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

This reverts CL 469095.

The newly added TestDisableContentLength is failing on all longtest
builders.

Change-Id: Id307df61c7bf80691d9c276e8d200eebf6d4a59c
Reviewed-on: https://go-review.googlesource.com/c/go/+/495017
Auto-Submit: Austin Clements <[email protected]>
Run-TryBot: Austin Clements <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/net/http/serve_test.go

@@ -6825,37 +6825,3 @@ 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

@@ -1320,7 +1320,7 @@ 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.has("Content-Length") && (!isHEAD || len(p) > 0) {
+	if w.handlerDone.Load() && !trailers && !hasTE && bodyAllowedForStatus(w.status) && header.get("Content-Length") == "" && (!isHEAD || len(p) > 0) {
 		w.contentLength = int64(len(p))
 		setHeader.contentLength = strconv.AppendInt(cw.res.clenBuf[:0], int64(len(p)), 10)
 	}