http: fix sniffing bug causing short writes

R=rsc
CC=golang-dev
https://golang.org/cl/5442045

src/pkg/net/http/server.go

@@ -467,7 +467,7 @@ func (w *response) Write(data []byte) (n int, err error) {
 		// determine the content type.  Accumulate the
 		// initial writes in w.conn.body.
 		// Cap m so that append won't allocate.
-		m := cap(w.conn.body) - len(w.conn.body)
+		m = cap(w.conn.body) - len(w.conn.body)
 		if m > len(data) {
 			m = len(data)
 		}

src/pkg/net/http/sniff_test.go

@@ -6,12 +6,14 @@ package http_test
 
 import (
 	"bytes"
+	"fmt"
 	"io"
 	"io/ioutil"
 	"log"
 	. "net/http"
 	"net/http/httptest"
 	"strconv"
+	"strings"
 	"testing"
 )
 
@@ -112,3 +114,24 @@ func TestContentTypeWithCopy(t *testing.T) {
 	}
 	resp.Body.Close()
 }
+
+func TestSniffWriteSize(t *testing.T) {
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		size, _ := strconv.Atoi(r.FormValue("size"))
+		written, err := io.WriteString(w, strings.Repeat("a", size))
+		if err != nil {
+			t.Errorf("write of %d bytes: %v", size, err)
+			return
+		}
+		if written != size {
+			t.Errorf("write of %d bytes wrote %d bytes", size, written)
+		}
+	}))
+	defer ts.Close()
+	for _, size := range []int{0, 1, 200, 600, 999, 1000, 1023, 1024, 512 << 10, 1 << 20} {
+		_, err := Get(fmt.Sprintf("%s/?size=%d", ts.URL, size))
+		if err != nil {
+			t.Fatalf("size %d: %v", size, err)
+		}
+	}
+}