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

Go's server reads the body left unread once the handler returns, but not
for a request with Connection: close: it closes the connection with the
rest unread, and the visitor, still sending, gets a reset. rproxy now
reads that rest itself once it has answered, up to the 256 KiB Go's
server reads, until the deadline already set on the connection.

Model: opus-5-5

internal/rproxy/limits_test.go

@@ -131,6 +131,67 @@ func TestADenyOutranksTheBodySizeLimit(t *testing.T) {
 	}
 }
 
+func TestAVisitorSendingConnectionCloseReadsAnAnswerBeforeTheRestOfItsBody(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	for _, tc := range []struct {
+		name   string
+		waf    http.HandlerFunc
+		status int
+	}{
+		{"over the size limit", allow, http.StatusRequestEntityTooLarge},
+		{"denied", deny, http.StatusForbidden},
+	} {
+		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: "1K"})
+
+			const size = 129 << 10
+
+			conn := dial(t, addr)
+			send(t, conn, post+"Connection: close\r\n"+
+				announcedLength(size)+
+				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 := questionBodyMaxBytes; sent < size; sent += 1 << 10 {
+				send(t, conn, strings.Repeat("x", 1<<10))
+			}
+
+			rest, err := io.ReadAll(reader)
+			if err != nil || len(rest) != 0 {
+				t.Errorf(
+					"after the answer, read %q and %v, want the end",
+					rest,
+					err,
+				)
+			}
+		})
+	}
+}
+
 func TestABodyNotAnnouncedOverItsLimitIsRefused(t *testing.T) {
 	t.Parallel()