Issue #7481: When a threading.Thread failed to start it would leave the

instance stuck in initial state and present in threading.enumerate().

Lib/test/test_threading.py

@@ -249,6 +249,25 @@ class ThreadTests(BaseTestCase):
             t.join()
         # else the thread is still running, and we have no way to kill it
 
+    def test_limbo_cleanup(self):
+        # Issue 7481: Failure to start thread should cleanup the limbo map.
+        def fail_new_thread(*args):
+            raise thread.error()
+        _start_new_thread = threading._start_new_thread
+        threading._start_new_thread = fail_new_thread
+        try:
+            t = threading.Thread(target=lambda: None)
+            try:
+                t.start()
+                assert False
+            except thread.error:
+                self.assertFalse(
+                    t in threading._limbo,
+                    "Failed to cleanup _limbo map on failure of Thread.start()."
+                )
+        finally:
+            threading._start_new_thread = _start_new_thread
+
     def test_finalize_runnning_thread(self):
         # Issue 1402: the PyGILState_Ensure / _Release functions may be called
         # very late on python exit: on deallocation of a running thread for

Lib/threading.py

@@ -469,7 +469,12 @@ class Thread(_Verbose):
             self._note("%s.start(): starting thread", self)
         with _active_limbo_lock:
             _limbo[self] = self
-        _start_new_thread(self.__bootstrap, ())
+        try:
+            _start_new_thread(self.__bootstrap, ())
+        except Exception:
+            with _active_limbo_lock:
+                del _limbo[self]
+            raise
         self.__started.wait()
 
     def run(self):

Misc/NEWS

@@ -72,6 +72,9 @@ Library
 
 - Issue #7427: improve the representation of httplib.BadStatusLine exceptions.
 
+- Issue #7481: When a threading.Thread failed to start it would leave the
+  instance stuck in initial state and present in threading.enumerate().
+
 Extension Modules
 -----------------