test,fix: Force pytest to exit zero when no tests are collected (#221)

* fix: Prevent pytest from exiting non-zero when there are no changed samples

    pytest considers a failure to collect any tests as an error to be
    investigated. In the context of "only running pytest tests for
    changed samples", we'd expect this to happen somewhat commonly.

    This PR makes pytest exit OK when no tests were collected if this
    plugin is active.

* docs: Fix docstrings

Author: kdestin

.infra/pytest_plugins/changed_samples/src/pytest_changed_samples/plugin.py

@@ -13,6 +13,11 @@
 PR_CHANGES_OPTION = "--changed-samples-only-from"
 
 
+def is_plugin_active(config: pytest.Config) -> bool:
+    """Return whether any of the plugin provided options were provided on commandline."""
+    return get_diff_paths_function(config) is not None
+
+
 def pytest_addoption(parser: pytest.Parser) -> None:
     parser.addoption(
         WORKING_TREE_CHANGES_OPTION,
@@ -42,10 +47,18 @@ def pytest_configure(config: pytest.Config) -> None:
 
 @pytest.hookimpl(hookwrapper=True)
 def pytest_collection(session: pytest.Session) -> None:
+    """Set up path filtering based on git diff."""
     config = session.config
     diff_path_trie = Trie()
 
-    for p in get_diff_paths_function(config)():
+    paths_filter = get_diff_paths_function(config)
+
+    if paths_filter is None:
+        # Exit early if there's no path filter
+        yield
+        return
+
+    for p in paths_filter():
         diff_path_trie.insert(p.parts)
 
     config.stash[DIFF_PATH_TRIE_KEY] = diff_path_trie
@@ -56,8 +69,9 @@ def pytest_collection(session: pytest.Session) -> None:
 
 
 def pytest_ignore_collect(collection_path: Path, config: pytest.Config) -> Optional[bool]:
+    """Ignore paths that were not touched by the current git diff."""
     if DIFF_PATH_TRIE_KEY not in config.stash:
-        # Occures when calling `pytest --fixtures`
+        # Occurs when calling `pytest --fixtures`
         return None
 
     diff_path_trie = config.stash[DIFF_PATH_TRIE_KEY]
@@ -72,7 +86,16 @@ def pytest_ignore_collect(collection_path: Path, config: pytest.Config) -> Optio
     return (not diff_path_trie.is_prefix(ignore_dir.resolve().parts)) or None
 
 
-def get_diff_paths_function(config: pytest.Config) -> Callable[[], Iterable[Path]]:
+@pytest.hookimpl(trylast=True)
+def pytest_sessionfinish(session: pytest.Session, exitstatus: int) -> None:
+    if not is_plugin_active(session.config):
+        return
+
+    if exitstatus == pytest.ExitCode.NO_TESTS_COLLECTED:
+        session.exitstatus = pytest.ExitCode.OK
+
+
+def get_diff_paths_function(config: pytest.Config) -> Optional[Callable[[], Iterable[Path]]]:
     """Get the function that returns paths present in a diff specfied by cmdline arguments
 
     :param pytest.Config config: The pytest config
@@ -87,7 +110,7 @@ def get_diff_paths_function(config: pytest.Config) -> Callable[[], Iterable[Path
     if ref := config.getoption(opt_var(PR_CHANGES_OPTION)):
         return lambda: get_branch_diff_paths(ref)
 
-    return lambda: ()
+    return None
 
 
 def opt_var(s: str) -> str: