Update config detection to allow absent files

Previously, if a non-existent config file was passed via CLI, the app
would error out. This will allow the app to continue even with a
non-existent config file.

This will prove useful for adwatchd where the app can be fully
configured via CLI, for the interactive root command which can accept a
`--config` argument (not necessarily present on the disk) and for cases
where the default config file doesn't exist.

internal/config/config.go

@@ -58,8 +58,8 @@ func Init(name string, rootCmd cobra.Command, vip *viper.Viper, configChanged fu
 
 	if err := vip.ReadInConfig(); err != nil {
 		var e viper.ConfigFileNotFoundError
-		if errors.As(err, &e) {
-			log.Infof(context.Background(), "No configuration file: %v.\nWe will only use the defaults, env variables or flags.", e)
+		if errors.As(err, &e) || !vip.IsSet("config") {
+			log.Infof(context.Background(), "No configuration file: %v.\nWe will only use the defaults, env variables or flags.", err)
 		} else {
 			return fmt.Errorf("invalid configuration file: %w", err)
 		}

internal/config/config_test.go

@@ -161,9 +161,11 @@ func TestInit(t *testing.T) {
 
 		// Error cases
 		"Error on no verbose flag set before Init is call": {noVerboseFlag: true, wantErr: true},
-		"Error on invalid configuration file":              {configFileContent: "invalidcontent", want: "filecontentvalue", wantErr: true},
 		"Error on callback returning error on first call":  {errFromCallbackOn: 1, wantErr: true},
-		"Error on config flag points to unexisting path":   {withConfigFlagSet: "DELETED.yaml", wantErr: true},
+
+		// Configuration file not found
+		"Config flag points to unexisting path": {withConfigFlagSet: "DELETED.yaml", wantCallbackCalled: 1},
+		"Invalid configuration file":            {configFileContent: "invalidcontent", wantCallbackCalled: 1},
 	}
 
 	for name, tc := range tests {