conftest: add docs and tests regarding CiTestCase's subp functionality (#343)

And raise TypeError when subp called with no args, which more accurately mirrors normal behaviour:

>>> from cloudinit.util import subp
>>> subp()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: subp() missing 1 required positional argument: 'args'

2 files changed, 25 insertions, 0 deletions

diff --git a/cloudinit/tests/helpers.py b/cloudinit/tests/helpers.py
index 477e14c2..b0af54c1 100644
--- a/cloudinit/tests/helpers.py
+++ b/cloudinit/tests/helpers.py
@@ -144,6 +144,9 @@ class CiTestCase(TestCase):
         if 'args' in kwargs:
             cmd = kwargs['args']
         else:
+            if not args:
+                raise TypeError(
+                    "subp() missing 1 required positional argument: 'args'")
             cmd = args[0]
 
         if not isinstance(cmd, str):
diff --git a/cloudinit/tests/test_conftest.py b/cloudinit/tests/test_conftest.py
index 62a5361d..11b91327 100644
--- a/cloudinit/tests/test_conftest.py
+++ b/cloudinit/tests/test_conftest.py
@@ -1,9 +1,11 @@
 import pytest
 
 from cloudinit import util
+from cloudinit.tests.helpers import CiTestCase
 
 
 class TestDisableSubpUsage:
+    """Test that the disable_subp_usage fixture behaves as expected."""
 
     def test_using_subp_raises_assertion_error(self):
         with pytest.raises(AssertionError):
@@ -16,3 +18,23 @@ class TestDisableSubpUsage:
     @pytest.mark.parametrize('disable_subp_usage', [False], indirect=True)
     def test_subp_usage_can_be_reenabled(self):
         util.subp(['whoami'])
+
+
+class TestDisableSubpUsageInTestSubclass(CiTestCase):
+    """Test that disable_subp_usage doesn't impact CiTestCase's subp logic."""
+
+    def test_using_subp_raises_exception(self):
+        with pytest.raises(Exception):
+            util.subp(["some", "args"])
+
+    def test_typeerrors_on_incorrect_usage(self):
+        with pytest.raises(TypeError):
+            util.subp()
+
+    def test_subp_usage_can_be_reenabled(self):
+        _old_allowed_subp = self.allow_subp
+        self.allowed_subp = True
+        try:
+            util.subp(['bash', '-c', 'true'])
+        finally:
+            self.allowed_subp = _old_allowed_subp