rproxy: read the rest of a Connection: close visitor's body after an early answer (closes #373)

Go's server reads up to 256 KiB of a body left unread once the handler
returns, but not a body with a declared length from a visitor that sent
Connection: close. It closes that connection with the rest unread, and
the visitor, still sending, gets a reset. rproxy now reads that rest
itself, until the deadline already set on the connection, through a
MaxBytesReader made with Go's own ResponseWriter. With more than 256 KiB
left, that has Go's server tell the visitor it sends no more and wait
500 ms before it closes, as for any other visitor.

Model: opus-5-5

internal/rproxy/limits_test.go

@@ -136,6 +136,73 @@ func TestADenyOutranksTheBodySizeLimit(t *testing.T) {
 	}
 }
 
+func TestAVisitorSendingConnectionCloseReadsAnAnswerBeforeTheRestOfItsBody(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	for _, tc := range []struct {
+		name   string
+		waf    http.HandlerFunc
+		rest   int
+		status int
+	}{
+		{"over the size limit", allow, 65 << 10, http.StatusRequestEntityTooLarge},
+		{"denied", deny, 65 << 10, http.StatusForbidden},
+		// rproxy reads 256 KiB of this rest. What it leaves unread must fit in the
+		// connection's buffers. Otherwise the visitor is still sending when the
+		// connection closes, and gets a reset.
+		{
+			"with more than 256 KiB left", allow, 272 << 10,
+			http.StatusRequestEntityTooLarge,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			standIn := startWAF(t, tc.waf)
+			app := startApp(t, func(http.ResponseWriter, *http.Request) {
+				t.Error("the app was asked")
+			})
+			addr, _ := startRProxy(t, standIn.URL, app.URL,
+				map[string]string{maxRequestSize: "128K"})
+
+			conn := dial(t, addr)
+			send(t, conn, post+"Connection: close\r\n"+
+				announcedLength(questionBodyMaxBytes+tc.rest)+
+				strings.Repeat("x", questionBodyMaxBytes))
+
+			reader := bufio.NewReader(conn)
+
+			res, err := http.ReadResponse(reader, nil)
+			if err != nil {
+				t.Fatalf("read the answer: %v", err)
+			}
+
+			_, _ = io.Copy(io.Discard, res.Body)
+			_ = res.Body.Close()
+
+			if res.StatusCode != tc.status {
+				t.Errorf("status %d, want %d", res.StatusCode, tc.status)
+			}
+
+			// The rest goes in small parts, so that a reset fails a later one.
+			for sent := 0; sent < tc.rest; sent += 1 << 10 {
+				send(t, conn, strings.Repeat("x", 1<<10))
+			}
+
+			got, err := io.ReadAll(reader)
+			if err != nil || len(got) != 0 {
+				t.Errorf(
+					"after the answer, read %q and %v, want the end",
+					got,
+					err,
+				)
+			}
+		})
+	}
+}
+
 func TestABodyNotAnnouncedOverItsLimitIsRefused(t *testing.T) {
 	t.Parallel()