Re: [go-nuts] exec.Cmd: if the child forks Wait may hang forever

[Steven Hartland]

If your forked process still has stdin / stdout open then I would say 
this is expected.


You could confirm this by adding the following to the python script.
import sys
...
if pid == 0:
   sys.stdin.close()
   sys.stdout.close()

On 13/07/2016 21:50, noxiouz wrote:

Good day!

I faced a corner case of cmd.Exec usage and want to find out if it's 
designed behavior.



go version go1.6.2 darwin/amd64 and linux


Let's assume we use cmd.Exec to start some process, that can fork and 
dies, but the fork will live forever. For example:


|
#!/usr/bin/env python

importos
importtime

pid =os.fork()
ifpid ==0:
whileTrue:
time.sleep(10)

print"main process has quited"

|

The real script is much more complicated but it's not important.

Also there is a go code:

|
packagemain

import(
"io"
"log"
"os/exec"
)

func main(){
cmd :=exec.Command("./forker.py")
rd,wr :=io.Pipe()
cmd.Stdout=wr
go func(){
b :=make([]byte,100)
for{
nn,err :=rd.Read(b)
ifnn >0{
log.Printf("%s",b[:nn])
}
iferr !=nil{
log.Fatal(err)
return
}
}
}()
iferr :=cmd.Start();err !=nil{
log.Fatal(err)
}

log.Printf("PID %d",cmd.Process.Pid)

iferr :=cmd.Wait();err !=nil{
log.Fatal(err)
}
}
|

The output would be like this:

2016/07/13 23:40:20 PID 90614
2016/07/13 23:40:20 main process has quited


but the Go process will hang forever, because of waiting on: 
https://github.com/golang/go/blob/release-branch.go1.6/src/os/exec/exec.go#L401

I think, because stdout fd of child process would not be closed .

On the other hand the next code works perfectly and does not hang:

|
packagemain

import(
"log"
"os/exec"
)

func main(){
cmd :=exec.Command("./forker.py")
rd,err :=cmd.StdoutPipe()
iferr !=nil{
log.Fatal(err)
}
go func(){
b :=make([]byte,100)
for{
nn,err :=rd.Read(b)
ifnn >0{
log.Printf("%s",b[:nn])
}
iferr !=nil{
log.Fatal(err)
return
}
}
}()
iferr :=cmd.Start();err !=nil{
log.Fatal(err)
}

log.Printf("PID %d",cmd.Process.Pid)

iferr :=cmd.Wait();err !=nil{
log.Fatal(err)
}
}

|

So I would like to find out if setting cmd.Stdout explicitly is not 
expected.



Thanks!




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Re: [go-nuts] exec.Cmd: if the child forks Wait may hang forever

[Ian Lance Taylor]

On Wed, Jul 13, 2016 at 1:50 PM, noxiouz  wrote:
>
> I faced a corner case of cmd.Exec usage and want to find out if it's
> designed behavior.

...

> So I would like to find out if setting cmd.Stdout explicitly is not
> expected.

Sounds like https://golang.org/issue/13155 .

Ian

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