哪里被赋值的。同样是在Start方法中
主要看第43行，学习到了，原来函数内还能定义类型。这里主要是调用了三个函数来初始化stdin, stderr, stdout.

这里主要先看下stdout、stderr

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func (c *Cmd) stdout() (f *os.File, err error) { return c.writerDescriptor(c.Stdout) } func (c *Cmd) stderr() (f *os.File, err error) { // 如果stdrr 和 stdout 是同一个输出目标 则跳过 if c.Stderr != nil && interfaceEqual(c.Stderr, c.Stdout) { return c.childFiles[1], nil } return c.writerDescriptor(c.Stderr) } func (c *Cmd) writerDescriptor(w io.Writer) (f *os.File, err error) { // case1 if w == nil { f, err = os.OpenFile(os.DevNull, os.O_WRONLY, 0) if err != nil { return } c.closeAfterStart = append(c.closeAfterStart, f) return } // case2 if f, ok := w.(*os.File); ok { return f, nil } // case3 pr, pw, err := os.Pipe() if err != nil { return } c.closeAfterStart = append(c.closeAfterStart, pw) c.closeAfterWait = append(c.closeAfterWait, pr) c.goroutine = append(c.goroutine, func() error { _, err := io.Copy(w, pr) pr.Close() // in case io.Copy stopped due to write error return err }) return pw, nil }

两个函数都调用了writerDescriptor，看代码主要由三个分支逻辑

• case1：如果没有制定stderr或者stdout，就直接写入os.DevNull 即 /dev/null
• case2：如果制定的stderr或stdout 是*os.File类型也直接返回，会把输出写入文件
• case3：最后一种情况，创建管道，返回写端点，在Start函数中的第59行，子进程和管道fd进行绑定。关键在第38行，goroutine中绑定了一个func()，从管道的读端点读取数据并copy到指定的stderr或者stdout(均实现Writer)。

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// Implementations must not retain p. type Writer interface { Write(p []byte) (n int, err error) }

io.Copy方法会一直阻塞到reader被关闭才会返回，这也是为什么会产生阻塞到原因了。在Wait方法到注释中可以得知

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// If any of c.Stdin, c.Stdout or c.Stderr are not an *os.File, Wait also waits // for the respective I/O loop copying to or from the process to complete. // // Wait releases any resources associated with the Cmd. func (c *Cmd) Wait() error {}

到这里了解到了阻塞到根本原因，那超时子进程被kill是在哪里触发的。在看Start方法时，可以看到最后一行有一个watchCtx调用

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// watchCtx conditionally starts a goroutine that waits until either c.ctx is // done or c.Process.Wait has completed (called from Wait). // If c.ctx is done first, the goroutine terminates c.Process. // // If a goroutine was started, watchCtx returns a channel on which its result // must be received. func (c *Cmd) watchCtx() <-chan error { if c.ctx == nil { return nil } errc := make(chan error) go func() { select { case errc <- nil: return case <-c.ctx.Done(): } var err error if killErr := c.Process.Kill(); killErr == nil { // We appear to have successfully delivered a kill signal, so any // program behavior from this point may be due to ctx. err = c.ctx.Err() } else if !errors.Is(killErr, os.ErrProcessDone) { err = wrappedError{ prefix: "exec: error sending signal to Cmd", err: killErr, } } errc <- err }() return errc }

可以看到，启动了一个goroutine，绑定了一个 error chan，selec监听ctx的状态，当ctx超时或者提前cancel了，则会出发process.kill，将子进程关闭。
正常情况下，当ctx超时，子进程会被kill，此时管道的写入端点自然会被关闭，io.Copy则会在copy完成后正常返回，给e.errch中发送一个nil，Wait方法则中c.errch中读取到nil。在Wait方法中的第26、40、44行可以看到，err最终被赋值为一个state第结构体对象。
但是在开始的demo中，除了sh这个子进程之外还启动了一个sleep子进程，context超时后，sleep进程依旧在运行，并持有管道的写端点，导致io.Copy阻塞。

2.2 相关流程#

3. 解决方案

根据上述分析可得，进入case3且产生了子进程才会导致阻塞。那么只要不满足其中一个条件即可。

3.1 使用*os.File类型接收输入输出#

可避免阻塞，但存在的问题：

• 需要额外处理输出，如: 从文件读取并写入到需要的地方
• 程序退出后，子子进程被1号进程托管称为僵尸进程

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func TestExecUseFileOutput(t *testing.T) { ctx, cancel := context.WithTimeout(context.Background(), 8*time.Second) defer cancel() cmd := exec.CommandContext(ctx, "sh", "-c", "echo start && sleep 15 && echo end.") combinedOutput, err := ioutil.TempFile("", "stdouterr") if err != nil { fmt.Println(err) return } defer func() { _ = os.Remove(combinedOutput.Name()) }() cmd.Stdout = combinedOutput cmd.Stderr = combinedOutput err = cmd.Run() if err != nil { fmt.Println(err) return } _, err = combinedOutput.Seek(0, 0) var b bytes.Buffer _, err = io.Copy(&b, combinedOutput) if err != nil { fmt.Println(err) return } fmt.Println("output:", b.String()) fmt.Printf("ctx.Err: [%v]\n", ctx.Err()) fmt.Printf("error: [%v]\n", err) }

3.2 避免产生子进程#

linux执行脚本的5种方式

• 使用绝对路径执行：/root/sleep.sh
• 使用相对路径：./sleep.sh 需要x权限
• 使用sh或者bash命令来执行：bash /root/sleep.sh
• 使用.(空格)脚本名称来执行: . ./sleep.sh
• 使用source来执行（一般用来生效配置文件）: source /root/sleep.sh

前三种方式都会产生bash 进程和sleep进程，后两种则只会产生sleep进程。
因为go中没有shell环境，如果执行复杂的命令，肯定只是用bash 或者 sh方式运行，肯定会产生一个新的进程，so这个方法无效

3.3 sh -c#

bash -c command / sh -c command 方式执行单条命令的时候有相关的优化，是不会产生多个进程的，因此如果将demo中的复杂命令或者脚本拆分成多个命令执行也可以实现，但这种方式不够优雅。
原理：
单条命令时：启动bash进程后发现是一个简单的命令，在不fork新进程的情况下直接调用exec执行命令，然后将子shell替换为sleep命令。
多条命令时：需要使用子shell 来处理 && 操作符，它需要等待第一个命令终止的SIGCHLD，然后根据第一个命令的exit status 决定是否需要运行第二个命令，因此不能将子shell替换为sleep命令。

具体见 shell.c 第 1370 行

3.4 kill -pid kill进程组#

linux kill(2) 指定 pid 为负数时会给这个进程组中的所有进程发送信号

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If pid is less than -1, then sig is sent to every process in the process group whose ID is -pid.

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func TestExecuteInOnPgid(t *testing.T) { ctx, cancel := context.WithTimeout(context.Background(), 8*time.Second) defer cancel() cmd := exec.CommandContext(ctx, "sh", "-c", "echo start && sleep 15 && echo end.") cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true} go func() { select { case <-ctx.Done(): if err := syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL); err != nil { fmt.Println("kill failed: ", err) } } }() output, err := cmd.CombinedOutput() if err != nil { fmt.Println(err) return } fmt.Println("output:", string(output)) fmt.Printf("ctx.Err : [%v]\n", ctx.Err()) fmt.Printf("error : [%v]\n", err) }

3.5 社区提案#

该问题其实很早就存在了，最早可以追溯到这个 2017 年的 Issue #23019，不过为了保持向后兼容，在方案上一直没有达成共识，最新提案见这个 Issue #50436，根据 #53400 中的最新消息，该提案可能会在 Go 1.20 中实现。
大致方案为在 exec.Cmd 中添加一个 Interrupt(os.Signal) 字段，在 context 超时后将这个信号发送给子进程以关闭所有子进程。

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// Context is the context that controls the lifetime of the command // (typically the one passed to CommandContext). Context context.Context // If Interrupt is non-nil, Context must also be non-nil and Interrupt will be // sent to the child process when Context is done. // // If the command exits with a success code after the Interrupt signal has // been sent, Wait and similar methods will return Context.Err() // instead of nil. // // If the Interrupt signal is not supported on the current platform // (for example, if it is os.Interrupt on Windows), Start may fail // (and return a non-nil error). Interrupt os.Signal // If WaitDelay is non-zero, the command's I/O pipes will be closed after // WaitDelay has elapsed after either the command's process has exited or // (if Context is non-nil) Context is done, whichever occurs first. // If the command's process is still running after WaitDelay has elapsed, // it will be terminated with os.Kill before the pipes are closed. // // If the command exits with a success code after pipes are closed due to // WaitDelay and no Interrupt signal has been sent, Wait and similar methods // will return ErrWaitDelay instead of nil. // // If WaitDelay is zero (the default), I/O pipes will be read until EOF, // which might not occur until orphaned subprocesses of the command have // also closed their descriptors for the pipes. WaitDelay time.Duration

4. 总结

现象#

使用 os/exec 执行 shell 脚本并设置超时时间，然后到超时时间之后程序并未超时退出，反而一直阻塞。

原因#

os/exec 包执行命令时会创建子进程，通过管道连接子进程以收集命令执行结果，goroutine 从管道中读取命令输出，超时后会 kill 掉子进程，从而关闭管道，管道被关闭后 goroutine 则自动退出。
如果存在子子进程，占有管道则会导致 kill 掉子进程后管道依旧未能释放，读取输出的 goroutine 被阻塞，最终导致程序超时后也无法返回。

解决#

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package ssh import "syscall" // 实现Cancel方法 func (e *SomeExecuteCmdForward) Cancel() { if pgid := -e.pid; pgid < 0 { _ = syscall.Kill(pgid, syscall.SIGKILL) e.pid = 0 } } // cmd.SysProcAttr = newSysProcAttr() func newSysProcAttr() *syscall.SysProcAttr { return &syscall.SysProcAttr{ Setpgid: true, } }

kill -- -pid

5. 思考

为什么在start 进程起来之后就把文件句柄close了c.closeDescriptors(c.closeAfterStart)，管道还可以继续读写？
在close之前已经将文件句柄传递给子进程了，相当于此时fd的引用计数为2，start调用的是关闭主进程引用的文件句柄，相当于引用计数-1。直到管道写入端close管道后，管道读取端就可以收到管道被关闭的信号，结束读取。
追踪closeDescriptors底层调用函数，可以看到也是进行引用计数-1，直到为0就关闭。

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// Close closes the FD. The underlying file descriptor is closed by the // destroy method when there are no remaining references. func (fd *FD) Close() error { if !fd.fdmu.increfAndClose() { return errClosing(fd.isFile) } // Unblock any I/O. Once it all unblocks and returns, // so that it cannot be referring to fd.sysfd anymore, // the final decref will close fd.sysfd. This should happen // fairly quickly, since all the I/O is non-blocking, and any // attempts to block in the pollDesc will return errClosing(fd.isFile). fd.pd.evict() // 关键代码 // The call to decref will call destroy if there are no other // references. err := fd.decref() // Wait until the descriptor is closed. If this was the only // reference, it is already closed. Only wait if the file has // not been set to blocking mode, as otherwise any current I/O // may be blocking, and that would block the Close. // No need for an atomic read of isBlocking, increfAndClose means // we have exclusive access to fd. if fd.isBlocking == 0 { runtime_Semacquire(&fd.csema) } return err } // decref removes a reference from fd. // It also closes fd when the state of fd is set to closed and there // is no remaining reference. func (fd *FD) decref() error { if fd.fdmu.decref() { return fd.destroy() } return nil }

go - What is the purpose of closeAfterStart in exec - Stack Overflow

6. exec案例

6.1 实时读取标准输出#

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func TestReadStdoutRealTime(t *testing.T) { cmd := exec.Command("ping", "-c", "5", "192.168.0.1") stdout, _ := cmd.StdoutPipe() cmd.Start() collect := func(output io.Reader) error { scanner := bufio.NewScanner(output) scanner.Split(bufio.SplitFunc(bufio.ScanLines)) for scanner.Scan() { if scanner.Err() != nil { fmt.Println(scanner.Err()) return scanner.Err() } line := scanner.Text() fmt.Println(line) } return nil } if err := collect(stdout); err != nil { fmt.Println("collect stdout failed", err) } if err := cmd.Wait(); err != nil { fmt.Println("wait exec failed", err.Error()) } }

6.2 实时读取标准输出(错误用法)#

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func collector(ctx context.Context, output io.Reader) error { scanner := bufio.NewReader(output) for { // 当异步收集日志比较慢，但命令执行很快就退出后，会将pipe关闭，导致管道无法读取， // 报错 read |0: file already closed // time.Sleep(2 * time.Second) select { case <-ctx.Done(): if ctx.Err() != nil { return ctx.Err() } return errors.New("process existed") default: readStr, readErr := scanner.ReadString('\n') if readErr != nil { if readErr != io.EOF { return readErr } return nil } fmt.Println(readStr) } } } func TestReadStdoutWrongExample(t *testing.T) { ctx, cancel := context.WithTimeout(context.Background(), 20*time.Second) defer cancel() cmd := exec.CommandContext(ctx, "ping", "-c", "5", "192.168.0.1") stdout, _ := cmd.StdoutPipe() cmd.Start() go func() { err := collector(ctx, stdout) if err != nil { fmt.Println(err) } }() if err := cmd.Wait(); err != nil { fmt.Println("wait exec failed", err) } }