gh-114423: Remove DummyThread from threading._active when thread dies (GH-114424)

Author: fabioz

Lib/test/test_threading.py

@@ -227,8 +227,6 @@ def f():
             tid = _thread.start_new_thread(f, ())
             done.wait()
             self.assertEqual(ident[0], tid)
-        # Kill the "immortal" _DummyThread
-        del threading._active[ident[0]]
 
     # run with a small(ish) thread stack size (256 KiB)
     def test_various_ops_small_stack(self):
@@ -256,32 +254,55 @@ def test_various_ops_large_stack(self):
 
     def test_foreign_thread(self):
         # Check that a "foreign" thread can use the threading module.
+        dummy_thread = None
+        error = None
         def f(mutex):
-            # Calling current_thread() forces an entry for the foreign
-            # thread to get made in the threading._active map.
-            threading.current_thread()
-            mutex.release()
+            try:
+                nonlocal dummy_thread
+                nonlocal error
+                # Calling current_thread() forces an entry for the foreign
+                # thread to get made in the threading._active map.
+                dummy_thread = threading.current_thread()
+                tid = dummy_thread.ident
+                self.assertIn(tid, threading._active)
+                self.assertIsInstance(dummy_thread, threading._DummyThread)
+                self.assertIs(threading._active.get(tid), dummy_thread)
+                # gh-29376
+                self.assertTrue(
+                    dummy_thread.is_alive(),
+                    'Expected _DummyThread to be considered alive.'
+                )
+                self.assertIn('_DummyThread', repr(dummy_thread))
+            except BaseException as e:
+                error = e
+            finally:
+                mutex.release()
 
         mutex = threading.Lock()
         mutex.acquire()
         with threading_helper.wait_threads_exit():
             tid = _thread.start_new_thread(f, (mutex,))
             # Wait for the thread to finish.
             mutex.acquire()
-        self.assertIn(tid, threading._active)
-        self.assertIsInstance(threading._active[tid], threading._DummyThread)
-        #Issue 29376
-        self.assertTrue(threading._active[tid].is_alive())
-        self.assertRegex(repr(threading._active[tid]), '_DummyThread')
-
+        if error is not None:
+            raise error
+        self.assertEqual(tid, dummy_thread.ident)
         # Issue gh-106236:
         with self.assertRaises(RuntimeError):
-            threading._active[tid].join()
-        threading._active[tid]._started.clear()
+            dummy_thread.join()
+        dummy_thread._started.clear()
         with self.assertRaises(RuntimeError):
-            threading._active[tid].is_alive()
-
-        del threading._active[tid]
+            dummy_thread.is_alive()
+        # Busy wait for the following condition: after the thread dies, the
+        # related dummy thread must be removed from threading._active.
+        timeout = 5
+        timeout_at = time.monotonic() + timeout
+        while time.monotonic() < timeout_at:
+            if threading._active.get(dummy_thread.ident) is not dummy_thread:
+                break
+            time.sleep(.1)
+        else:
+            self.fail('It was expected that the created threading._DummyThread was removed from threading._active.')
 
     # PyThreadState_SetAsyncExc() is a CPython-only gimmick, not (currently)
     # exposed at the Python level.  This test relies on ctypes to get at it.

Lib/threading.py

@@ -54,6 +54,13 @@
 TIMEOUT_MAX = _thread.TIMEOUT_MAX
 del _thread
 
+# get thread-local implementation, either from the thread
+# module, or from the python fallback
+
+try:
+    from _thread import _local as local
+except ImportError:
+    from _threading_local import local
 
 # Support for profile and trace hooks
 
@@ -1476,10 +1483,36 @@ def __init__(self):
             _active[self._ident] = self
 
 
+# Helper thread-local instance to detect when a _DummyThread
+# is collected. Not a part of the public API.
+_thread_local_info = local()
+
+
+class _DeleteDummyThreadOnDel:
+    '''
+    Helper class to remove a dummy thread from threading._active on __del__.
+    '''
+
+    def __init__(self, dummy_thread):
+        self._dummy_thread = dummy_thread
+        self._tident = dummy_thread.ident
+        # Put the thread on a thread local variable so that when
+        # the related thread finishes this instance is collected.
+        #
+        # Note: no other references to this instance may be created.
+        # If any client code creates a reference to this instance,
+        # the related _DummyThread will be kept forever!
+        _thread_local_info._track_dummy_thread_ref = self
+
+    def __del__(self):
+        with _active_limbo_lock:
+            if _active.get(self._tident) is self._dummy_thread:
+                _active.pop(self._tident, None)
+
+
 # Dummy thread class to represent threads not started here.
-# These aren't garbage collected when they die, nor can they be waited for.
-# If they invoke anything in threading.py that calls current_thread(), they
-# leave an entry in the _active dict forever after.
+# These should be added to `_active` and removed automatically
+# when they die, although they can't be waited for.
 # Their purpose is to return *something* from current_thread().
 # They are marked as daemon threads so we won't wait for them
 # when we exit (conform previous semantics).
@@ -1495,6 +1528,7 @@ def __init__(self):
             self._set_native_id()
         with _active_limbo_lock:
             _active[self._ident] = self
+        _DeleteDummyThreadOnDel(self)
 
     def _stop(self):
         pass
@@ -1676,14 +1710,6 @@ def main_thread():
     # XXX Figure this out for subinterpreters.  (See gh-75698.)
     return _main_thread
 
-# get thread-local implementation, either from the thread
-# module, or from the python fallback
-
-try:
-    from _thread import _local as local
-except ImportError:
-    from _threading_local import local
-
 
 def _after_fork():
     """

Misc/NEWS.d/next/Library/2024-01-22-11-43-38.gh-issue-114423.6mMoPH.rst

@@ -0,0 +1 @@
+``_DummyThread`` entries in ``threading._active`` are now automatically removed when the related thread dies.