net/http: do not force the Content-Length header if nilled

According to the ResponseWriter documentation:

  To suppress automatic response headers (such as "Date"), set
  their value to nil.

In some cases, this documentation is incorrect: chunkWriter writes
a Content-Length header even if the value was set to nil. Meaning
there is no way to suppress this header.

This patch replaces the empty string comparison with a call to
`header.has` which takes into account nil values as expected.
This is similar to the way we handle the "Date" header.

Change-Id: Ie10d54ab0bb7d41270bc944ff867e035fe2bd0c5
GitHub-Last-Rev: e0616dd463
GitHub-Pull-Request: golang/go#58578
Reviewed-on: https://go-review.googlesource.com/c/go/+/469095
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Damien Neil <dneil@google.com>
Run-TryBot: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>

src/net/http/serve_test.go

@@ -6756,3 +6756,37 @@ func testHeadBody(t *testing.T, mode testMode, chunked bool, method string) {
 		}
 	}
 }
+
+// TestContentLengthResponseCanBeNilled verifies that the Content-Length is set by default
+// or disabled when the header is set to nil.
+func TestDisableContentLength(t *testing.T) { run(t, testDisableContentLength) }
+func testDisableContentLength(t *testing.T, mode testMode) {
+	if mode == http2Mode {
+		t.Skip("skipping until h2_bundle.go is updated; see https://go-review.googlesource.com/c/net/+/471535")
+	}
+
+	noCL := newClientServerTest(t, mode, HandlerFunc(func(w ResponseWriter, r *Request) {
+		w.Header()["Content-Length"] = nil // disable the default Content-Length response
+		fmt.Fprintf(w, "OK")
+	}))
+
+	res, err := noCL.c.Get(noCL.ts.URL)
+	if err != nil {
+		t.Error(err)
+	}
+	if got, haveCL := res.Header["Content-Length"]; haveCL {
+		t.Errorf("Unexpected Content-Length: %q", got)
+	}
+
+	withCL := newClientServerTest(t, mode, HandlerFunc(func(w ResponseWriter, r *Request) {
+		fmt.Fprintf(w, "OK")
+	}))
+
+	res, err = withCL.c.Get(withCL.ts.URL)
+	if err != nil {
+		t.Error(err)
+	}
+	if got := res.Header.Get("Content-Length"); got != "2" {
+		t.Errorf("Content-Length: %q; want 2", got)
+	}
+}

src/net/http/server.go

@@ -1317,7 +1317,7 @@ func (cw *chunkWriter) writeHeader(p []byte) {
 	// send a Content-Length header.
 	// Further, we don't send an automatic Content-Length if they
 	// set a Transfer-Encoding, because they're generally incompatible.
-	if w.handlerDone.Load() && !trailers && !hasTE && bodyAllowedForStatus(w.status) && header.get("Content-Length") == "" && (!isHEAD || len(p) > 0) {
+	if w.handlerDone.Load() && !trailers && !hasTE && bodyAllowedForStatus(w.status) && !header.has("Content-Length") && (!isHEAD || len(p) > 0) {
 		w.contentLength = int64(len(p))
 		setHeader.contentLength = strconv.AppendInt(cw.res.clenBuf[:0], int64(len(p)), 10)
 	}