Use isctest.asyncserver in the "tsig" test

Replace the custom DNS server used in the "tsig" system test with
new code based on the isctest.asyncserver module.

bin/tests/system/tsig/ans2/ans.pl

@@ -1,52 +0,0 @@
-# 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.
-
-#
-# An adhoc server that returns a TC=1 response with the final byte
-# removed to generate UNEXPECTEDEND form dns_message_parse.
-#
-
-use IO::File;
-use IO::Socket;
-
-my $localport = int($ENV{'PORT'});
-if (!$localport) { $localport = 5300; }
-printf "localport %u\n", $localport;
-
-my $sock = IO::Socket::INET->new(LocalAddr => "10.53.0.2",
-   LocalPort => $localport, Proto => "udp") or die "$!";
-
-my $pidf = new IO::File "ans.pid", "w" or die "cannot open pid file: $!";
-print $pidf "$$\n" or die "cannot write pid file: $!";
-$pidf->close or die "cannot close pid file: $!";
-sub rmpid { unlink "ans.pid"; exit 1; };
-
-$SIG{INT} = \&rmpid;
-$SIG{TERM} = \&rmpid;
-
-sub arraystring {
-    my $string = join("", @_);
-    return $string;
-}
-
-for (;;) {
-	$from = $sock->recv($buf, 512);
-	($port, $ip_address) = unpack_sockaddr_in($from);
-	$l = length($buf);
-	printf "received %u bytes from %s#%u\n", $l, inet_ntoa($ip_address), $port;
-	@up = unpack("C[$l]", $buf);
-	$up[2] |= 0x80; # QR
-	$up[2] |= 0x02; # TC
-	$up[3] |= 0x80; # RA
-	$l -= 1;	# truncate the response 1 byte
-	$replydata = pack("C[$l]", @up);
-	printf "sent %u bytes\n", $sock->send($replydata);
-}

bin/tests/system/tsig/ans2/ans.py

@@ -0,0 +1,49 @@
+"""
+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.
+"""
+
+from typing import AsyncGenerator
+
+import dns.flags
+
+from isctest.asyncserver import (
+    AsyncDnsServer,
+    BytesResponseSend,
+    QueryContext,
+    ResponseHandler,
+)
+
+
+class TruncatedWithLastByteDroppedHandler(ResponseHandler):
+    """
+    Return a TC=1 response with the final byte removed to make
+    dns_message_parse() return ISC_R_UNEXPECTEDEND.
+    """
+
+    async def get_responses(
+        self, qctx: QueryContext
+    ) -> AsyncGenerator[BytesResponseSend, None]:
+        tc_response = qctx.query
+        tc_response.flags |= dns.flags.QR
+        tc_response.flags |= dns.flags.TC
+        tc_response.flags |= dns.flags.RA
+        yield BytesResponseSend(tc_response.to_wire()[:-1])
+
+
+def main() -> None:
+    server = AsyncDnsServer(acknowledge_tsig_dnspython_hacks=True)
+    server.install_response_handler(TruncatedWithLastByteDroppedHandler())
+    server.run()
+
+
+if __name__ == "__main__":
+    main()

bin/tests/system/tsig/tests_sh_tsig.py

@@ -21,6 +21,9 @@ pytestmark = pytest.mark.extra_artifacts(
     ]
 )
 
+# TSIG queries not supported by isctest.asyncserver with old dnspython
+pytest.importorskip("dns", minversion="2.0.0")
+
 
 def test_tsig(run_tests_sh):
     run_tests_sh()