强制关闭SSH客户端会话

Question

我已经编写了一个SSH客户端来连接到网络设备，一旦运行命令超过25秒，我就通过"select“设置超时。我注意到有一些设备有另一个IOS，一旦超时被触发，它就不能通过Close()方法丢弃SSH会话，这会导致goroutinge泄漏。我需要保持客户端状态并断开会话，以便为下一条命令做好准备。看起来goroutine在那个时候不会永远终止！你们有什么想法吗？

    go func() {
       r <- s.Run(cmd)
    }()

    select {
       case err := <-r:
         return err
       case <-time.After(time.Duration(timeout) * time.Second):
         s.Close()
         return fmt.Errorf("timeout after %d seconds", timeout)
    }

通过堆分析，我看到了以下内容:2.77 saw 99.44% 99.44% 2.77 saw 99.44% bytes.makeSlice

     0     0% 99.44%     2.77GB 99.44%  bytes.(*Buffer).ReadFrom

     0     0% 99.44%     2.77GB 99.44%  golang.org/x/crypto/ssh.(*Session).start.func1

     0     0% 99.44%     2.77GB 99.44%  golang.org/x/crypto/ssh.(*Session).stdout.func1

     0     0% 99.44%     2.77GB 99.44%  io.Copy

     0     0% 99.44%     2.77GB 99.44%  io.copyBuffer

     0     0% 99.44%     2.78GB 99.93%  runtime.goexit

在/usr/ ======================== /go/src/ runtime.goexit /asm_amd64.s中的例程运行时

     0     2.78GB (flat, cum) 99.93% of Total

     .          .   1993:   RET

     .          .   1994:

     .          .   1995:// The top-most function running on a goroutine

     .          .   1996:// returns to goexit+PCQuantum.

     .          .   1997:TEXT runtime·goexit(SB),NOSPLIT,$0-0

     .     2.78GB   1998:   BYTE    $0x90   // NOP

     .          .   1999:   CALL    runtime·goexit1(SB) // does not return

     .          .   2000:   // traceback from goexit1 must hit code range of goexit

     .          .   2001:   BYTE    $0x90   // NOP

     .          .   2002:


     .          .   2003:TEXT runtime·prefetcht0(SB),NOSPLIT,$0-8

Answer 1

通道r阻止Go例程返回，因为它没有被清空。我已经编写了您的代码的改编版本，并插入了一个等待组来演示这个问题：

func main() {
    var wg sync.WaitGroup // This is only added for demonstration purposes
    s := new(clientSession)

    r := make(chan error)

    go func(s *clientSession) {
        wg.Add(1)
        r <- s.Run()
        wg.Done() // Will only be called after s.Run() is able to return
    }(s)

    fmt.Println("Client has been opened")

    select {
    case err := <-r:
        fmt.Println(err)
    case <-time.After(1 * time.Second):
        s.Close()
        fmt.Println("Timed out, closing")
    }

    wg.Wait() // Waits until wg.Done() is called.

    fmt.Println("Main finished successfully")
}

Go playground似乎会终止程序，所以我用完整的可运行代码创建了一个gist。当我们运行incorrect.go时

$ go run incorrect.go
Client has been opened
Timed out, closing
fatal error: all goroutines are asleep - deadlock!
....

这是因为我们的代码在wg.Wait()行上死锁了。这表明Go例程中的wg.Done()从未达到。

正如评论指出的那样，缓冲通道在这方面可以有所帮助。但前提是您在调用s.Close()之后不再关心错误

r := make(chan error, 1)

buffered.go运行正常，但错误丢失：

$ go run buffered.go
Client has been opened
Timed out, closing
Main finished successfully

另一种选择是将通道正好排出一次：

select {
    case err := <-r:
        fmt.Println(err)
    case <-time.After(1 * time.Second):
        s.Close()
        fmt.Println("Timed out, closing")
        fmt.Println(<-r)
    }

或者通过将select包装在for循环中(无缓冲通道)：

X:
    for {
        select {
        case err := <-r:
            fmt.Println(err)
            break X // because we are in main(). Normally `return err`
        case <-time.After(1 * time.Second):
            s.Close()
            fmt.Println("Timed out, closing")
        }
    }

当我们运行drain.go时，我们看到错误也被打印出来：

$ go run incorrect.go
Client has been opened
Timed out, closing
Run() closed
Main finished successfully

在现实世界中，人们可能会运行多个围棋例程。因此，您将希望在for循环上使用一些计数器，或者进一步利用等待组功能。