gh-127586: properly restore blocked signals in resource_tracker.py (GH-127587)

* 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.

* Adding resource_tracker blocked signals test

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>

3 files changed, 22 insertions, 3 deletions

diff --git a/Lib/multiprocessing/resource_tracker.py b/Lib/multiprocessing/resource_tracker.py
index 20ddd9c..90e036a 100644
--- a/Lib/multiprocessing/resource_tracker.py
+++ b/Lib/multiprocessing/resource_tracker.py
@@ -155,13 +155,14 @@ class ResourceTracker(object):
                 # 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 80b08b8..01e92f0 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -6044,6 +6044,21 @@ class TestResourceTracker(unittest.TestCase):
                 self._test_resource_tracker_leak_resources(
                     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
+        signals = {signal.SIGTERM, signal.SIGINT, signal.SIGUSR1}
+
+        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()
 
 class TestSimpleQueue(unittest.TestCase):
 
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 0000000..80217bd
--- /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*.