test: move conftest.py to top-level, to cover tests/ also (#414)

* test_opennebula: convert TestParseShellConfig to a pytest test

And allow it to run bash.

(We aren't aiming to convert TestCase tests to pytest tests as a rule.
In this case, I needed to change its implementation to limit subp usage,
and I chose pytest over CiTestCase.)

* test: move conftest.py to top-level, to cover tests/ also

This gives us a single conftest.py which is shared by all tests in the
project.

1 files changed, 72 insertions, 0 deletions

diff --git a/conftest.py b/conftest.py
new file mode 100644
index 00000000..251bca59
--- /dev/null
+++ b/conftest.py
@@ -0,0 +1,72 @@
+from unittest import mock
+
+import pytest
+
+from cloudinit import subp
+
+
+@pytest.yield_fixture(autouse=True)
+def disable_subp_usage(request):
+    """
+    Across all (pytest) tests, ensure that subp.subp is not invoked.
+
+    Note that this can only catch invocations where the util module is imported
+    and ``subp.subp(...)`` is called.  ``from cloudinit.subp mport 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 subp.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):
+            subp.subp(["whoami"])
+
+    To instead allow subp.subp usage for a specific command, you can set the
+    parameter passed to this fixture to that command:
+
+        @pytest.mark.parametrize("disable_subp_usage", ["bash"], indirect=True)
+        def test_bash(self):
+            subp.subp(["bash"])
+
+    To specify multiple commands, set the parameter to a list (note the
+    double-layered list: we specify a single parameter that is itself a list):
+
+        @pytest.mark.parametrize(
+            "disable_subp_usage", ["bash", "whoami"], indirect=True)
+        def test_several_things(self):
+            subp.subp(["bash"])
+            subp.subp(["whoami"])
+
+    This fixture (roughly) mirrors the functionality of
+    CiTestCase.allowed_subp.  N.B. While autouse fixtures do affect non-pytest
+    tests, CiTestCase's allowed_subp does take precedence (and we have
+    TestDisableSubpUsageInTestSubclass to confirm that).
+    """
+    should_disable = getattr(request, "param", True)
+    if should_disable:
+        if not isinstance(should_disable, (list, str)):
+            def side_effect(args, *other_args, **kwargs):
+                raise AssertionError("Unexpectedly used subp.subp")
+        else:
+            # Look this up before our patch is in place, so we have access to
+            # the real implementation in side_effect
+            real_subp = subp.subp
+
+            if isinstance(should_disable, str):
+                should_disable = [should_disable]
+
+            def side_effect(args, *other_args, **kwargs):
+                cmd = args[0]
+                if cmd not in should_disable:
+                    raise AssertionError(
+                        "Unexpectedly used subp.subp to call {} (allowed:"
+                        " {})".format(cmd, ",".join(should_disable))
+                    )
+                return real_subp(args, *other_args, **kwargs)
+
+        with mock.patch('cloudinit.subp.subp', autospec=True) as m_subp:
+            m_subp.side_effect = side_effect
+            yield
+    else:
+        yield