aperture: add config file flag and parse command line flags

aperture.go

@@ -102,8 +102,7 @@ func run() error {
 	}
 
 	// Next, parse configuration file and set up logging.
-	configFile := filepath.Join(apertureDataDir, defaultConfigFilename)
-	cfg, err := getConfig(configFile)
+	cfg, err := getConfig()
 	if err != nil {
 		return fmt.Errorf("unable to parse config file: %w", err)
 	}
@@ -319,15 +318,44 @@ func fileExists(name string) bool {
 
 // getConfig loads and parses the configuration file then checks it for valid
 // content.
-func getConfig(configFile string) (*Config, error) {
+func getConfig() (*Config, error) {
+	// Pre-parse command line flags to determine whether we've been pointed
+	// to a custom config file.
 	cfg := &Config{}
-	b, err := ioutil.ReadFile(configFile)
-	if err != nil {
+	if _, err := flags.Parse(cfg); err != nil {
 		return nil, err
 	}
-	err = yaml.Unmarshal(b, cfg)
-	if err != nil {
+
+	// If a custom config file is provided, we require that it exists.
+	var mustExist bool
+
+	configFile := filepath.Join(apertureDataDir, defaultConfigFilename)
+	if cfg.ConfigFile != "" {
+		configFile = lnd.CleanAndExpandPath(cfg.ConfigFile)
+		mustExist = true
+	}
+
+	// Read our config file, either from the custom path provided or our
+	// default location.
+	b, err := ioutil.ReadFile(configFile)
+	switch {
+	// If the file was found, unmarshal it.
+	case err == nil:
+		err = yaml.Unmarshal(b, cfg)
+		if err != nil {
+			return nil, err
+		}
+
+	// If the error is unrelated to the existence of the file, we must
+	// always return it.
+	case !os.IsNotExist(err):
 		return nil, err
+
+	// If we require that the config file exists and we got an error
+	// related to file existence, we must fail.
+	case mustExist && os.IsNotExist(err):
+		return nil, fmt.Errorf("config file: %v must exist: %w",
+			configFile, err)
 	}
 
 	// Finally, parse the remaining command line options again to ensure

config.go

@@ -104,6 +104,9 @@ type Config struct {
 	// DebugLevel is a string defining the log level for the service either
 	// for all subsystems the same or individual level by subsystem.
 	DebugLevel string `long:"debuglevel" description:"Debug level for the Aperture application and its subsystems."`
+
+	// ConfigFile points aperture to an alternative config file.
+	ConfigFile string `long:"configfile" description:"Custom path to a config file."`
 }
 
 func (c *Config) validate() error {