hashicorp/nomad · error

attempted to resize a non-tty session

Error message

attempted to resize a non-tty session

What it means

On Unix, setTTYSize resizes a terminal via pty.Setsize, which requires the writer to be the *os.File backing a real PTY. This error is returned when a resize request targets a session whose output writer is not a PTY file (i.e., the exec session is not a TTY).

Source

Thrown at drivers/shared/executor/pty_unix.go:30

	"os"
	"strings"
	"syscall"

	"github.com/creack/pty"
	"golang.org/x/sys/unix"
)

func sessionCmdAttr(tty *os.File) *syscall.SysProcAttr {
	return &syscall.SysProcAttr{
		Setsid:  true,
		Setctty: true,
	}
}

func setTTYSize(w io.Writer, height, width int32) error {
	f, ok := w.(*os.File)
	if !ok {
		return fmt.Errorf("attempted to resize a non-tty session")
	}

	return pty.Setsize(f, &pty.Winsize{
		Rows: uint16(height),
		Cols: uint16(width),
	})

}

func isUnixEIOErr(err error) bool {
	if err == nil {
		return false
	}

	return strings.Contains(err.Error(), unix.EIO.Error())
}

View on GitHub (pinned to 482b49bf1a)

Solutions

1. Only request terminal resize for sessions created with tty=true
2. Check the session's Tty flag before calling resize and skip silently when false
3. Allocate the session with TTY enabled if interactive resize is required
4. Fix caller code to pass the original PTY *os.File, not a wrapper writer

Example fix

// before
setTTYSize(session.Output, h, w) // Output is a pipe
// after
if session.Tty {
    setTTYSize(session.Output, h, w)
}

if !session.Tty { skip resize }

func isPTYFile(w io.Writer) (*os.File, bool) {
    f, ok := w.(*os.File)
    return f, ok
}

if err := session.Resize(h, w); err != nil && strings.Contains(err.Error(), "non-tty") {
    // ignore: session has no PTY
}

When it happens

Trigger: Calling TerminalResize on a non-TTY exec session (task exec started with tty=false), or passing a buffered/pipe writer instead of the PTY *os.File.

Common situations: Terminal UI (e.g. nomad monitor/exec clients) sending SIGWINCH-driven resize events for sessions allocated without a TTY; drivers that don't allocate PTYs on the given platform.

Related errors

• failed to open a tty: %v
• failed to get pty: %v
• failed to resize tty: %v
• not a terminal
• error creating fifo: %w

AI-assisted analysis of hashicorp/nomad@482b49bf1a (2026-09-04). Data as JSON: /api/errors/781c3c9f304b2921. Report an issue: GitHub.