compute/metadata: workaround for App Engine SDK's broken build tags i…

…n dev

See comments from Péter Szilágyi and David Symonds at
https://code-review.googlesource.com/#/c/1670/

This new arrangement will work, even with the App Engine SDK's broken
behavior where it uploads a set of files thinking it's Go 1.2, and
then later compiles those files thinking it's Go 1.4.

Change-Id: Ieb43ebb0925354539f63075b94a63d8e8dede58c

compute/metadata/go13.go

@@ -27,9 +27,11 @@ import (
 //
 // TODO(bradfitz,jbd,adg): remove this once App Engine supports Go
 // 1.3+.
-func metaClientDialer() *net.Dialer {
-	return &net.Dialer{
-		Timeout:   750 * time.Millisecond,
-		KeepAlive: 30 * time.Second,
+func init() {
+	go13Dialer = func() *net.Dialer {
+		return &net.Dialer{
+			Timeout:   750 * time.Millisecond,
+			KeepAlive: 30 * time.Second,
+		}
 	}
 }

compute/metadata/metadata.go

@@ -23,6 +23,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
+	"net"
 	"net/http"
 	"strings"
 	"sync"
@@ -47,12 +48,31 @@ var (
 var metaClient = &http.Client{
 	Transport: &internal.Transport{
 		Base: &http.Transport{
-			Dial: metaClientDialer().Dial,
+			Dial: dialer().Dial,
 			ResponseHeaderTimeout: 750 * time.Millisecond,
 		},
 	},
 }
 
+// go13Dialer is nil until we're using Go 1.3+.
+// This is a workaround for https://github.com/golang/oauth2/issues/70, where
+// net.Dialer.KeepAlive is unavailable on Go 1.2 (which App Engine as of
+// Jan 2015 still runs).
+//
+// TODO(bradfitz,jbd,adg,dsymonds): remove this once App Engine supports Go
+// 1.3+ and go-app-builder also supports 1.3+, or when Go 1.2 is no longer an
+// option on App Engine.
+var go13Dialer func() *net.Dialer
+
+func dialer() *net.Dialer {
+	if fn := go13Dialer; fn != nil {
+		return fn()
+	}
+	return &net.Dialer{
+		Timeout: 750 * time.Millisecond,
+	}
+}
+
 // Get returns a value from the metadata service.
 // The suffix is appended to "http://metadata/computeMetadata/v1/".
 func Get(suffix string) (string, error) {