From 2018de6474b53b867e3de070a5ce447228b7220d Mon Sep 17 00:00:00 2001
From: Steven Silvester <steve.silvester@mongodb.com>
Date: Thu, 3 Sep 2026 14:10:53 -0500
Subject: [PATCH 1/2] PYTHON-5874 Fix test_fork.py failures on Python 3.15 due
 to fork() DeprecationWarning

Python 3.12 emits a DeprecationWarning when os.fork() runs in a multi-threaded process. The fork tests intentionally fork with the client's background threads running, so the warning is expected; suppress it at the fork site so the error-level warning filter does not fail the tests. Removes the 3.15 skipIf stopgap added in PYTHON-5837.
---
 test/__init__.py              | 12 +++++++++++-
 test/asynchronous/__init__.py | 12 +++++++++++-
 test/test_fork.py             |  6 ++----
 3 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/test/__init__.py b/test/__init__.py
index 7aff0a4620..2de2681ac8 100644
--- a/test/__init__.py
+++ b/test/__init__.py
@@ -950,7 +950,17 @@ def _target() -> None:
 
         ctx = multiprocessing.get_context("fork")
         proc = ctx.Process(target=_target)
-        proc.start()
+        # Python 3.12+ warns when os.fork() runs in a multi-threaded process. The
+        # warning is a general thread-count heuristic; benign here because the only
+        # extra threads are pymongo's, whose locks register_at_fork resets in the
+        # child. Suppressed only in this helper, not globally (PYTHON-5874).
+        with warnings.catch_warnings():
+            warnings.filterwarnings(
+                "ignore",
+                message=r".*use of fork\(\) may lead to deadlocks.*",
+                category=DeprecationWarning,
+            )
+            proc.start()
         try:
             yield proc  # type: ignore
         finally:
diff --git a/test/asynchronous/__init__.py b/test/asynchronous/__init__.py
index 4ecaa589cf..bf5c1df8b2 100644
--- a/test/asynchronous/__init__.py
+++ b/test/asynchronous/__init__.py
@@ -950,7 +950,17 @@ def _target() -> None:
 
         ctx = multiprocessing.get_context("fork")
         proc = ctx.Process(target=_target)
-        proc.start()
+        # Python 3.12+ warns when os.fork() runs in a multi-threaded process. The
+        # warning is a general thread-count heuristic; benign here because the only
+        # extra threads are pymongo's, whose locks register_at_fork resets in the
+        # child. Suppressed only in this helper, not globally (PYTHON-5874).
+        with warnings.catch_warnings():
+            warnings.filterwarnings(
+                "ignore",
+                message=r".*use of fork\(\) may lead to deadlocks.*",
+                category=DeprecationWarning,
+            )
+            proc.start()
         try:
             yield proc  # type: ignore
         finally:
diff --git a/test/test_fork.py b/test/test_fork.py
index 4b33dc76dc..e8a551d058 100644
--- a/test/test_fork.py
+++ b/test/test_fork.py
@@ -29,6 +29,8 @@
 from test.utils_shared import is_greenthread_patched
 
 
+# self.fork suppresses the Python 3.12+ fork() DeprecationWarning; it is
+# benign for these intentional forks (PYTHON-5874).
 @unittest.skipIf(
     not hasattr(os, "register_at_fork"), "register_at_fork not available in this version of Python"
 )
@@ -36,10 +38,6 @@
     is_greenthread_patched(),
     "gevent does not support POSIX-style forking.",
 )
-@unittest.skipIf(
-    sys.version_info >= (3, 15),
-    "fork() in multi-threaded processes is deprecated in Python 3.15+ (PYTHON-5874)",
-)
 class TestFork(IntegrationTest):
     def test_lock_client(self):
         # Forks the client with some items locked.

From 6c9882029fac77d943cf841adfea5301b2713fd6 Mon Sep 17 00:00:00 2001
From: Steven Silvester <steve.silvester@mongodb.com>
Date: Thu, 3 Sep 2026 18:43:31 -0500
Subject: [PATCH 2/2] PYTHON-5874 Reword fork() DeprecationWarning comment

Clarify that pymongo's locks are reset via register_at_fork in the child.
---
 test/__init__.py              | 4 ++--
 test/asynchronous/__init__.py | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/test/__init__.py b/test/__init__.py
index 2de2681ac8..eb588abec9 100644
--- a/test/__init__.py
+++ b/test/__init__.py
@@ -952,8 +952,8 @@ def _target() -> None:
         proc = ctx.Process(target=_target)
         # Python 3.12+ warns when os.fork() runs in a multi-threaded process. The
         # warning is a general thread-count heuristic; benign here because the only
-        # extra threads are pymongo's, whose locks register_at_fork resets in the
-        # child. Suppressed only in this helper, not globally (PYTHON-5874).
+        # extra threads are pymongo's, whose locks are reset via register_at_fork
+        # in the child. Suppressed only in this helper, not globally (PYTHON-5874).
         with warnings.catch_warnings():
             warnings.filterwarnings(
                 "ignore",
diff --git a/test/asynchronous/__init__.py b/test/asynchronous/__init__.py
index bf5c1df8b2..d31e653705 100644
--- a/test/asynchronous/__init__.py
+++ b/test/asynchronous/__init__.py
@@ -952,8 +952,8 @@ def _target() -> None:
         proc = ctx.Process(target=_target)
         # Python 3.12+ warns when os.fork() runs in a multi-threaded process. The
         # warning is a general thread-count heuristic; benign here because the only
-        # extra threads are pymongo's, whose locks register_at_fork resets in the
-        # child. Suppressed only in this helper, not globally (PYTHON-5874).
+        # extra threads are pymongo's, whose locks are reset via register_at_fork
+        # in the child. Suppressed only in this helper, not globally (PYTHON-5874).
         with warnings.catch_warnings():
             warnings.filterwarnings(
                 "ignore",