Fix altsrc nil source flag

altsrc/default_input_source.go

@@ -0,0 +1,6 @@
+package altsrc
+
+// defaultInputSource creates a default InputSourceContext.
+func defaultInputSource() (InputSourceContext, error) {
+	return &MapInputSource{file: "", valueMap: map[interface{}]interface{}{}}, nil
+}

altsrc/json_source_context.go

@@ -17,7 +17,11 @@ import (
 // by the given flag.
 func NewJSONSourceFromFlagFunc(flag string) func(c *cli.Context) (InputSourceContext, error) {
 	return func(context *cli.Context) (InputSourceContext, error) {
-		return NewJSONSourceFromFile(context.String(flag))
+		if context.IsSet(flag) {
+			return NewJSONSourceFromFile(context.String(flag))
+		} else {
+			return defaultInputSource()
+		}
 	}
 }
 

altsrc/toml_file_loader.go

@@ -87,8 +87,12 @@ func NewTomlSourceFromFile(file string) (InputSourceContext, error) {
 // NewTomlSourceFromFlagFunc creates a new TOML InputSourceContext from a provided flag name and source context.
 func NewTomlSourceFromFlagFunc(flagFileName string) func(context *cli.Context) (InputSourceContext, error) {
 	return func(context *cli.Context) (InputSourceContext, error) {
-		filePath := context.String(flagFileName)
-		return NewTomlSourceFromFile(filePath)
+		if context.IsSet(flagFileName) {
+			filePath := context.String(flagFileName)
+			return NewTomlSourceFromFile(filePath)
+		} else {
+			return defaultInputSource()
+		}
 	}
 }
 

altsrc/yaml_file_loader.go

@@ -33,8 +33,12 @@ func NewYamlSourceFromFile(file string) (InputSourceContext, error) {
 // NewYamlSourceFromFlagFunc creates a new Yaml InputSourceContext from a provided flag name and source context.
 func NewYamlSourceFromFlagFunc(flagFileName string) func(context *cli.Context) (InputSourceContext, error) {
 	return func(context *cli.Context) (InputSourceContext, error) {
-		filePath := context.String(flagFileName)
-		return NewYamlSourceFromFile(filePath)
+		if context.IsSet(flagFileName) {
+			filePath := context.String(flagFileName)
+			return NewYamlSourceFromFile(filePath)
+		} else {
+			return defaultInputSource()
+		}
 	}
 }