What causes "data cannot be nil" in OpenNHP/opennhp?

Check the value is non-nil before calling, or return an error upstream instead

data cannot be nil

Error message

data cannot be nil

What it means

SaveStructAsJsonFile refuses to serialize a nil value: if the data parameter is nil it returns 'data cannot be nil' before any file I/O. This guards against writing a literal 'null' JSON file or a confusing marshal outcome. Callers (e.g. registerTAService) must pass a concrete struct/map value.

Solutions

Check the value is non-nil before calling, or return an error upstream instead
Initialize the struct with defaults when source data is missing
Handle the error from SaveStructAsJsonFile rather than ignoring it

Example fix

// before
var taService *TAService // nil after failed lookup
SaveStructAsJsonFile(path, taService)
// after
if taService == nil { return fmt.Errorf("ta service not found") }
SaveStructAsJsonFile(path, taService)

Defensive patterns

Strategy: validation

Validate before calling

if data == nil { return fmt.Errorf("refusing to save nil payload to %s", filePath) }

Type guard

func nonNil(v any) bool { return v != nil }

Try / catch

if err := SaveStructAsJsonFile(path, payload); err != nil { return fmt.Errorf("save ta service: %w", err) }

Prevention

Never forward possibly-nil lookup results straight into save helpers
Return errors from upstream lookups instead of nil values
Check pointer receivers for nil before building payload structs

When it happens

Trigger: Passing an untyped nil, a nil pointer of concrete type passed as any and checked as nil only for untyped nil is NOT caught — but direct nil, or a variable that is literally nil (nil map assigned to any interface value is non-nil interface; untyped nil is caught), typically from a failed lookup upstream returning nil data.

Common situations: Upstream API/registry call returned nil and the result was forwarded unchecked to SaveStructAsJsonFile; forgotten struct initialization; error path that skips population of the payload.

Related errors

AI-assisted analysis of OpenNHP/opennhp@6e04ca5ff0 (2026-09-07). Data as JSON: /api/errors/f05b6dffec75593e. Report an issue: GitHub.

Appendix: source

Thrown at nhp/utils/utils.go:114

func GenerateTempFilePath(pattern string) (string, error) {
	file, err := os.CreateTemp("", pattern)
	if err != nil {
		return "", err
	}

	tempPath := file.Name()

	if err := file.Close(); err != nil {
		return "", err
	}

	return tempPath, nil
}

func SaveStructAsJsonFile(filePath string, data any) error {
	if data == nil {
		return fmt.Errorf("data cannot be nil")
	}
	if filePath == "" {
		return fmt.Errorf("file path cannot be empty")
	}

	jsonData, err := json.MarshalIndent(data, "", "  ")
	if err != nil {
		return fmt.Errorf("failed to marshal data to JSON: %w", err)
	}

	err = os.WriteFile(filePath, jsonData, 0644) //nolint:gosec // G306: Generic utility - callers determine sensitivity
	if err != nil {
		return fmt.Errorf("failed to write JSON to file: %w", err)
	}

	return nil
}

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