net/http: use atomic.Value for Transport's alternate protocol map

Fix an old TODO and use atomic.Value for holding the Transport's
alternate protocol map. It is very frequently accessed and almost
never set or updated.

Change-Id: Ic5a71c504bdac76678114c6390d1fc0673e07aa9
Reviewed-on: https://go-review.googlesource.com/29967
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/transport.go

@@ -25,6 +25,7 @@ import (
 	"os"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"golang_org/x/net/lex/httplex"
@@ -79,8 +80,8 @@ type Transport struct {
 	reqMu       sync.Mutex
 	reqCanceler map[*Request]func(error)
 
-	altMu    sync.RWMutex
-	altProto map[string]RoundTripper // nil or map of URI scheme => RoundTripper
+	altMu    sync.Mutex   // guards changing altProto only
+	altProto atomic.Value // of nil or map[string]RoundTripper, key is URI scheme
 
 	// Proxy specifies a function to return a proxy for a given
 	// Request. If the function returns a non-nil error, the
@@ -331,11 +332,9 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 			}
 		}
 	}
-	// TODO(bradfitz): switch to atomic.Value for this map instead of RWMutex
-	t.altMu.RLock()
-	altRT := t.altProto[scheme]
-	t.altMu.RUnlock()
-	if altRT != nil {
+
+	altProto, _ := t.altProto.Load().(map[string]RoundTripper)
+	if altRT := altProto[scheme]; altRT != nil {
 		if resp, err := altRT.RoundTrip(req); err != ErrSkipAltProtocol {
 			return resp, err
 		}
@@ -460,13 +459,16 @@ var ErrSkipAltProtocol = errors.New("net/http: skip alternate protocol")
 func (t *Transport) RegisterProtocol(scheme string, rt RoundTripper) {
 	t.altMu.Lock()
 	defer t.altMu.Unlock()
-	if t.altProto == nil {
-		t.altProto = make(map[string]RoundTripper)
-	}
-	if _, exists := t.altProto[scheme]; exists {
+	oldMap, _ := t.altProto.Load().(map[string]RoundTripper)
+	if _, exists := oldMap[scheme]; exists {
 		panic("protocol " + scheme + " already registered")
 	}
-	t.altProto[scheme] = rt
+	newMap := make(map[string]RoundTripper)
+	for k, v := range oldMap {
+		newMap[k] = v
+	}
+	newMap[scheme] = rt
+	t.altProto.Store(newMap)
 }
 
 // CloseIdleConnections closes any connections which were previously