test: acme --log flag asserts explicit log file path

tests/test_acme.py

@@ -61,14 +61,19 @@ class TestRunAcme:
     @patch("lib.acme._find_acme")
     @patch("lib.acme.subprocess.run")
     def test_log_flag_is_last(self, mock_run, mock_find):
-        # --log must trail the subcommand args: acme.sh would otherwise
+        # --log <file> must trail the subcommand args: acme.sh would
-        # consume the first subcommand arg as its (optional) file argument.
+        # otherwise consume the first subcommand arg as its file argument.
+        # The explicit file path (not a bare trailing --log) is required
+        # because a valueless trailing --log makes acme.sh's arg loop
+        # double-shift under dash and fail with "shift: can't shift that
+        # many".
         mock_find.return_value = "/usr/local/bin/acme.sh"
         mock_run.return_value = MagicMock(returncode=0, stdout="", stderr="")
         acme._run_acme(["--issue", "-d", "example.com"])
         cmd = mock_run.call_args[0][0]
         assert cmd.count("--log") == 1
-        assert cmd[-1] == "--log"
+        assert cmd[-2] == "--log"
+        assert cmd[-1].endswith("acme.sh.log")
         assert cmd.index("--issue") < cmd.index("--log")
         assert "example.com" in cmd