What causes "failed to create stderr: %v" in hashicorp/nomad?

Verify the stderr fifo exists in the alloc dir; restart the allocation if missing

failed to create stderr: %v

Error message

failed to create stderr: %v

What it means

ExecCommand.Stderr() mirrors Stdout(): it lazily opens a FIFO writer at StderrPath to stream task stderr. If fifo.OpenWriter on the stderr path fails, the error is wrapped with this message and Launch fails.

Source

Thrown at drivers/shared/executor/executor.go:285

			f, err := fifo.OpenWriter(c.StdoutPath)
			if err != nil {
				return nil, fmt.Errorf("failed to create stdout: %v", err)
			}
			c.stdout = f
		} else {
			c.stdout = nopCloser{io.Discard}
		}
	}
	return c.stdout, nil
}

// Stderr returns a writer for the configured file descriptor
func (c *ExecCommand) Stderr() (io.WriteCloser, error) {
	if c.stderr == nil {
		if c.StderrPath != "" && c.StderrPath != os.DevNull {
			f, err := fifo.OpenWriter(c.StderrPath)
			if err != nil {
				return nil, fmt.Errorf("failed to create stderr: %v", err)
			}
			c.stderr = f
		} else {
			c.stderr = nopCloser{io.Discard}
		}
	}
	return c.stderr, nil
}

func (c *ExecCommand) Close() {
	if c.stdout != nil {
		c.stdout.Close()
	}
	if c.stderr != nil {
		c.stderr.Close()
	}
}

View on GitHub (pinned to 482b49bf1a)

Solutions

Verify the stderr fifo exists in the alloc dir; restart the allocation if missing
Correct ownership/permissions on the alloc/log directories
Check audit logs for SELinux/apparmor denials on the path
Compare with the stdout error — if both fail, the alloc dir is the problem

Example fix

// before
// SELinux: 'denied { write } for comm=nomad path=.../stderr.fifo'
// after
// semanage fcontext -a -t container_file_t '.../stderr.fifo' && restorecon -v .../stderr.fifo

Defensive patterns

Strategy: try-catch

Validate before calling

func stderrFifoReady(path string) error {
  fi, err := os.Stat(path)
  if err != nil { return fmt.Errorf("stderr fifo missing: %w", err) }
  if fi.Mode()&os.ModeNamedPipe == 0 { return fmt.Errorf("%s is not a fifo", path) }
  f, err := os.OpenFile(path, os.O_WRONLY, 0)
  if err != nil { return err }
  f.Close()
  return nil
}

Try / catch

if err := launchTask(); err != nil {
  if strings.Contains(err.Error(), "failed to create stderr") {
    log.Printf("stderr fifo broken: %v — check alloc dir perms", err)
    return restartAllocation()
  }
  return err
}

Prevention

Verify fifo writability before deploying permission changes
Consistent UID/GID between client and executor users
Audit MAC (SELinux) policy changes affecting alloc dirs
Monitor alloc-dir tampering by external log shippers

When it happens

Trigger: Launch() calls command.Stderr() and fifo.OpenWriter(c.StderrPath) errors — missing fifo, permission denied, or path deleted between stdout and stderr opens.

Common situations: Same as stdout fifo failures: allocation dir permission problems, premature cleanup, MAC policy (SELinux) denials, disk full preventing pipe allocation is rare but possible.

Related errors

AI-assisted analysis of hashicorp/nomad@482b49bf1a (2026-09-04). Data as JSON: /api/errors/bc95b046155eeb69. Report an issue: GitHub.