net/http: set/override Content-Length for encoded range requests

Currently, http.ServeContent returns invalid Content-Length header if:

* Request is a range request.
* Content is encoded (e.g., gzip compressed).
* Content-Length of the encoded content has been set before calling
  http.ServeContent, as suggested in https://github.com/golang/go/issues/19420.

Example:

	w.Header().Set("Content-Type", "application/json")
	w.Header().Set("Content-Length", strconv.Itoa(len(compressedJsonBody)))
	w.Header().Set("Content-Encoding", "gzip")
	w.Header().Set("Etag", etag)
	http.ServeContent(
		w, req, "", time.Time{},
		bytes.NewReader(compressedJsonBody),
	)

The issue is that http.ServeContent currently sees Content-Length as
something optional when Content-Encoding is set, but that is a problem
with range request which can send a payload of different size. So this
reverts https://go.dev/cl/4538111 and makes Content-Length be set
always to the number of bytes which will actually be send (both for
range and non-range requests).

Without this fix, this is an example response:

	HTTP/1.1 206 Partial Content
	Accept-Ranges: bytes
	Content-Encoding: gzip
	Content-Length: 351
	Content-Range: bytes 100-350/351
	Content-Type: application/json; charset=UTF-8
	Etag: "amCTP_vgT5PQt5OsAEI7NFJ6Hx1UfEpR5nIaYEInfOA"
	Date: Sat, 29 Jan 2022 14:42:15 GMT

As you see, Content-Length is invalid and should be 251.

Change-Id: I4d2ea3a8489a115f92ef1f7e98250d555b47a94e
GitHub-Last-Rev: 3aff9126f5
GitHub-Pull-Request: golang/go#50904
Reviewed-on: https://go-review.googlesource.com/c/go/+/381956
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: t hepudds <thepudds1460@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/net/http/fs.go

@@ -343,9 +343,7 @@ func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time,
 	}
 
 	w.Header().Set("Accept-Ranges", "bytes")
-	if w.Header().Get("Content-Encoding") == "" {
+	w.Header().Set("Content-Length", strconv.FormatInt(sendSize, 10))
-		w.Header().Set("Content-Length", strconv.FormatInt(sendSize, 10))
-	}
 
 	w.WriteHeader(code)
 

src/net/http/fs_test.go

@@ -571,7 +571,7 @@ func testServeDirWithoutTrailingSlash(t *testing.T, mode testMode) {
 	}
 }
 
-// Tests that ServeFile doesn't add a Content-Length if a Content-Encoding is
+// Tests that ServeFile adds a Content-Length even if a Content-Encoding is
 // specified.
 func TestServeFileWithContentEncoding(t *testing.T) { run(t, testServeFileWithContentEncoding) }
 func testServeFileWithContentEncoding(t *testing.T, mode testMode) {
@@ -593,7 +593,7 @@ func testServeFileWithContentEncoding(t *testing.T, mode testMode) {
 		t.Fatal(err)
 	}
 	resp.Body.Close()
-	if g, e := resp.ContentLength, int64(-1); g != e {
+	if g, e := resp.ContentLength, int64(11); g != e {
 		t.Errorf("Content-Length mismatch: got %d, want %d", g, e)
 	}
 }