docs: describe Python tests and fixtures

Signed-off-by: Alfredo Deza <adeza@redhat.com>

docs/source/testing/tests.rst

@@ -2,9 +2,90 @@
 
 Tests
 =====
+Actual tests are written in Python methods that accept optional fixtures. These
+fixtures come with interesting attributes to help with remote assertions.
+
+As described in :ref:`test_conventions`, tests need to go into
+``tests/functional/tests/``. These are collected and *mapped* to a distinct
+node type, or *mapped* to run on all nodes.
+
+Simple Python asserts are used (these tests do not need to follow the Python
+``unittest.TestCase`` base class) that make it easier to reason about failures
+and errors.
+
+The test run is handled by ``py.test`` along with :ref:`testinfra` for handling
+remote execution.
+
 
 .. _test_files:
 
 Test Files
 ----------
 
+
+
+.. _test_fixtures:
+
+Test Fixtures
+=============
+Test fixtures are a powerful feature of ``py.test`` and most tests depend on
+this for making assertions about remote nodes. To request them in a test
+method, all that is needed is to require it as an argument.
+
+Fixtures are detected by name, so as long as the argument being used has the
+same name, the fixture will be passed in (see `pytest fixtures`_ for more
+in-depth examples). The code that follows shows a test method that will use the
+``node`` fixture that contains useful information about a node in a ceph
+cluster::
+
+    def test_ceph_conf(self, node):
+        assert node['conf_path'] == "/etc/ceph/ceph.conf"
+
+The test is naive (the configuration path might not exist remotely) but
+explains how simple it is to "request" a fixture.
+
+For remote execution, we can rely further on other fixtures (tests can have as
+many fixtures as needed) like ``File``::
+
+    def test_ceph_config_has_inital_members_line(self, node, File):
+        assert File(node["conf_path"]).contains("^mon initial members = .*$")
+
+
+.. node:
+
+``node`` fixture
+----------------
+The ``node`` fixture contains a few useful pieces of information about the node
+where the test is being executed, this is captured once, before tests run:
+
+* ``address``: The IP for the ``eth1`` interface
+* ``subnet``: The subnet that ``address`` belongs to
+* ``vars``: all the ansible vars set for the current run
+* ``osd_ids``: a list of all the OSD IDs
+* ``num_mons``: the total number of monitors for the current environment
+* ``num_devices``: the number of devices for the current node
+* ``num_osd_hosts``: the total number of OSD hosts
+* ``total_osds``: total number of OSDs on the current node
+* ``cluster_name``: the name of the Ceph cluster (which defaults to 'ceph')
+* ``conf_path``: since the cluster name can change the file path for the Ceph
+  configuration, this gets sets according to the cluster name.
+* ``cluster_address``: the address used for cluster communication. All
+  environments are set up with 2 interfaces, 1 being used exclusively for the
+  cluster
+* ``docker``: A boolean that identifies a Ceph docker cluster
+* ``osds``: A list of OSD IDs, unless it is a docker cluster, where it gets the
+  name of the devices (e.g. ``sda1``)
+
+
+Other Fixtures
+--------------
+There are a lot of other fixtures provided by :ref:`testinfra` as well as
+``py.test``. The full list of ``testinfra`` fixtures are available in
+`testinfra_fixtures`_
+
+``py.test`` builtin fixtures can be listed with ``pytest -q --fixtures`` and
+they are described in `pytest builtin fixtures`_
+
+.. _pytest fixtures: https://docs.pytest.org/en/latest/fixture.html
+.. _pytest builtin fixtures: https://docs.pytest.org/en/latest/builtin.html#builtin-fixtures-function-arguments
+.. _testinfra_fixtures: https://testinfra.readthedocs.io/en/latest/modules.html#modules