doc: move testing section from HACKING to its own doc (#739)

Our HACKING doc is very long, making it hard to justify expanding or
restructuring its content too much. This moves the testing section to
its own doc, and adds section headers in to make finding/linking to
testing guidelines easier.

To avoid confusion, the title of debugging.rst is changed.

3 files changed, 177 insertions, 4 deletions

diff --git a/doc/rtd/topics/debugging.rst b/doc/rtd/topics/debugging.rst
index 0d416f32..fb3006fe 100644
--- a/doc/rtd/topics/debugging.rst
+++ b/doc/rtd/topics/debugging.rst
@@ -1,6 +1,6 @@
-********************************
-Testing and debugging cloud-init
-********************************
+********************
+Debugging cloud-init
+********************
 
 Overview
 ========
diff --git a/doc/rtd/topics/integration_tests.rst b/doc/rtd/topics/integration_tests.rst
index 3cfca31e..6c124ad9 100644
--- a/doc/rtd/topics/integration_tests.rst
+++ b/doc/rtd/topics/integration_tests.rst
@@ -9,7 +9,7 @@ Overview
 
 Integration tests are written using pytest and are located at
 ``tests/integration_tests``. General design principles
-laid out in :ref:`unit_testing` should be followed for integration tests.
+laid out in :ref:`testing` should be followed for integration tests.
 
 Setup is accomplished via a set of fixtures located in
 ``tests/integration_tests/conftest.py``.
diff --git a/doc/rtd/topics/testing.rst b/doc/rtd/topics/testing.rst
new file mode 100644
index 00000000..5b702bd2
--- /dev/null
+++ b/doc/rtd/topics/testing.rst
@@ -0,0 +1,173 @@
+*******
+Testing
+*******
+
+cloud-init has both unit tests and integration tests. Unit tests can
+be found in-tree alongside the source code, as well as
+at ``tests/unittests``. Integration tests can be found at
+``tests/integration_tests``. Documentation specifically for integration
+tests can be found on the :ref:`integration_tests` page, but
+the guidelines specified below apply to both types of tests.
+
+cloud-init uses `pytest`_ to run its tests, and has tests written both
+as ``unittest.TestCase`` sub-classes and as un-subclassed pytest tests.
+
+Guidelines
+==========
+
+The following guidelines should be followed.
+
+Test Layout
+-----------
+
+* For ease of organisation and greater accessibility for developers not
+  familiar with pytest, all cloud-init unit tests must be contained
+  within test classes
+
+  * Put another way, module-level test functions should not be used
+
+* As all tests are contained within classes, it is acceptable to mix
+  ``TestCase`` test classes and pytest test classes within the same
+  test file
+
+  * These can be easily distinguished by their definition: pytest
+    classes will not use inheritance at all (e.g.
+    `TestGetPackageMirrorInfo`_), whereas ``TestCase`` classes will
+    subclass (indirectly) from ``TestCase`` (e.g.
+    `TestPrependBaseCommands`_)
+
+``pytest`` Tests
+----------------
+
+* pytest test classes should use `pytest fixtures`_ to share
+  functionality instead of inheritance
+
+* pytest tests should use bare ``assert`` statements, to take advantage
+  of pytest's `assertion introspection`_
+
+  * For ``==`` and other commutative assertions, the expected value
+    should be placed before the value under test:
+    ``assert expected_value == function_under_test()``
+
+
+``pytest`` Version Gotchas
+--------------------------
+
+As we still support Ubuntu 16.04 (Xenial Xerus), we can only use pytest
+features that are available in v2.8.7.  This is an inexhaustive list of
+ways in which this may catch you out:
+
+* Support for using ``yield`` in ``pytest.fixture`` functions was only
+  introduced in `pytest 3.0`_.  Such functions must instead use the
+  ``pytest.yield_fixture`` decorator.
+
+* Only the following built-in fixtures are available [#fixture-list]_:
+
+  * ``cache``
+  * ``capfd``
+  * ``caplog`` (provided by ``python3-pytest-catchlog`` on xenial)
+  * ``capsys``
+  * ``monkeypatch``
+  * ``pytestconfig``
+  * ``record_xml_property``
+  * ``recwarn``
+  * ``tmpdir_factory``
+  * ``tmpdir``
+
+* On xenial, the objects returned by the ``tmpdir`` fixture cannot be
+  used where paths are required; they are rejected as invalid paths.
+  You must instead use their ``.strpath`` attribute.
+
+  * For example, instead of ``util.write_file(tmpdir.join("some_file"),
+    ...)``, you should write
+    ``util.write_file(tmpdir.join("some_file").strpath, ...)``.
+
+* The `pytest.param`_ function cannot be used. It was introduced in
+  pytest 3.1, which means it is not available on xenial.  The more
+  limited mechanism it replaced was removed in pytest 4.0, so is not
+  available in focal or later.  The only available alternatives are to
+  write mark-requiring test instances as completely separate tests,
+  without utilising parameterisation, or to apply the mark to the
+  entire parameterized test (and therefore every test instance).
+
+Mocking and Assertions
+----------------------
+
+* Variables/parameter names for ``Mock`` or ``MagicMock`` instances
+  should start with ``m_`` to clearly distinguish them from non-mock
+  variables
+
+  * For example, ``m_readurl`` (which would be a mock for ``readurl``)
+
+* The ``assert_*`` methods that are available on ``Mock`` and
+  ``MagicMock`` objects should be avoided, as typos in these method
+  names may not raise ``AttributeError`` (and so can cause tests to
+  silently pass).  An important exception: if a ``Mock`` is
+  `autospecced`_ then misspelled assertion methods *will* raise an
+  ``AttributeError``, so these assertion methods may be used on
+  autospecced ``Mock`` objects.
+
+  For non-autospecced ``Mock`` s, these substitutions can be used
+  (``m`` is assumed to be a ``Mock``):
+
+  * ``m.assert_any_call(*args, **kwargs)`` => ``assert
+    mock.call(*args, **kwargs) in m.call_args_list``
+  * ``m.assert_called()`` => ``assert 0 != m.call_count``
+  * ``m.assert_called_once()`` => ``assert 1 == m.call_count``
+  * ``m.assert_called_once_with(*args, **kwargs)`` => ``assert
+    [mock.call(*args, **kwargs)] == m.call_args_list``
+  * ``m.assert_called_with(*args, **kwargs)`` => ``assert
+    mock.call(*args, **kwargs) == m.call_args_list[-1]``
+  * ``m.assert_has_calls(call_list, any_order=True)`` => ``for call in
+    call_list: assert call in m.call_args_list``
+
+    * ``m.assert_has_calls(...)`` and ``m.assert_has_calls(...,
+      any_order=False)`` are not easily replicated in a single
+      statement, so their use when appropriate is acceptable.
+
+  * ``m.assert_not_called()`` => ``assert 0 == m.call_count``
+
+* When there are multiple patch calls in a test file for the module it
+  is testing, it may be desirable to capture the shared string prefix
+  for these patch calls in a module-level variable.  If used, such
+  variables should be named ``M_PATH`` or, for datasource tests,
+  ``DS_PATH``.
+
+Test Argument Ordering
+----------------------
+
+* Test arguments should be ordered as follows:
+
+  * ``mock.patch`` arguments.  When used as a decorator, ``mock.patch``
+    partially applies its generated ``Mock`` object as the first
+    argument, so these arguments must go first.
+  * ``pytest.mark.parametrize`` arguments, in the order specified to
+    the ``parametrize`` decorator.  These arguments are also provided
+    by a decorator, so it's natural that they sit next to the
+    ``mock.patch`` arguments.
+  * Fixture arguments, alphabetically.  These are not provided by a
+    decorator, so they are last, and their order has no defined
+    meaning, so we default to alphabetical.
+
+* It follows from this ordering of test arguments (so that we retain
+  the property that arguments left-to-right correspond to decorators
+  bottom-to-top) that test decorators should be ordered as follows:
+
+  * ``pytest.mark.parametrize``
+  * ``mock.patch``
+
+.. [#fixture-list] This list of fixtures (with markup) can be
+   reproduced by running::
+
+     py.test-3 --fixtures -q | grep "^[^ -]" | grep -v '\(no\|capturelog\)' | sort | sed 's/.*/* ``\0``/'
+
+   in a xenial lxd container with python3-pytest-catchlog installed.
+
+.. _pytest: https://docs.pytest.org/
+.. _pytest fixtures: https://docs.pytest.org/en/latest/fixture.html
+.. _TestGetPackageMirrorInfo: https://github.com/canonical/cloud-init/blob/42f69f410ab8850c02b1f53dd67c132aa8ef64f5/cloudinit/distros/tests/test_init.py\#L15
+.. _TestPrependBaseCommands: https://github.com/canonical/cloud-init/blob/master/cloudinit/tests/test_subp.py#L9
+.. _assertion introspection: https://docs.pytest.org/en/latest/assert.html
+.. _pytest 3.0: https://docs.pytest.org/en/latest/changelog.html#id1093
+.. _pytest.param: https://docs.pytest.org/en/latest/reference.html#pytest-param
+.. _autospecced: https://docs.python.org/3.8/library/unittest.mock.html#autospeccing