Update federation target to etcd/clientv3 (#6119)

With CoreDNS now supporting etcdv3 as the DNS backend, we
can update our federation target to etcdv3. Users will now be
able to use etcdv3 server as the federation backbone.

Minio will update bucket data to etcdv3 and CoreDNS can pick
that data up and serve it as bucket style DNS path.

vendor/github.com/golang/protobuf/proto/clone.go

@@ -35,22 +35,39 @@
 package proto
 
 import (
+	"fmt"
 	"log"
 	"reflect"
 	"strings"
 )
 
 // Clone returns a deep copy of a protocol buffer.
-func Clone(pb Message) Message {
-	in := reflect.ValueOf(pb)
+func Clone(src Message) Message {
+	in := reflect.ValueOf(src)
 	if in.IsNil() {
-		return pb
+		return src
 	}
-
 	out := reflect.New(in.Type().Elem())
-	// out is empty so a merge is a deep copy.
-	mergeStruct(out.Elem(), in.Elem())
-	return out.Interface().(Message)
+	dst := out.Interface().(Message)
+	Merge(dst, src)
+	return dst
+}
+
+// Merger is the interface representing objects that can merge messages of the same type.
+type Merger interface {
+	// Merge merges src into this message.
+	// Required and optional fields that are set in src will be set to that value in dst.
+	// Elements of repeated fields will be appended.
+	//
+	// Merge may panic if called with a different argument type than the receiver.
+	Merge(src Message)
+}
+
+// generatedMerger is the custom merge method that generated protos will have.
+// We must add this method since a generate Merge method will conflict with
+// many existing protos that have a Merge data field already defined.
+type generatedMerger interface {
+	XXX_Merge(src Message)
 }
 
 // Merge merges src into dst.
@@ -58,17 +75,24 @@ func Clone(pb Message) Message {
 // Elements of repeated fields will be appended.
 // Merge panics if src and dst are not the same type, or if dst is nil.
 func Merge(dst, src Message) {
+	if m, ok := dst.(Merger); ok {
+		m.Merge(src)
+		return
+	}
+
 	in := reflect.ValueOf(src)
 	out := reflect.ValueOf(dst)
 	if out.IsNil() {
 		panic("proto: nil destination")
 	}
 	if in.Type() != out.Type() {
-		// Explicit test prior to mergeStruct so that mistyped nils will fail
-		panic("proto: type mismatch")
+		panic(fmt.Sprintf("proto.Merge(%T, %T) type mismatch", dst, src))
 	}
 	if in.IsNil() {
-		// Merging nil into non-nil is a quiet no-op
+		return // Merge from nil src is a noop
+	}
+	if m, ok := dst.(generatedMerger); ok {
+		m.XXX_Merge(src)
 		return
 	}
 	mergeStruct(out.Elem(), in.Elem())
@@ -84,7 +108,7 @@ func mergeStruct(out, in reflect.Value) {
 		mergeAny(out.Field(i), in.Field(i), false, sprop.Prop[i])
 	}
 
-	if emIn, ok := extendable(in.Addr().Interface()); ok {
+	if emIn, err := extendable(in.Addr().Interface()); err == nil {
 		emOut, _ := extendable(out.Addr().Interface())
 		mIn, muIn := emIn.extensionsRead()
 		if mIn != nil {