fix: move multiprocessing_start_method_fork fixture to shared conftest

On Python 3.14, the default multiprocessing start method changed from
fork to forkserver (python/cpython#132898), which requires all Process
arguments to be picklable. The unprivileged_client Kubernetes client
object contains thread locks and closures that cannot be pickled,
causing _pickle.PicklingError in test_successful_concurrent_uploads.

Move the multiprocessing_start_method_fork fixture from
tests/chaos/conftest.py to tests/conftest.py so it can be reused
across test directories, and apply it to the affected upload test.

Changes:
- Move fixture to tests/conftest.py (shared across all test dirs)
- Remove duplicate from tests/chaos/conftest.py
- Add @pytest.mark.usefixtures("multiprocessing_start_method_fork")
  to test_successful_concurrent_uploads in test_upload.py
- Existing chaos consumers (test_standard, test_snapshot,
  test_migration) resolve the fixture from the parent conftest

Signed-off-by: Jathavedhan M <jathavedhan.m@ibm.com>

Author: carterpewpew

tests/chaos/conftest.py

@@ -406,14 +406,3 @@ def deleted_pod_by_name_prefix(admin_client, cnv_pod_deletion_test_matrix__class
         namespace=pod_deletion_config["namespace_name"],
         pod_prefix=pod_deletion_config["pod_prefix"],
     )
-
-
-@pytest.fixture(scope="module")
-def multiprocessing_start_method_fork():
-    # Use fork context to avoid pickling issues with nested functions
-    # https://docs.python.org/3/library/multiprocessing.html#multiprocessing.Process
-    # https://github.com/python/cpython/issues/132898
-    original_start_method = multiprocessing.get_start_method()
-    multiprocessing.set_start_method("fork", force=True)
-    yield
-    multiprocessing.set_start_method(original_start_method, force=True)

tests/conftest.py

@@ -4,6 +4,7 @@
 
 import copy
 import logging
+import multiprocessing
 import os
 import os.path
 import re
@@ -227,6 +228,22 @@
 AUDIT_LOG_PATTERN = re.compile(r"audit-(\d{4}-\d{2}-\d{2})T(\d{2})-(\d{2})-(\d{2}\.\d{3})\.log")
 
 
+@pytest.fixture(scope="module")
+def multiprocessing_start_method_fork():
+    """Temporarily set multiprocessing start method to ``fork``.
+
+    Side effects:
+        Changes the process-global multiprocessing start method to ``fork``
+        for the module lifetime, then restores the previous method on teardown.
+    """
+    # https://docs.python.org/3/library/multiprocessing.html#multiprocessing.Process
+    # https://github.com/python/cpython/issues/132898
+    original_start_method = multiprocessing.get_start_method()
+    multiprocessing.set_start_method("fork", force=True)
+    yield
+    multiprocessing.set_start_method(original_start_method, force=True)
+
+
 @pytest.fixture(scope="session")
 def junitxml_polarion(record_testsuite_property):
     """

tests/storage/cdi_upload/test_upload.py

@@ -284,6 +284,7 @@ def _upload_image(dv_name, namespace, storage_class, local_name, client):
 @pytest.mark.sno
 @pytest.mark.s390x
 @pytest.mark.polarion("CNV-2015")
+@pytest.mark.usefixtures("multiprocessing_start_method_fork")
 @pytest.mark.parametrize(
     "upload_file_path",
     [