Fix race condition in Thread.join()

Author: pitrou

Lib/test/test_threading.py

@@ -482,6 +482,32 @@ def test_enumerate_after_join(self):
         finally:
             sys.setswitchinterval(old_interval)
 
+    def test_join_from_multiple_threads(self):
+        # Thread.join() should be thread-safe
+        errors = []
+
+        def worker():
+            time.sleep(0.005)
+
+        def joiner(thread):
+            try:
+                thread.join()
+            except Exception as e:
+                errors.append(e)
+
+        for N in range(2, 20):
+            threads = [threading.Thread(target=worker)]
+            for i in range(N):
+                threads.append(threading.Thread(target=joiner,
+                                                args=(threads[0],)))
+            for t in threads:
+                t.start()
+            time.sleep(0.01)
+            for t in threads:
+                t.join()
+            if errors:
+                raise errors[0]
+
     def test_no_refcycle_through_target(self):
         class RunSelfFunction(object):
             def __init__(self, should_raise):

Lib/threading.py

@@ -1165,13 +1165,15 @@ def join(self, timeout=None):
 
     def _join_os_thread(self):
         join_lock = self._join_lock
-        if join_lock is not None:
-            # Calling join() multiple times simultaneously would result in early
-            # return for one of the callers.
-            with join_lock:
+        if join_lock is None:
+            return
+        with join_lock:
+            # Calling join() multiple times simultaneously would raise
+            # an exception in one of the callers.
+            if self._join_lock is not None:
                 _join_thread(self._ident)
-            self._join_lock = None
-            self._non_joined_finalizer = None
+                self._join_lock = None
+                self._non_joined_finalizer = None
 
     def _wait_for_tstate_lock(self, block=True, timeout=-1):
         # Issue #18808: wait for the thread state to be gone.