resolv: Add tst-resolv-byaddr for testing reverse lookup

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

resolv/tst-resolv-maybe_insert_sig.h

@@ -0,0 +1,32 @@
+/* Code snippet for optionally inserting ignored SIG records in resolver tests.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* Set to true for an alternative pass that inserts (ignored) SIG
+   records.  This does not alter the response, so this property is not
+   encoded in the QNAME.  The variable needs to be volatile because
+   leaf attributes tell GCC that the response function is not
+   called.  */
+static volatile bool insert_sig;
+
+static void
+maybe_insert_sig (struct resolv_response_builder *b, const char *owner)
+{
+  resolv_response_open_record (b, owner, C_IN, T_SIG, 60);
+  resolv_response_add_data (b, "", 1);
+  resolv_response_close_record (b);
+}