Issue #21963: backout issue #1856 patch (avoid crashes and lockups when

daemon threads run while the interpreter is shutting down; instead,
these threads are now killed when they try to take the GIL), as it seems
to break some existing code.

Include/pythonrun.h

@@ -147,8 +147,6 @@ PyAPI_FUNC(void) PyOS_FiniInterrupts(void);
 PyAPI_FUNC(void) PyByteArray_Fini(void);
 PyAPI_FUNC(void) _PyRandom_Fini(void);
 
-PyAPI_DATA(PyThreadState *) _Py_Finalizing;
-
 /* Stuff with no proper home (yet) */
 PyAPI_FUNC(char *) PyOS_Readline(FILE *, FILE *, char *);
 PyAPI_DATA(int) (*PyOS_InputHook)(void);

Lib/test/test_threading.py

@@ -700,49 +700,6 @@ class ThreadJoinOnShutdown(BaseTestCase):
         output = "end of worker thread\nend of main thread\n"
         self.assertScriptHasOutput(script, output)
 
-    @unittest.skipIf(sys.platform in platforms_to_skip, "due to known OS bug")
-    def test_6_daemon_threads(self):
-        # Check that a daemon thread cannot crash the interpreter on shutdown
-        # by manipulating internal structures that are being disposed of in
-        # the main thread.
-        script = """if True:
-            import os
-            import random
-            import sys
-            import time
-            import threading
-
-            thread_has_run = set()
-
-            def random_io():
-                '''Loop for a while sleeping random tiny amounts and doing some I/O.'''
-                while True:
-                    in_f = open(os.__file__, 'rb')
-                    stuff = in_f.read(200)
-                    null_f = open(os.devnull, 'wb')
-                    null_f.write(stuff)
-                    time.sleep(random.random() / 1995)
-                    null_f.close()
-                    in_f.close()
-                    thread_has_run.add(threading.current_thread())
-
-            def main():
-                count = 0
-                for _ in range(40):
-                    new_thread = threading.Thread(target=random_io)
-                    new_thread.daemon = True
-                    new_thread.start()
-                    count += 1
-                while len(thread_has_run) < count:
-                    time.sleep(0.001)
-                # Trigger process shutdown
-                sys.exit(0)
-
-            main()
-            """
-        rc, out, err = assert_python_ok('-c', script)
-        self.assertFalse(err)
-
     @unittest.skipUnless(hasattr(os, 'fork'), "needs os.fork()")
     @unittest.skipIf(sys.platform in platforms_to_skip, "due to known OS bug")
     def test_reinit_tls_after_fork(self):

Misc/NEWS

@@ -10,6 +10,11 @@ What's New in Python 2.7.9?
 Core and Builtins
 -----------------
 
+- Backout issue #1856 patch (avoid crashes and lockups when daemon threads
+  run while the interpreter is shutting down; instead, these threads are
+  now killed when they try to take the GIL), as it seems to break some
+  existing code.
+
 - Issue #22604: Fix assertion error in debug mode when dividing a complex
   number by (nan+0j).
 
@@ -360,10 +365,6 @@ Core and Builtins
 - Issue #21831: Avoid integer overflow when large sizes and offsets are given to
   the buffer type. CVE-2014-7185.
 
-- Issue #1856: Avoid crashes and lockups when daemon threads run while the
-  interpreter is shutting down; instead, these threads are now killed when they
-  try to take the GIL.
-
 - Issue #19656: Running Python with the -3 option now also warns about
   non-ascii bytes literals.
 

Python/ceval.c

@@ -355,12 +355,6 @@ PyEval_RestoreThread(PyThreadState *tstate)
     if (interpreter_lock) {
         int err = errno;
         PyThread_acquire_lock(interpreter_lock, 1);
-        /* _Py_Finalizing is protected by the GIL */
-        if (_Py_Finalizing && tstate != _Py_Finalizing) {
-            PyThread_release_lock(interpreter_lock);
-            PyThread_exit_thread();
-            assert(0);  /* unreachable */
-        }
         errno = err;
     }
 #endif
@@ -1025,12 +1019,6 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 
                 PyThread_acquire_lock(interpreter_lock, 1);
 
-                /* Check if we should make a quick exit. */
-                if (_Py_Finalizing && _Py_Finalizing != tstate) {
-                    PyThread_release_lock(interpreter_lock);
-                    PyThread_exit_thread();
-                }
-
                 if (PyThreadState_Swap(tstate) != NULL)
                     Py_FatalError("ceval: orphan tstate");
 

Python/pythonrun.c

@@ -91,8 +91,6 @@ int _Py_QnewFlag = 0;
 int Py_NoUserSiteDirectory = 0; /* for -s and site.py */
 int Py_HashRandomizationFlag = 0; /* for -R and PYTHONHASHSEED */
 
-PyThreadState *_Py_Finalizing = NULL;
-
 
 /* Hack to force loading of object files */
 int (*_PyOS_mystrnicmp_hack)(const char *, const char *, Py_ssize_t) = \
@@ -165,7 +163,6 @@ Py_InitializeEx(int install_sigs)
     if (initialized)
         return;
     initialized = 1;
-    _Py_Finalizing = NULL;
 
     if ((p = Py_GETENV("PYTHONDEBUG")) && *p != '\0')
         Py_DebugFlag = add_flag(Py_DebugFlag, p);
@@ -425,16 +422,12 @@ Py_Finalize(void)
      * the threads created via Threading.
      */
     call_sys_exitfunc();
+    initialized = 0;
 
     /* Get current thread state and interpreter pointer */
     tstate = PyThreadState_GET();
     interp = tstate->interp;
 
-    /* Remaining threads (e.g. daemon threads) will automatically exit
-       after taking the GIL (in PyEval_RestoreThread()). */
-    _Py_Finalizing = tstate;
-    initialized = 0;
-
     /* Disable signal handling */
     PyOS_FiniInterrupts();
 

Python/thread_pthread.h

@@ -242,9 +242,9 @@ void
 PyThread_exit_thread(void)
 {
     dprintf(("PyThread_exit_thread called\n"));
-    if (!initialized)
+    if (!initialized) {
         exit(0);
-    pthread_exit(0);
+    }
 }
 
 #ifdef USE_SEMAPHORES