containerd/containerd · error

failed to parse output: %q

Error message

failed to parse output: %q

What it means

Status() parses `dmsetup status` output, which must contain at least 4 space-separated fields (offset, length, target type, params). This error is thrown when the output has fewer fields, meaning the status line is malformed or not a valid table status.

Source

Thrown at plugins/snapshots/devmapper/dmsetup/dmsetup.go:311

		err    error
		status DeviceStatus
	)

	output, err := dmsetup("status", deviceName)
	if err != nil {
		return nil, err
	}
	status.RawOutput = output

	// Status output format:
	//  Offset (int64)
	//  Length (int64)
	//  Target type (string)
	//  Params (Array of strings)
	const MinParseCount = 4
	parts := strings.Split(output, " ")
	if len(parts) < MinParseCount {
		return nil, fmt.Errorf("failed to parse output: %q", output)
	}

	status.Offset, err = strconv.ParseInt(parts[0], 10, 64)
	if err != nil {
		return nil, fmt.Errorf("failed to parse offset: %q: %w", parts[0], err)
	}

	status.Length, err = strconv.ParseInt(parts[1], 10, 64)
	if err != nil {
		return nil, fmt.Errorf("failed to parse length: %q: %w", parts[1], err)
	}

	status.Target = parts[2]
	status.Params = parts[3:]

	return &status, nil
}

View on GitHub (pinned to 4246446a2b)

Solutions

1. Run `dmsetup status <name>` manually to see the actual output shape.
2. Ensure the thin-pool/device is created and activated before querying status.
3. Treat a missing device as a distinct case: check Info() first or handle exit codes from dmsetup.
4. Confirm dmsetup version compatibility with the expected 4+-field status format.
5. Check the wrapped dmsetup exec error path — stderr may explain why the output is malformed.

Example fix

// before
usage, err := pool.GetUsage() // device not activated yet
// after
info, err := devInfo() // wrap dmsetup Info
if err != nil || !info.TableLive { return nil, fmt.Errorf("device %s not active", name) }
usage, err := pool.GetUsage()

func ensureDeviceActive(name string) error {
    out, err := exec.Command("dmsetup", "info", name).Output()
    if err != nil { return err }
    return nil // exists; proceed to status
}
// call ensureDeviceActive before Status/GetUsage

usage, err := pool.GetUsage()
if err != nil {
    if strings.Contains(err.Error(), "failed to parse output") {
        return fmt.Errorf("device not active or status malformed; run 'dmsetup status %s' to inspect: %w", name, err)
    }
    return err
}

When it happens

Trigger: Status -> GetUsage or createSnapshot receives truncated or unexpected `dmsetup status` output: device not yet activated, error line from dmsetup, or empty output passed to the parser.

Common situations: Querying a device before it is created/activated; dmsetup reporting an error state whose status line has a different shape; version mismatch changing column count.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• failed to parse line %q: %w
• failed to parse offset: %q: %w
• failed to parse length: %q: %w
• failed to get block device size: %s: %w
• failed to seek on %q: %w

AI-assisted analysis of containerd/containerd@4246446a2b (2026-09-02). Data as JSON: /api/errors/873e6cebdcc97a8c. Report an issue: GitHub.