Add a reusable, bare-bones AsyncDnsServer

Add bin/tests/system/ans.py, a bare-bones DNS server that can be used in
system tests instead of full-blown named instances when a server is only
required to return zone-based data.  Where applicable, this reduces load
on the test host and the amount of generated logs.

diff --git a/bin/tests/system/ans.py b/bin/tests/system/ans.py
new file mode 100644 (file)
index 0000000..9dbdee1
--- /dev/null
+++ b/bin/tests/system/ans.py
@@ -0,0 +1,48 @@
+"""
+Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+
+SPDX-License-Identifier: MPL-2.0
+
+This Source Code Form is subject to the terms of the Mozilla Public
+License, v. 2.0.  If a copy of the MPL was not distributed with this
+file, you can obtain one at https://mozilla.org/MPL/2.0/.
+
+See the COPYRIGHT file distributed with this work for additional
+information regarding copyright ownership.
+"""
+
+"""
+This is a bare-bones DNS server that only serves data from zone files.  It is
+meant to be used as a replacement for full-blown named instances in system
+tests when a given server is only required to return zone-based data.
+
+!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+ BEWARE!  THIS SERVER DOES NOT NECESSARILY RETURN PROTOCOL-COMPLIANT ANSWERS!
+!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+See AsyncDnsServer._abort_if_*() methods in isctests/asyncserver.py for more
+details.  Use a regular named instance for anything non-trivial.
+
+!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+ DO NOT ADD CUSTOM LOGIC TO THIS FILE.  IT IS ONLY MEANT TO BE SYMLINKED INTO
+ansX/ SUBDIRECTORIES IN SYSTEM TESTS TO REDUCE THE AMOUNT OF BOILERPLATE CODE.
+!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+If you need to customize server behavior, implement it in a dedicated ans.py
+server in the system test at hand.  If an extension you are working on can be
+useful in other system tests, please consider opening a merge request extending
+isctest/asyncserver.py.
+"""
+
+from isctest.asyncserver import (
+    AsyncDnsServer,
+)
+
+
+def main() -> None:
+    server = AsyncDnsServer()
+    server.run()
+
+
+if __name__ == "__main__":
+    main()