http2: reduce init-time work & allocations

Updates golang/go#26775

Change-Id: Iea95ea07bb0fed42410efb4e8420d8e9a17704fe
Reviewed-on: https://go-review.googlesource.com/127664
Reviewed-by: Ian Lance Taylor <iant@golang.org>

http2/headermap.go

@@ -7,15 +7,21 @@ package http2
 import (
 	"net/http"
 	"strings"
+	"sync"
 )
 
 var (
-	commonLowerHeader = map[string]string{} // Go-Canonical-Case -> lower-case
-	commonCanonHeader = map[string]string{} // lower-case -> Go-Canonical-Case
+	commonBuildOnce   sync.Once
+	commonLowerHeader map[string]string // Go-Canonical-Case -> lower-case
+	commonCanonHeader map[string]string // lower-case -> Go-Canonical-Case
 )
 
-func init() {
-	for _, v := range []string{
+func buildCommonHeaderMapsOnce() {
+	commonBuildOnce.Do(buildCommonHeaderMaps)
+}
+
+func buildCommonHeaderMaps() {
+	common := []string{
 		"accept",
 		"accept-charset",
 		"accept-encoding",
@@ -63,14 +69,18 @@ func init() {
 		"vary",
 		"via",
 		"www-authenticate",
-	} {
+	}
+	commonLowerHeader = make(map[string]string, len(common))
+	commonCanonHeader = make(map[string]string, len(common))
+	for _, v := range common {
 		chk := http.CanonicalHeaderKey(v)
 		commonLowerHeader[chk] = v
 		commonCanonHeader[v] = chk
 	}
 }
 
 func lowerHeader(v string) string {
+	buildCommonHeaderMapsOnce()
 	if s, ok := commonLowerHeader[v]; ok {
 		return s
 	}

http2/http2.go

@@ -201,19 +201,12 @@ func validWireHeaderFieldName(v string) bool {
 	return true
 }
 
-var httpCodeStringCommon = map[int]string{} // n -> strconv.Itoa(n)
-
-func init() {
-	for i := 100; i <= 999; i++ {
-		if v := http.StatusText(i); v != "" {
-			httpCodeStringCommon[i] = strconv.Itoa(i)
-		}
-	}
-}
-
 func httpCodeString(code int) string {
-	if s, ok := httpCodeStringCommon[code]; ok {
-		return s
+	switch code {
+	case 200:
+		return "200"
+	case 404:
+		return "404"
 	}
 	return strconv.Itoa(code)
 }

http2/server.go

@@ -674,6 +674,7 @@ func (sc *serverConn) condlogf(err error, format string, args ...interface{}) {
 
 func (sc *serverConn) canonicalHeader(v string) string {
 	sc.serveG.check()
+	buildCommonHeaderMapsOnce()
 	cv, ok := commonCanonHeader[v]
 	if ok {
 		return cv