sync: support Pool under race detector

Fixes #7203.

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/53020044

src/pkg/sync/pool.go

@@ -72,6 +72,12 @@ func init() {
 
 // Put adds x to the pool.
 func (p *Pool) Put(x interface{}) {
+	if raceenabled {
+		// Under race detector the Pool degenerates into no-op.
+		// It's conforming, simple and does not introduce excessive
+		// happens-before edges between unrelated goroutines.
+		return
+	}
 	if x == nil {
 		return
 	}
@@ -95,6 +101,12 @@ func (p *Pool) Put(x interface{}) {
 // If Get would otherwise return nil and p.New is non-nil, Get returns
 // the result of calling p.New.
 func (p *Pool) Get() interface{} {
+	if raceenabled {
+		if p.New != nil {
+			return p.New()
+		}
+		return nil
+	}
 	l := p.pin()
 	t := l.tail
 	if t > 0 {

src/pkg/sync/pool_test.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Pool is no-op under race detector, so all these tests do not work.
+// +build !race
+
 package sync_test
 
 import (