tests: detect stale Cython .so files at test startup

Add a pytest_configure hook in tests/conftest.py that compares mtime
of each compiled .so against its .py source and warns when the source
is newer. This prevents silently testing stale compiled code after
editing a Cython-compiled module without rebuilding.

Also document the rebuild requirement in CONTRIBUTING.rst.

Author: mykaul

CONTRIBUTING.rst

@@ -40,6 +40,16 @@ When modifying driver files, rebuilding Cython modules is often necessary.
 Without caching, each such rebuild may take over a minute. Caching usually brings it
 down to about 2-3 seconds.
 
+**Important:** After modifying any ``.py`` file under ``cassandra/`` that is
+Cython-compiled (such as ``query.py``, ``protocol.py``, ``cluster.py``, etc.),
+you **must** rebuild extensions before running tests::
+
+    python setup.py build_ext --inplace
+
+Without rebuilding, Python will load the stale compiled ``.so`` file instead of
+your modified ``.py`` source, and your changes will not actually be tested.
+The test suite will emit a warning if it detects this situation.
+
 Building the Docs
 =================
 

tests/conftest.py

@@ -0,0 +1,55 @@
+# Copyright ScyllaDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import glob
+import os
+import warnings
+
+# Directory containing the Cython-compiled driver modules.
+_CASSANDRA_DIR = os.path.join(os.path.dirname(__file__), os.pardir, "cassandra")
+
+
+def pytest_configure(config):
+    """Warn when a compiled Cython .so is older than its .py source.
+
+    Python's import system prefers compiled extensions (.so / .pyd) over pure
+    Python (.py) files.  If a developer edits a .py file without rebuilding
+    the Cython extensions (``python setup.py build_ext --inplace``), the tests
+    will silently run the *old* compiled code, masking any regressions in the
+    Python source.
+
+    This hook detects such staleness at test-session startup so the developer
+    is alerted immediately.
+    """
+    stale = []
+    for so_path in glob.glob(os.path.join(_CASSANDRA_DIR, "*.so")):
+        # Extension filenames look like  query.cpython-314-x86_64-linux-gnu.so
+        # The corresponding source is    query.py
+        basename = os.path.basename(so_path)
+        module_name = basename.split(".")[0]
+        py_path = os.path.join(_CASSANDRA_DIR, module_name + ".py")
+        if os.path.exists(py_path) and os.path.getmtime(py_path) > os.path.getmtime(
+            so_path
+        ):
+            stale.append((module_name, so_path, py_path))
+
+    if stale:
+        names = ", ".join(m for m, _, _ in stale)
+        warnings.warn(
+            f"Stale Cython extension(s) detected: {names}. "
+            f"The .py source is newer than the compiled .so — tests will run "
+            f"the OLD compiled code, not your latest changes. "
+            f"Rebuild with:  python setup.py build_ext --inplace",
+            stacklevel=1,
+        )