What causes "failed to seek in file: %w" in ahmetb/kubectx?

Check the wrapped error — "file already closed" means another path closed the handle; remove the duplicate Close

failed to seek in file: %w

Error message

failed to seek in file: %w

What it means

kubeconfigFile.Reset calls Truncate(0) then Seek(0, 0) to reposition the file offset at the start before rewriting. This error wraps any Seek failure; it happens after truncation, so the file may already be empty when this error surfaces.

Source

Thrown at internal/kubeconfig/kubeconfigloader.go:68

				continue
			}
			return nil, fmt.Errorf("failed to open file %q: %w", p, err)
		}
		files = append(files, &kubeconfigFile{File: f, path: p})
	}
	if len(files) == 0 {
		return nil, fmt.Errorf("kubeconfig file not found: %w",
			&os.PathError{Op: "open", Path: paths[0], Err: os.ErrNotExist})
	}
	return files, nil
}

func (kf *kubeconfigFile) Reset() error {
	if err := kf.Truncate(0); err != nil {
		return fmt.Errorf("failed to truncate file: %w", err)
	}
	if _, err := kf.Seek(0, 0); err != nil {
		return fmt.Errorf("failed to seek in file: %w", err)
	}
	return nil
}

func kubeconfigPaths() ([]string, error) {
	// KUBECONFIG env var
	if v := os.Getenv("KUBECONFIG"); v != "" {
		return filepath.SplitList(v), nil
	}

	// default path
	home := cmdutil.HomeDir()
	if home == "" {
		return nil, errors.New("HOME or USERPROFILE environment variable not set")
	}
	return []string{filepath.Join(home, ".kube", "config")}, nil
}

View on GitHub (pinned to 12ad6fb22e)

Solutions

Check the wrapped error — "file already closed" means another path closed the handle; remove the duplicate Close
Ensure Reset and Save run sequentially from one owner of the file handle, not concurrently
Verify the file is a regular file (special devices may not support seek)
Reload the kubeconfig from disk (construct a new loader) if the handle is in an unknown state

Example fix

// before
go k.Close()
k.Save()   // handle closed mid-way; seek fails
// after
if err := k.Save(); err != nil { return err }
if err := k.Close(); err != nil { return err }

Defensive patterns

Strategy: try-catch

Validate before calling

// Go
// Ensure the handle is still valid before Reset
if _, err := kf.Seek(0, io.SeekCurrent); err != nil {
    return fmt.Errorf("kubeconfig handle invalid: %w", err)
}

Type guard

func isClosedErr(err error) bool {
    return errors.Is(err, os.ErrClosed)
}

Try / catch

if err := kf.Reset(); err != nil {
    if errors.Is(err, os.ErrClosed) {
        return fmt.Errorf("kubeconfig file handle already closed — reload before saving: %w", err)
    }
    return err
}

Prevention

Define strict ownership: only one place closes the file, after all writes
Avoid concurrent goroutines sharing one kubeconfigFile handle
Reload from disk if handle state is uncertain instead of reusing it
Only use regular files for kubeconfigs

When it happens

Trigger: Calling Reset when the underlying file descriptor is invalid or already closed, causing os.File.Seek to fail (the underlying error is an *os.PathError with Op "seek").

Common situations: Double-close of the shared *os.File from concurrent code paths; using a file opened against a special file/device that does not support seeking; file descriptor corruption after an earlier I/O failure.

Related errors

AI-assisted analysis of ahmetb/kubectx@12ad6fb22e (2026-09-02). Data as JSON: /api/errors/2c9dd64671db4899. Report an issue: GitHub.