GH-110829: Ensure Thread.join() joins the OS thread (#110848)

Joining a thread now ensures the underlying OS thread has exited. This is required for safer fork() in multi-threaded processes.

---------

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>

Lib/test/test_thread.py

@@ -160,6 +160,132 @@ class ThreadRunningTests(BasicThreadTest):
                              f"Exception ignored in thread started by {task!r}")
             self.assertIsNotNone(cm.unraisable.exc_traceback)
 
+    def test_join_thread(self):
+        finished = []
+
+        def task():
+            time.sleep(0.05)
+            finished.append(thread.get_ident())
+
+        with threading_helper.wait_threads_exit():
+            handle = thread.start_joinable_thread(task)
+            handle.join()
+            self.assertEqual(len(finished), 1)
+            self.assertEqual(handle.ident, finished[0])
+
+    def test_join_thread_already_exited(self):
+        def task():
+            pass
+
+        with threading_helper.wait_threads_exit():
+            handle = thread.start_joinable_thread(task)
+            time.sleep(0.05)
+            handle.join()
+
+    def test_join_several_times(self):
+        def task():
+            pass
+
+        with threading_helper.wait_threads_exit():
+            handle = thread.start_joinable_thread(task)
+            handle.join()
+            with self.assertRaisesRegex(ValueError, "not joinable"):
+                handle.join()
+
+    def test_joinable_not_joined(self):
+        handle_destroyed = thread.allocate_lock()
+        handle_destroyed.acquire()
+
+        def task():
+            handle_destroyed.acquire()
+
+        with threading_helper.wait_threads_exit():
+            handle = thread.start_joinable_thread(task)
+            del handle
+            handle_destroyed.release()
+
+    def test_join_from_self(self):
+        errors = []
+        handles = []
+        start_joinable_thread_returned = thread.allocate_lock()
+        start_joinable_thread_returned.acquire()
+        task_tried_to_join = thread.allocate_lock()
+        task_tried_to_join.acquire()
+
+        def task():
+            start_joinable_thread_returned.acquire()
+            try:
+                handles[0].join()
+            except Exception as e:
+                errors.append(e)
+            finally:
+                task_tried_to_join.release()
+
+        with threading_helper.wait_threads_exit():
+            handle = thread.start_joinable_thread(task)
+            handles.append(handle)
+            start_joinable_thread_returned.release()
+            # Can still join after joining failed in other thread
+            task_tried_to_join.acquire()
+            handle.join()
+
+        assert len(errors) == 1
+        with self.assertRaisesRegex(RuntimeError, "Cannot join current thread"):
+            raise errors[0]
+
+    def test_detach_from_self(self):
+        errors = []
+        handles = []
+        start_joinable_thread_returned = thread.allocate_lock()
+        start_joinable_thread_returned.acquire()
+        thread_detached = thread.allocate_lock()
+        thread_detached.acquire()
+
+        def task():
+            start_joinable_thread_returned.acquire()
+            try:
+                handles[0].detach()
+            except Exception as e:
+                errors.append(e)
+            finally:
+                thread_detached.release()
+
+        with threading_helper.wait_threads_exit():
+            handle = thread.start_joinable_thread(task)
+            handles.append(handle)
+            start_joinable_thread_returned.release()
+            thread_detached.acquire()
+            with self.assertRaisesRegex(ValueError, "not joinable"):
+                handle.join()
+
+        assert len(errors) == 0
+
+    def test_detach_then_join(self):
+        lock = thread.allocate_lock()
+        lock.acquire()
+
+        def task():
+            lock.acquire()
+
+        with threading_helper.wait_threads_exit():
+            handle = thread.start_joinable_thread(task)
+            # detach() returns even though the thread is blocked on lock
+            handle.detach()
+            # join() then cannot be called anymore
+            with self.assertRaisesRegex(ValueError, "not joinable"):
+                handle.join()
+            lock.release()
+
+    def test_join_then_detach(self):
+        def task():
+            pass
+
+        with threading_helper.wait_threads_exit():
+            handle = thread.start_joinable_thread(task)
+            handle.join()
+            with self.assertRaisesRegex(ValueError, "not joinable"):
+                handle.detach()
+
 
 class Barrier:
     def __init__(self, num_threads):