Extend test guidelines with implementation notes for unit/regression tests

contributor-docs/development/index.md

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+# Developer documentation
+
+Welcome to the developer section of the contributor docs. Here you will find
+guidelines for the implementation of tooling such as the SCS conformance tests.

contributor-docs/development/tests/test-implementation-guide.md

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+---
+title: SCS Conformance Test Implementation Guide
+type:
+status: Draft
+track: Global
+---
+
+SovereignCloudStack (SCS) uses [conformance tests][gh-scripts] to certify
+compliance of a given cloud offering with respect to a given [certificate
+scope][cert-scopes] such as *SCS Compatible IaaS v4*.
+Our aim is that these tests are reliable, consistent and comprehensible for
+the users.
+This document is a guideline for conformance test authors and summarizes the
+current best practices to achieve this goal.
+
+## 1. Mapping of RFC2119 Keywords
+
+Test authors working on new and existing conformance tests for an SCS standard
+must implement the keywords such as MUST and SHOULD according to the [SCS
+RFC2119 Keyword Test Guide][scs-rfc2119-guide].
+
+## 2. Unit and Regression Tests
+
+Test authors are *strongly* encouraged to include unit and regression tests for
+the conformance test's logic.
+Conformance tests will inevitably contain some non-trivial algorithms, be it for
+parsing flavor names or evaluating CVE vulnerability reports.
+Testing them automatically and regularly against valid and invalid inputs helps
+test authors to find programming mistakes early on.
+As a bonus, a well written unit test also makes it easier for reviewers to cross
+check a given pull request with new, enhanced or refactored conformance tests.
+
+In general, unit and regression tests for the conformance tests are located in
+the same location as the conformance tests, that is, the `Tests/` directory of
+the [SCS standards repository][scs-standards].
+Setup and development of the unit tests is described in the [SCS conformance
+tests README][tests-readme].
+
+### Naming Conventions
+
+Any module that ends in `_test` will be picked up as a unit test module by
+pytest in our CI pipeline (in other words, all files matched by the pattern
+`Tests/**/*_test.py`).
+Pytest will execute any function prefixed with `test_` as a test function, or,
+alternatively, tests based on the `unittest` module from the Python standard
+library.
+
+Follow these naming rules:
+
+* The name of a unit test module is constructed from the module to be unit
+  tested and the `_test` suffix. For example, unit tests for `flavor_names.py`
+  should reside in `flavor_names_test.py`.
+* The name of a Python module in general should be a valid [Python
+  identifier][python-identifiers], to allow imports via the `import` statement.
+  For example, use `flavor_names.py` instead of `flavor-names.py`.
+
+### Write Testable Conformance Tests
+
+Software design fills complete book shelves and this section can only cover some
+basic best practices.
+As a general rule of thumb, the earlier you begin writing unit tests, the better
+– it will force you to write modules that are easy to test.
+
+Divide conformance tests scripts into smaller, loosely coupled units, i.e.,
+functions that serve one purpose each.
+Use the [*dependency injection*][wiki-di] technique, i.e., pass externally
+retrieved data as function arguments instead of hardcoding the calls to the
+retrieval functions inside a function.
+Avoid logging calls deep down in the call hierarchy and use explicit return
+values or raise exceptions, instead.
+Here is an abstract example of a conformance test which does exactly that:
+
+```python
+# property_compliance.py
+
+def retrieve(location):
+    """Retrieve raw data via network."""
+    ...
+
+def parse(data):
+    """Parse raw data and return a dict."""
+    ...
+
+def handle(...):
+    """Evaluate parsed data."""
+    ...
+
+def main(args):
+    ...
+    data = retrieve(location)
+    parsed = parse(data)
+    result = handle(parsed)
+    ...
+    if result.some_prop != expected_value:
+        logging.warn("some_prop is not as expected: %s (vs. %s)", result.some_prop, expected_value)
+    ...
+    return result.success
+
+if __name__ == "__main__":
+    # using sys.exit(…) only here makes it possible to unit test main(…)
+    sys.exit(main(sys.argv))
+```
+
+Adhering to this style makes it easier to test an algorithm in isolation,
+without actually making a call to some external service.
+
+### Pytest Test Example
+
+Assuming we want to unit test some members of the module `property_compliance`
+from the previous section, we would create a file `property_compliance_test.py`
+with the following content as a starting point:
+
+```python
+"""Unit tests for property_compliance.
+
+(c) Your Name <your.name@example.com>, 4/2024
+SPDX-License-Identifier: CC-BY-SA-4.0
+"""
+
+import pytest
+
+from property_compliance import parse
+
+
+def test_success():
+    assert parse("some valid input") == "expected output"
+
+
+def test_failure():
+    with pytest.raises(ValueError):
+        parse("invalid input")
+```
+
+## 3. Conformance Tests Shouldn't Require Admin Privileges
+
+Conformance tests are expected to be executable without admin privileges (see §2 of
+[Regulations for achieving SCS-compatible certification][scs-0004-v1]).
+In particular, this means:
+
+* The cloud credentials (e.g., kubeconfig and OpenStack `clouds.yaml`) passed to
+  the scripts are non-admin credentials.
+* Conformance tests scripts should not require root privileges, except for the
+  installation of operating system prerequisites (e.g., a Python interpreter).
+
+[scs-standards]: https://github.com/SovereignCloudStack/standards/
+[tests-readme]: https://github.com/SovereignCloudStack/standards/blob/main/Tests/README.md
+[python-identifiers]: https://docs.python.org/3/reference/lexical_analysis.html#identifiers
+[gh-scripts]: https://github.com/SovereignCloudStack/standards/tree/main/Tests
+[cert-scopes]: https://docs.scs.community/standards/certification/scopes-versions
+[scs-rfc2119-guide]: https://docs.scs.community/contributor-docs/operations/tests/rfc2119-keyword-test-guide
+[scs-0004-v1]: https://github.com/SovereignCloudStack/standards/blob/main/Standards/scs-0004-v1-achieving-certification.md
+[wiki-di]: https://en.wikipedia.org/wiki/Dependency_injection