net/http/internal/http2: prevent alloc when writing status code for responses

Previously, writing responses with non-200 and non-404 status code
requires an allocation. Now that CL 762040 prevents header names and
values from escaping, we can modify writeFrame to not allocate status
codes on the heap.

Change-Id: I230bed1b83627c1fb389c0507106d8e16a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/762140
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: nicholashusin

src/net/http/internal/http2/http2.go

@@ -21,7 +21,6 @@ import (
 	"net"
 	"os"
 	"slices"
-	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -215,16 +214,6 @@ func validWireHeaderFieldName(v string) bool {
 	return true
 }
 
-func httpCodeString(code int) string {
-	switch code {
-	case 200:
-		return "200"
-	case 404:
-		return "404"
-	}
-	return strconv.Itoa(code)
-}
-
 // A closeWaiter is like a sync.WaitGroup but only goes 1 to 0 (open to closed).
 type closeWaiter chan struct{}
 

src/net/http/internal/http2/write.go

@@ -10,6 +10,8 @@ import (
 	"log"
 	"net/http/internal/httpcommon"
 	"net/url"
+	"strconv"
+	"strings"
 
 	"golang.org/x/net/http/httpguts"
 	"golang.org/x/net/http2/hpack"
@@ -200,7 +202,7 @@ type writeResHeaders struct {
 
 func encKV(enc *hpack.Encoder, k, v string) {
 	if VerboseLogs {
-		log.Printf("http2: server encoding header %q = %q", k, v)
+		log.Printf("http2: server encoding header %q = %q", strings.Clone(k), strings.Clone(v))
 	}
 	enc.WriteField(hpack.HeaderField{Name: k, Value: v})
 }
@@ -221,7 +223,8 @@ func (w *writeResHeaders) writeFrame(ctx writeContext) error {
 	buf.Reset()
 
 	if w.httpResCode != 0 {
-		encKV(enc, ":status", httpCodeString(w.httpResCode))
+		codeBuf := strconv.AppendInt(make([]byte, 0, 3), int64(w.httpResCode), 10)
+		encKV(enc, ":status", string(codeBuf))
 	}
 
 	encodeHeaders(enc, w.h, w.trailers)