conftest: introduce disable_subp_usage autouse fixture (#304)

This mirrors the behaviour of CiTestCase.allowed_subp, by causing all
calls to util.subp to raise an AssertionError.

1 files changed, 35 insertions, 0 deletions

diff --git a/cloudinit/conftest.py b/cloudinit/conftest.py
new file mode 100644
index 00000000..6a1c63e9
--- /dev/null
+++ b/cloudinit/conftest.py
@@ -0,0 +1,35 @@
+from unittest import mock
+
+import pytest
+
+
+@pytest.yield_fixture(autouse=True)
+def disable_subp_usage(request):
+    """
+    Across all (pytest) tests, ensure that util.subp is not invoked.
+
+    Note that this can only catch invocations where the util module is imported
+    and ``util.subp(...)`` is called.  ``from cloudinit.util import subp``
+    imports happen before the patching here (or the CiTestCase monkey-patching)
+    happens, so are left untouched.
+
+    To allow a particular test method or class to use util.subp you can set the
+    parameter passed to this fixture to False using pytest.mark.parametrize::
+
+        @pytest.mark.parametrize("disable_subp_usage", [False], indirect=True)
+        def test_whoami(self):
+            util.subp(["whoami"])
+
+    This fixture (roughly) mirrors the functionality of
+    CiTestCase.allowed_subp.
+
+    TODO:
+        * Enable select subp usage (i.e. allowed_subp=[...])
+    """
+    should_disable = getattr(request, "param", True)
+    if should_disable:
+        with mock.patch('cloudinit.util.subp', autospec=True) as m_subp:
+            m_subp.side_effect = AssertionError("Unexpectedly used util.subp")
+            yield
+    else:
+        yield