Extract connection rotating dialer into a package

This will be re-used for exec auth plugin to rotate connections on
credential change.

Kubernetes-commit: 85a61ff3aa7df8d83af304ebffab780fabd028a8

util/connrotation/connrotation_test.go

@@ -0,0 +1,61 @@
+/*
+Copyright 2018 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package connrotation
+
+import (
+	"context"
+	"net"
+	"testing"
+	"time"
+)
+
+func TestCloseAll(t *testing.T) {
+	closed := make(chan struct{})
+	dialFn := func(ctx context.Context, network, address string) (net.Conn, error) {
+		return closeOnlyConn{onClose: func() { closed <- struct{}{} }}, nil
+	}
+	dialer := NewDialer(dialFn)
+
+	const numConns = 10
+
+	// Outer loop to ensure Dialer is re-usable after CloseAll.
+	for i := 0; i < 5; i++ {
+		for j := 0; j < numConns; j++ {
+			if _, err := dialer.Dial("", ""); err != nil {
+				t.Fatal(err)
+			}
+		}
+		dialer.CloseAll()
+		for j := 0; j < numConns; j++ {
+			select {
+			case <-closed:
+			case <-time.After(time.Second):
+				t.Fatalf("iteration %d: 1s after CloseAll only %d/%d connections closed", i, j, numConns)
+			}
+		}
+	}
+}
+
+type closeOnlyConn struct {
+	net.Conn
+	onClose func()
+}
+
+func (c closeOnlyConn) Close() error {
+	go c.onClose()
+	return nil
+}