mcfg: add ParamJSON and fix required params error message

mcfg/mcfg.go

@@ -161,6 +161,14 @@ func (c *Cfg) populateParams(src Source) error {
 		Path []string `json:",omitempty"`
 		Name string
 	}
+	paramFullName := func(p param) string {
+		if len(p.Path) == 0 {
+			return p.Name
+		}
+		slice := append(make([]string, 0, len(p.Path)+1), p.Name)
+		slice = append(slice, p.Path...)
+		return strings.Join(slice, "-")
+	}
 
 	pvM := map[string]ParamValue{}
 	for _, pv := range pvs {
@@ -193,7 +201,7 @@ func (c *Cfg) populateParams(src Source) error {
 		if err != nil {
 			return err
 		} else if _, ok := pvM[string(keyB)]; !ok {
-			return fmt.Errorf("param %s is required but wasn't populated by any configuration source", keyB)
+			return fmt.Errorf("param %s is required but wasn't populated by any configuration source", paramFullName(reqP))
 		}
 	}
 

mcfg/param.go

@@ -113,3 +113,16 @@ func (c *Cfg) ParamDuration(name string, defaultVal mtime.Duration, usage string
 	c.ParamAdd(Param{Name: name, Usage: usage, IsString: true, Into: &d})
 	return &d
 }
+
+// ParamJSON reads the parameter value as a JSON value and unmarshals it into
+// the given interface{} (which should be a pointer). The receiver (into) is
+// also used to determine the default value.
+func (c *Cfg) ParamJSON(name string, into interface{}, usage string) {
+	c.ParamAdd(Param{Name: name, Usage: usage, Into: into})
+}
+
+// ParamRequiredJSON reads the parameter value as a JSON value and unmarshals it
+// into the given interface{} (which should be a pointer).
+func (c *Cfg) ParamRequiredJSON(name string, into interface{}, usage string) {
+	c.ParamAdd(Param{Name: name, Required: true, Usage: usage, Into: into})
+}