os/exec: run "echo xxx" hung up

[mei-rune]

What version of Go are you using (go version)?

go version go1.8.1 windows/amd64

What operating system and processor architecture are you using (go env)?

set GOARCH=amd64
set GOBIN=
set GOEXE=.exe
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=d:\developing\go\meijing;d:\developing\go\meijing\tpt_vendor;d:\dev
eloping\go\meijing\src\cn\com\hengwei\Godeps_workspace
set GORACE=
set GOROOT=d:\tools\go_amd64
set GOTOOLDIR=d:\tools\go_amd64\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\mei
fakun\AppData\Local\Temp\go-build280858219=/tmp/go-build -gno-record-gcc-switche
s
set CXX=g++
set CGO_ENABLED=1
set PKG_CONFIG=pkg-config
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2

What did you do?

run "echo xxx" command and read console ouput of comand

https://play.golang.org/p/Dob0vjP-L6

What did you expect to see?

excpect output: abc
don't output: timeout

What did you see instead?

process is hung up util timeout

[davecheney]

createPipe is not creating a pipe.

…

On 31 Jul 2017, at 22:38, runner ***@***.***> wrote: What version of Go are you using (go version)? go version go1.8.1 windows/amd64 What operating system and processor architecture are you using (go env)? set GOARCH=amd64 set GOBIN= set GOEXE=.exe set GOHOSTARCH=amd64 set GOHOSTOS=windows set GOOS=windows set GOPATH=d:\developing\go\meijing;d:\developing\go\meijing\tpt_vendor;d:\dev eloping\go\meijing\src\cn\com\hengwei\Godeps_workspace set GORACE= set GOROOT=d:\tools\go_amd64 set GOTOOLDIR=d:\tools\go_amd64\pkg\tool\windows_amd64 set GCCGO=gccgo set CC=gcc set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\mei fakun\AppData\Local\Temp\go-build280858219=/tmp/go-build -gno-record-gcc-switche s set CXX=g++ set CGO_ENABLED=1 set PKG_CONFIG=pkg-config set CGO_CFLAGS=-g -O2 set CGO_CPPFLAGS= set CGO_CXXFLAGS=-g -O2 set CGO_FFLAGS=-g -O2 set CGO_LDFLAGS=-g -O2 What did you do? run "echo xxx" command and read console ouput of comand https://play.golang.org/p/Dob0vjP-L6 What did you expect to see? excpect output: abc don't output: timeout What did you see instead? process is hung up util timeout — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub, or mute the thread.

[mei-rune]

@davecheney Yes, it is a tcp conn.

I rename createPipe to createConn
https://play.golang.org/p/67tDZbIKzd

[davecheney]

Pipes have two ends, createPipe only returns one end of the pipe, the other is lost during the call to accept.

[mei-rune]

@davecheney Yes, the other end should give to my another app1 in my real world, it will send some chars to "echo" command by the pipe, "echo" is my another app2.
I hope "if app2 is exited, then cmd.Wait() is return, and I should close the conn.

[davecheney]

I don't understand what the goal of running echo is. Echo will just write back to stout whatever is written to standard in. If feels like this is an over complicated solution, and possibly not the actual problem you are trying to solve.

…

On 31 Jul 2017, at 22:51, runner ***@***.***> wrote: @davecheney Yes, the other end should give to my another app1, it will send some chars to "echo" command, "echo" is my another app2. I hope "if app2 is exited, then cmd.Wait() is return, and I should close the conn. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub, or mute the thread.

[mei-rune]

@davecheney No, "echo" is not echo of linux(windows), echo is a my app, it read message from stdio and do some things. I replace my app name with "echo" in the example.
sometimes the my app will crash, but cmd.Wait() will hung up.

[mei-rune]

I hope Wait() don't wait stdin if process is exited, and I self close the stdin.
Becase sometimes stdin may not close forever

[mattn]

Because the client connection that returned from l.Accept() is not GC collected. You can confirm connection will be closed with

func createPipe() net.Conn {
	defer runtime.GC()
	l, err := net.Listen("tcp", "127.0.0.1:52354")
	if err != nil {
		panic(err)
	}
	go func() {
		l.Accept()
		// Client Conneciton will be closed by runtime.GC above
	}()
	fmt.Println(l.Addr().String())

	conn, err := net.Dial("tcp", "127.0.0.1:52354")
	if err != nil {
		panic(err)
	}
	return conn
}

[mei-rune]

@mattn Yes, the client connection is lose and not close in the example, But the client connection should give to my another app1 in my real world, it will send some chars to the "echo" app by the pipe, "echo" is my another app2.

I hope Wait() don't wait stdin if process is exited, and I self close the stdin.

I am chinese, english is bad, I draw a picture to explain it at tomorrow.

[mattn]

cmd.Wait wait goroutines which is spawned for reading pipes. If you want to just wait the process without waiting pipes, please use cmd.Process.Wait

// You can edit this code!
// Click here and start typing.
package main

import (
	"bytes"
	"fmt"
	"net"
	"os/exec"
	"runtime"
	"time"
)

func createPipe() net.Conn {
	l, err := net.Listen("tcp", "127.0.0.1:52354")
	if err != nil {
		panic(err)
	}
	go func() {
		l.Accept()
	}()
	fmt.Println(l.Addr().String())

	conn, err := net.Dial("tcp", "127.0.0.1:52354")
	if err != nil {
		panic(err)
	}
	return conn
}

func main() {
	var buf bytes.Buffer
	var cmd *exec.Cmd
	if runtime.GOOS == "windows" {
		cmd = exec.Command("cmd", "/c", "echo", "abc")
	} else {
		cmd = exec.Command("echo", "abc")
	}

	pipe := createPipe() // cmd.Wait() will not exit if pipe don't close()

	cmd.Stdin = pipe
	cmd.Stdout = &buf
	cmd.Stderr = &buf
	if err := cmd.Start(); err != nil {
		panic(err)
	}
	time.AfterFunc(1*time.Minute, func() {
		cmd.Process.Kill()
		fmt.Println("timeout")
	})
	if _, err := cmd.Process.Wait(); err != nil {
		panic(err)
	} else {
		fmt.Println(buf.String())
	}
}

[mei-rune]

@mattn Yes, it is ok.

Please add a document to describe it.
in addition you cannot call cmd.Wait () after calling cmd.Process.Wait (), otherwise it will blocked， it should be warned in the document.

[ianlancetaylor]

Merging into #23019.