From aeb9b65aa26444529e4adc7d6e5b0d3dd9889ec2 Mon Sep 17 00:00:00 2001
From: Stephen Hansen <stephen.paul.hansen@gmail.com>
Date: Fri, 27 Dec 2024 17:09:01 -0500
Subject: [PATCH] gh-127586: multiprocessing.Pool does not properly restore
 blocked signals (try 2) (GH-128011)

Correct pthread_sigmask in resource_tracker to restore old signals

Using SIG_UNBLOCK to remove blocked "ignored signals" may accidentally
cause side effects if the calling parent already had said signals
blocked to begin with and did not intend to unblock them when
creating a pool. Use SIG_SETMASK instead with the previous mask of
blocked signals to restore the original blocked set.

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>
---
 Lib/multiprocessing/resource_tracker.py       |  7 ++++---
 Lib/test/_test_multiprocessing.py             | 21 +++++++++++++++++++
 ...-12-03-20-28-08.gh-issue-127586.zgotYF.rst |  3 +++
 3 files changed, 28 insertions(+), 3 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2024-12-03-20-28-08.gh-issue-127586.zgotYF.rst

diff --git a/Lib/multiprocessing/resource_tracker.py b/Lib/multiprocessing/resource_tracker.py
index 20ddd9c50e3d88..90e036ae905afa 100644
--- a/Lib/multiprocessing/resource_tracker.py
+++ b/Lib/multiprocessing/resource_tracker.py
@@ -155,13 +155,14 @@ def ensure_running(self):
                 # that can make the child die before it registers signal handlers
                 # for SIGINT and SIGTERM. The mask is unregistered after spawning
                 # the child.
+                prev_sigmask = None
                 try:
                     if _HAVE_SIGMASK:
-                        signal.pthread_sigmask(signal.SIG_BLOCK, _IGNORED_SIGNALS)
+                        prev_sigmask = signal.pthread_sigmask(signal.SIG_BLOCK, _IGNORED_SIGNALS)
                     pid = util.spawnv_passfds(exe, args, fds_to_pass)
                 finally:
-                    if _HAVE_SIGMASK:
-                        signal.pthread_sigmask(signal.SIG_UNBLOCK, _IGNORED_SIGNALS)
+                    if prev_sigmask is not None:
+                        signal.pthread_sigmask(signal.SIG_SETMASK, prev_sigmask)
             except:
                 os.close(w)
                 raise
diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index 80b08b8ac66899..38a03f3391d31d 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -6045,6 +6045,27 @@ def test_resource_tracker_exit_code(self):
                     cleanup=cleanup,
                 )
 
+    @unittest.skipUnless(hasattr(signal, "pthread_sigmask"), "pthread_sigmask is not available")
+    def test_resource_tracker_blocked_signals(self):
+        #
+        # gh-127586: Check that resource_tracker does not override blocked signals of caller.
+        #
+        from multiprocessing.resource_tracker import ResourceTracker
+        orig_sigmask = signal.pthread_sigmask(signal.SIG_BLOCK, set())
+        signals = {signal.SIGTERM, signal.SIGINT, signal.SIGUSR1}
+
+        try:
+            for sig in signals:
+                signal.pthread_sigmask(signal.SIG_SETMASK, {sig})
+                self.assertEqual(signal.pthread_sigmask(signal.SIG_BLOCK, set()), {sig})
+                tracker = ResourceTracker()
+                tracker.ensure_running()
+                self.assertEqual(signal.pthread_sigmask(signal.SIG_BLOCK, set()), {sig})
+                tracker._stop()
+        finally:
+            # restore sigmask to what it was before executing test
+            signal.pthread_sigmask(signal.SIG_SETMASK, orig_sigmask)
+
 class TestSimpleQueue(unittest.TestCase):
 
     @classmethod
diff --git a/Misc/NEWS.d/next/Library/2024-12-03-20-28-08.gh-issue-127586.zgotYF.rst b/Misc/NEWS.d/next/Library/2024-12-03-20-28-08.gh-issue-127586.zgotYF.rst
new file mode 100644
index 00000000000000..80217bd4a10503
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-12-03-20-28-08.gh-issue-127586.zgotYF.rst
@@ -0,0 +1,3 @@
+:class:`multiprocessing.pool.Pool` now properly restores blocked signal handlers
+of the parent thread when creating processes via either *spawn* or
+*forkserver*.