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/test/_test_multiprocessing.py

@@ -6045,6 +6045,27 @@ class TestResourceTracker(unittest.TestCase):
                     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