lib/structs: factor reflection based structure manipulation into a library

lib/structs/structs.go

@@ -0,0 +1,57 @@
+// Package structs is for manipulating structures with reflection
+package structs
+
+import (
+	"reflect"
+)
+
+// SetFrom sets the public members of a from b
+//
+// a and b should be pointers to structs
+//
+// a can be a different type from b
+//
+// Only the Fields which have the same name and assignable type on a
+// and b will be set.
+//
+// This is useful for copying between almost identical structures that
+// are requently present in auto generated code for cloud storage
+// interfaces.
+func SetFrom(a, b interface{}) {
+	ta := reflect.TypeOf(a).Elem()
+	tb := reflect.TypeOf(b).Elem()
+	va := reflect.ValueOf(a).Elem()
+	vb := reflect.ValueOf(b).Elem()
+	for i := 0; i < tb.NumField(); i++ {
+		bField := vb.Field(i)
+		tbField := tb.Field(i)
+		name := tbField.Name
+		aField := va.FieldByName(name)
+		taField, found := ta.FieldByName(name)
+		if found && aField.IsValid() && bField.IsValid() && aField.CanSet() && tbField.Type.AssignableTo(taField.Type) {
+			aField.Set(bField)
+		}
+	}
+}
+
+// SetDefaults for a from b
+//
+// a and b should be pointers to the same kind of struct
+//
+// This copies the public members only from b to a.  This is useful if
+// you can't just use a struct copy because it contains a private
+// mutex, eg as http.Transport.
+func SetDefaults(a, b interface{}) {
+	pt := reflect.TypeOf(a)
+	t := pt.Elem()
+	va := reflect.ValueOf(a).Elem()
+	vb := reflect.ValueOf(b).Elem()
+	for i := 0; i < t.NumField(); i++ {
+		aField := va.Field(i)
+		// Set a from b if it is public
+		if aField.CanSet() {
+			bField := vb.Field(i)
+			aField.Set(bField)
+		}
+	}
+}