net: add lookup IP hook for testing

This change adds testHookLookIP to enable to inject DNS name to IP
address mappings for Happ{y,yish,ier} Eyeballs dial testing.

Change-Id: I8ac04a594e1e2bd77909528df0552889914a7790
Reviewed-on: https://go-review.googlesource.com/8399Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/hook.go

+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package net
+
+var testHookLookupIP = func(fn func(string) ([]IPAddr, error), host string) ([]IPAddr, error) { return fn(host) }

src/net/lookup.go

@@ -46,7 +46,7 @@ var lookupGroup singleflight
 // is always owned by the caller.
 func lookupIPMerge(host string) (addrs []IPAddr, err error) {
 	addrsi, err, shared := lookupGroup.Do(host, func() (interface{}, error) {
-		return lookupIP(host)
+		return testHookLookupIP(lookupIP, host)
 	})
 	return lookupIPReturn(addrsi, err, shared)
 }
@@ -84,7 +84,7 @@ func lookupIPDeadline(host string, deadline time.Time) (addrs []IPAddr, err erro
 	defer t.Stop()
 
 	ch := lookupGroup.DoChan(host, func() (interface{}, error) {
-		return lookupIP(host)
+		return testHookLookupIP(lookupIP, host)
 	})
 
 	select {