Begin multiprocessing misadventure

EliahKagan · EliahKagan · commit 602de0c93572 · 2024-03-28T23:26:10.000-04:00
There is no per-instance state involved in USE_SHELL, so pickling
is far less directly relevant than usual to multiprocessing: the
spawn and forkserver methods will not preserve a subsequently
changed attribute value unless side effects of loading a module (or
other unpickling of a function or its arguments that are submitted
to run on a worker subprocess) causes it to run again; the fork
method will.

This will be (automatically) the same with any combination of
metaclasses, properties, and custom descriptors as in the more
straightforward case of a simple class attribute. Subtleties arise
in the code that uses GitPython and multiprocessing, but should not
arise unintentionally from the change in implementation of USE_SHELL
done to add deprecation warnings, except possibly with respect to
whether warnings will be repeated in worker processes, which is
less important than whether the actual state is preserved.
diff --git a/test/deprecation/test_cmd_git.py b/test/deprecation/test_cmd_git.py
@@ -6,6 +6,7 @@
 access are not inadvertently broken by mechanisms introduced to issue the warnings.
 """
 
+from concurrent.futures import ProcessPoolExecutor
 import contextlib
 import sys
 from typing import Generator
@@ -36,7 +37,7 @@ def _suppress_deprecation_warning() -> Generator[None, None, None]:
 
 
 @pytest.fixture
-def try_restore_use_shell_state() -> Generator[None, None, None]:
+def restore_use_shell_state() -> Generator[None, None, None]:
     """Fixture to attempt to restore state associated with the ``USE_SHELL`` attribute.
 
     This is used to decrease the likelihood of state changes leaking out and affecting
@@ -142,7 +143,7 @@ def _assert_use_shell_full_results(
     assert recheck_message.startswith(_USE_SHELL_DEPRECATED_FRAGMENT)
 
 
-def test_use_shell_set_and_get_on_class(try_restore_use_shell_state: None) -> None:
+def test_use_shell_set_and_get_on_class(restore_use_shell_state: None) -> None:
     """USE_SHELL can be set and re-read as a class attribute, always warning."""
     with pytest.deprecated_call() as setting:
         Git.USE_SHELL = True
@@ -163,7 +164,7 @@ def test_use_shell_set_and_get_on_class(try_restore_use_shell_state: None) -> No
     )
 
 
-def test_use_shell_set_on_class_get_on_instance(try_restore_use_shell_state: None) -> None:
+def test_use_shell_set_on_class_get_on_instance(restore_use_shell_state: None) -> None:
     """USE_SHELL can be set on the class and read on an instance, always warning.
 
     This is like test_use_shell_set_and_get_on_class but it performs reads on an
@@ -196,14 +197,31 @@ class (or an instance) is needed first before a read on an instance (or the clas
 @pytest.mark.parametrize("value", [False, True])
 def test_use_shell_cannot_set_on_instance(
     value: bool,
-    try_restore_use_shell_state: None,  # In case of a bug where it does set USE_SHELL.
+    restore_use_shell_state: None,  # In case of a bug where it does set USE_SHELL.
 ) -> None:
     instance = Git()
     with pytest.raises(AttributeError):
         instance.USE_SHELL = value
 
 
-# FIXME: Test behavior with multiprocessing (the attribute needs to pickle properly).
+def _check_use_shell_in_worker(value: bool) -> None:
+    # USE_SHELL should have the value set in the parent before starting the worker.
+    assert Git.USE_SHELL is value
+
+    # FIXME: Check that mutation still works and raises the warning.
+
+
+@pytest.mark.filterwarnings("ignore::DeprecationWarning")
+@pytest.mark.parametrize("value", [False, True])
+def test_use_shell_preserved_in_multiprocessing(
+    value: bool,
+    restore_use_shell_state: None,
+) -> None:
+    """The USE_SHELL class attribute pickles accurately for multiprocessing."""
+    Git.USE_SHELL = value
+    with ProcessPoolExecutor(max_workers=1) as executor:
+        # Calling result() marshals any exception back to this process and raises it.
+        executor.submit(_check_use_shell_in_worker, value).result()
 
 
 _EXPECTED_DIR_SUBSET = {
