package webui

import (
	"crypto/rand"
	"fmt"
	"os"
	"path/filepath"
)

const appSecretSize = 32

// LoadOrCreateAppSecret reads the app's CSRF-signing secret from path, generating a
// fresh random one on first run if the file doesn't exist yet — mirrors
// mailstore.LoadOrCreateMasterKey's identical generate-if-missing pattern for the
// mailstore encryption key. Unlike that key, losing this one has no data-loss
// consequence: every outstanding CSRF token just stops validating, so users get
// logged-out-feeling form-submit errors until they reload a page for a fresh one.
func LoadOrCreateAppSecret(path string) ([]byte, error) {
	if b, err := os.ReadFile(path); err == nil {
		if len(b) != appSecretSize {
			return nil, fmt.Errorf("app secret at %s is %d bytes, want %d", path, len(b), appSecretSize)
		}
		return b, nil
	} else if !os.IsNotExist(err) {
		return nil, err
	}

	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
		return nil, err
	}
	secret := make([]byte, appSecretSize)
	if _, err := rand.Read(secret); err != nil {
		return nil, err
	}
	if err := os.WriteFile(path, secret, 0o600); err != nil {
		return nil, err
	}
	return secret, nil
}