Add mock to avoid loading guestfs in unit test

We recently discovered that when the perfect conditions are present
where:

  * libguestfs-dev/el and guestfs python bindings are installed

and

  * unit tests are not being run in a venv or guestfs python bindings
    are installed in the tox venv

the test will end up loading the guestfs module and try to call the
real guestfs and possibly libvirt and fail because of it.

Our unit tests shouldn't be loading modules like guestfs, so this adds
proper mocking to the test along with a poison fixture that will
prevent future accidental imports of such modules.

Closes-Bug: #1994913

Change-Id: I676ee1fd33cf053681a07448759c28f0f2ad79d1

nova/test.py

@@ -288,6 +288,10 @@ class TestCase(base.BaseTestCase):
         self.useFixture(nova_fixtures.GenericPoisonFixture())
         self.useFixture(nova_fixtures.SysFsPoisonFixture())
 
+        # Additional module names can be added to this set if needed
+        self.useFixture(nova_fixtures.ImportModulePoisonFixture(
+            set(['guestfs', 'libvirt'])))
+
         # make sure that the wsgi app is fully initialized for all testcase
         # instead of only once initialized for test worker
         wsgi_app.init_global_data.reset()

nova/tests/fixtures/nova.py

@@ -20,8 +20,10 @@ import collections
 import contextlib
 from contextlib import contextmanager
 import functools
+from importlib.abc import MetaPathFinder
 import logging as std_logging
 import os
+import sys
 import time
 from unittest import mock
 import warnings
@@ -1798,3 +1800,51 @@ class SysFsPoisonFixture(fixtures.Fixture):
         # a bunch of test to fail
         # self.inject_poison("os.path", "exists")
         # self.inject_poison("os", "stat")
+
+
+class ImportModulePoisonFixture(fixtures.Fixture):
+    """Poison imports of modules unsuitable for the test environment.
+
+    Examples are guestfs and libvirt. Ordinarily, these would not be installed
+    in the test environment but if they _are_ present, it can result in
+    actual calls to libvirt, for example, which could cause tests to fail.
+
+    This fixture will inspect module imports and if they are in the disallowed
+    list, it will fail the test with a helpful message about mocking needed in
+    the test.
+    """
+
+    class ForbiddenModules(MetaPathFinder):
+        def __init__(self, test, modules):
+            super().__init__()
+            self.test = test
+            self.modules = modules
+
+        def find_spec(self, fullname, path, target=None):
+            if fullname in self.modules:
+                self.test.fail_message = (
+                    f"This test imports the '{fullname}' module, which it "
+                    f'should not in the test environment. Please add '
+                    f'appropriate mocking to this test.'
+                )
+                raise ImportError(fullname)
+
+    def __init__(self, module_names):
+        self.module_names = module_names
+        self.fail_message = ''
+        if isinstance(module_names, str):
+            self.module_names = set([module_names])
+        sys.meta_path.insert(0, self.ForbiddenModules(self, self.module_names))
+
+    def setUp(self):
+        super().setUp()
+        self.addCleanup(self.cleanup)
+
+    def cleanup(self):
+        # We use a flag and check it during the cleanup phase to fail the test
+        # if needed. This is done because some module imports occur inside of a
+        # try-except block that ignores all exceptions, so raising an exception
+        # there (which is also what self.assert* and self.fail() do underneath)
+        # will not work to cause a failure in the test.
+        if self.fail_message:
+            raise ImportError(self.fail_message)

nova/tests/unit/virt/disk/test_api.py

@@ -40,6 +40,7 @@ class FakeMount(object):
 
 
 class APITestCase(test.NoDBTestCase):
+    @mock.patch('nova.virt.disk.vfs.guestfs.VFSGuestFS', new=mock.Mock())
     def test_can_resize_need_fs_type_specified(self):
         imgfile = tempfile.NamedTemporaryFile()
         self.addCleanup(imgfile.close)