Darwin: Freezes if not reading stdout.

[Mattemagikern]

Hello!

I've found a difference in behaviour between Mac and Linux using your module.

package main

import (
	"github.com/creack/pty"

	"os/exec"
	"testing"
)

func TestCommand(t *testing.T) {
	c := exec.Command("./echo.sh", "TestCommand")
	pty, err := pty.Start(c)
	if err != nil {
		t.Fatal(err)
	}
	defer pty.Close()
	err = c.Wait()
	if err != nil {
		t.Fatal(err)
	}
}

echo.sh:

#!/bin/bash
echo $@

If this is run on linux it will pass, on darwin it will freeze.
However If I add a buffer to c.stdout it will pass.
My guess is that the master file descriptor isn't buffering output from the slave. Which is really strange.
On the otherhand if you replace ./echo.sh to echo it will pass.

I'm not sure what I'm seeing or how to amend it.
My guess is a combination between differences in the os/exec between linux and darwin and file settings differences between darwin and linux regarding the master terminal.

Do you have any idea what this might be?

Best regargs,
Måns Ansgariusson

Go version: 1.14
pty version: v1.1.11

[creack]

I am sorry, I didn't have time to look into this just yet. Will do asap, I should have time over the weekend.

[Mattemagikern]

No stress :)

[creack]

I managed to reproduce.

To be honest, I am not 100% sure of the reason.

Adding a sink instead of a buffer is probably better (go io.Copy(ioutil.Discard, pty)).

I think you are right and it as to do with some lower-level difference on how the tty driver works between linux and darwin.

That being said, it looks quite specific to bash. if you switch to zsh, it doesn't block.

Do you have a specific use-case where is could cause concern? From the looks of it, I'd be inclined to simply close this as a non-issue.
As a rule of thumb, even in tty mode, when the process writes, something should read it.

[Mattemagikern]

Sweet!

Adding a sink instead of a buffer is probably better (go io.Copy(ioutil.Discard, pty)).

This solved it for me as well.

That being said, it looks quite specific to bash. if you switch to zsh, it doesn't block.

That is super interesting! I'll defenetly dig into that, didn't think of testing another shell.

Do you have a specific use-case where is could cause concern? From the looks of it, I'd be inclined to simply close this as a non-issue.
As a rule of thumb, even in tty mode, when the process writes, something should read it.

Yes, close it. I agree that it is expected that something should read what is outputed from the pty.
I was making test-cases for a project of mine when i stumbled upon this.

Thank you for your time debugging this :)

Best regards!