Add test for missing service arguments

The test currently fails due to QubesOS/qubes-issues#9089.  The test for
socket services will be added along with the fix, as without the fix it
hangs.

qrexec/tests/socket/agent.py

@@ -26,6 +26,7 @@
 import struct
 import getpass
 import itertools
+import asyncio
 
 import psutil
 import pytest
@@ -55,6 +56,10 @@ def setUp(self):
 
     def start_agent(self):
         env = os.environ.copy()
+        try:
+            del os.environ["QREXEC_SERVICE_ARGUMENT"]
+        except KeyError:
+            pass
         env["LD_LIBRARY_PATH"] = os.path.join(ROOT_PATH, "libqrexec")
         env["VCHAN_DOMAIN"] = str(self.domain)
         env["VCHAN_SOCKET_DIR"] = self.tempdir
@@ -579,6 +584,38 @@ def test_exec_broken_specific_service(self):
         )
         self.check_dom0(dom0)
 
+    @unittest.expectedFailure
+    def test_exec_null_argument_finds_service_for_empty_argument(self):
+        self.make_executable_service(
+            "local-rpc",
+            "qubes.Service+",
+            """\
+#!/bin/sh --
+echo "${QREXEC_SERVICE_ARGUMENT+specific} service: $QREXEC_SERVICE_FULL_NAME"
+""",
+        )
+        self.make_executable_service(
+            "rpc",
+            "qubes.Service",
+            """\
+#!/bin/sh
+echo "general service"
+""",
+        )
+        target, dom0 = self.execute_qubesrpc("qubes.Service", "domX")
+        target.send_message(qrexec.MSG_DATA_STDIN, b"")
+        messages = target.recv_all_messages()
+        self.assertListEqual(
+            util.sort_messages(messages),
+            [
+                (qrexec.MSG_DATA_STDOUT, b"specific service: qubes.Service+\n"),
+                (qrexec.MSG_DATA_STDOUT, b""),
+                (qrexec.MSG_DATA_STDERR, b""),
+                (qrexec.MSG_DATA_EXIT_CODE, b"\0\0\0\0"),
+            ],
+        )
+        self.check_dom0(dom0)
+
     def test_connect_socket_no_metadata(self):
         socket_path = os.path.join(
             self.tempdir, "rpc", "qubes.SocketService+arg2"