Fix missing defaults in empty configurations

config/config.go

@@ -26,6 +26,11 @@ var (
 // Load parses the YAML input s into a Config.
 func Load(s string) (*Config, error) {
 	cfg := &Config{}
+	// If the entire config body is empty the UnmarshalYAML method is
+	// never called. We thus have to set the DefaultConfig at the entry
+	// point as well.
+	*cfg = DefaultConfig
+
 	err := yaml.Unmarshal([]byte(s), cfg)
 	if err != nil {
 		return nil, err

config/config_test.go

@@ -249,3 +249,15 @@ func TestBadTargetGroup(t *testing.T) {
 		t.Errorf("Expected unmarshal error but got none.")
 	}
 }
+
+func TestEmptyConfig(t *testing.T) {
+	c, err := Load("")
+	if err != nil {
+		t.Fatalf("Unexpected error parsing empty config file: %s", err)
+	}
+	exp := DefaultConfig
+
+	if !reflect.DeepEqual(*c, exp) {
+		t.Fatalf("want %v, got %v", exp, c)
+	}
+}