net/http: ignore ranges if the content is empty in serveContent

Fixes #54794

Change-Id: I6f2b7b86b82ea27b9d53cf989daa21cb8ace13da
Reviewed-on: https://go-review.googlesource.com/c/go/+/427195
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/net/http/fs.go

@@ -254,19 +254,34 @@ func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time,
 		Error(w, err.Error(), StatusInternalServerError)
 		return
 	}
+	if size < 0 {
+		// Should never happen but just to be sure
+		Error(w, "negative content size computed", StatusInternalServerError)
+		return
+	}
 
 	// handle Content-Range header.
 	sendSize := size
 	var sendContent io.Reader = content
-	if size >= 0 {
 	ranges, err := parseRange(rangeReq, size)
-		if err != nil {
-			if err == errNoOverlap {
-				w.Header().Set("Content-Range", fmt.Sprintf("bytes */%d", size))
+	switch err {
+	case nil:
+	case errNoOverlap:
+		if size == 0 {
+			// Some clients add a Range header to all requests to
+			// limit the size of the response. If the file is empty,
+			// ignore the range header and respond with a 200 rather
+			// than a 416.
+			ranges = nil
+			break
 		}
+		w.Header().Set("Content-Range", fmt.Sprintf("bytes */%d", size))
+		fallthrough
+	default:
 		Error(w, err.Error(), StatusRequestedRangeNotSatisfiable)
 		return
 	}
+
 	if sumRangesSize(ranges) > size {
 		// The total number of bytes in all the ranges
 		// is larger than the size of the file by
@@ -329,7 +344,6 @@ func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time,
 	if w.Header().Get("Content-Encoding") == "" {
 		w.Header().Set("Content-Length", strconv.FormatInt(sendSize, 10))
 	}
-	}
 
 	w.WriteHeader(code)
 

src/net/http/fs_test.go

@@ -218,6 +218,27 @@ func TestServeFileDirPanicEmptyPath(t *testing.T) {
 	}
 }
 
+// Tests that ranges are ignored with serving empty content. (Issue 54794)
+func TestServeContentWithEmptyContentIgnoreRanges(t *testing.T) {
+	for _, r := range []string{
+		"bytes=0-128",
+		"bytes=1-",
+	} {
+		rec := httptest.NewRecorder()
+		req := httptest.NewRequest("GET", "/", nil)
+		req.Header.Set("Range", r)
+		ServeContent(rec, req, "nothing", time.Now(), bytes.NewReader(nil))
+		res := rec.Result()
+		if res.StatusCode != 200 {
+			t.Errorf("code = %v; want 200", res.Status)
+		}
+		bodyLen := rec.Body.Len()
+		if bodyLen != 0 {
+			t.Errorf("body.Len() = %v; want 0", res.Status)
+		}
+	}
+}
+
 var fsRedirectTestData = []struct {
 	original, redirect string
 }{