Setctty set but Ctty not valid in child with Go +be08e10b3b

[myitcv]

Steps to reproduce:

go version
go run .

-- go.mod --
module mod.com

go 1.15

require (
        github.com/creack/pty v1.1.9
        golang.org/x/sys v0.0.0-20200501145240-bc7a7d42d5c3 // indirect
)
-- main.go --
package main

import (
        "io"
        "io/ioutil"
        "os/exec"

        "github.com/creack/pty"
)

func main() {
        cmd := exec.Command("vim")
        thepty, err := pty.Start(cmd)
        if err != nil {
                panic(err)
        }
        io.Copy(ioutil.Discard, thepty)
}

What did you expect to see?

No error

What did you see instead?

Error:

fork/exec /home/myitcv/dev/vim/src/vim: Setctty set but Ctty not valid in child

Bisected to this change: https://go-review.googlesource.com/c/go/+/231638/

FYI @ianlancetaylor @neild @mvdan

[myitcv]

Previous discussion in golang/go#29458

[creack]

reproduced. Investigating.

[ianlancetaylor]

Note copied from golang/go#29458:

If tty is going to be open in the child process, then it must have a file descriptor in the child process. When using os/exec.Cmd as the creack/pty code does, tty must be in Stdin or Stdout or Stderr or ExtraFiles. If it isn't in any of those, then it will be closed when the child process runs. If it is in one of those, that gives you the file descriptor number that you should use in Ctty.

Right now the creack code is setting Ctty to the parent's file descriptor number. That is working (assuming that it is working) by accident. The correct fix is to ensure that tty is passed to the child somewhere, and set Ctty to the appropriate descriptor number. If it is possible for all of Stdin, Stdout, Stderr to be set, the simplest approach would be to always add tty to ExtraFiles, and set Ctty to 3. That will work for old versions of Go and also for the upcoming 1.15 release.