2 files changed, 35 insertions, 1 deletions
diff --git a/src/net/http/serve_test.go b/src/net/http/serve_test.go
index 819152658b..a21518b563 100644
--- a/src/net/http/serve_test.go
+++ b/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)
+	}
+}
diff --git a/src/net/http/server.go b/src/net/http/server.go
index 9bd381ff48..e82669a180 100644
--- a/src/net/http/server.go
+++ b/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)
 	}