net/http: remove Content-Length header in http.Error

Error replies to a request with an error message and HTTP code.
Delete any preexisting Content-Length header before writing the header;
if a Content-Length is present, it's probably for content that the
caller has given up on writing.

For #50905

Change-Id: Ia3d4ca008be46fa5d41afadf29ca5cacb1c47660
Reviewed-on: https://go-review.googlesource.com/c/go/+/554216
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

src/net/http/serve_test.go

@@ -7052,3 +7052,24 @@ func testDisableContentLength(t *testing.T, mode testMode) {
 		t.Fatal(err)
 	}
 }
+
+func TestErrorContentLength(t *testing.T) { run(t, testErrorContentLength) }
+func testErrorContentLength(t *testing.T, mode testMode) {
+	const errorBody = "an error occurred"
+	cst := newClientServerTest(t, mode, HandlerFunc(func(w ResponseWriter, r *Request) {
+		w.Header().Set("Content-Length", "1000")
+		Error(w, errorBody, 400)
+	}))
+	res, err := cst.c.Get(cst.ts.URL)
+	if err != nil {
+		t.Fatalf("Get(%q) = %v", cst.ts.URL, err)
+	}
+	defer res.Body.Close()
+	body, err := io.ReadAll(res.Body)
+	if err != nil {
+		t.Fatalf("io.ReadAll(res.Body) = %v", err)
+	}
+	if string(body) != errorBody+"\n" {
+		t.Fatalf("read body: %q, want %q", string(body), errorBody)
+	}
+}

src/net/http/server.go

@@ -2173,6 +2173,7 @@ func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
 // writes are done to w.
 // The error message should be plain text.
 func Error(w ResponseWriter, error string, code int) {
+	w.Header().Del("Content-Length")
 	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
 	w.Header().Set("X-Content-Type-Options", "nosniff")
 	w.WriteHeader(code)