avoid crashes and lockups from daemon threads during interpreter shutdown (#1856)

Python/ceval.c

@@ -355,6 +355,12 @@ 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

Python/pythonrun.c

@@ -91,6 +91,8 @@ 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) = \
@@ -163,6 +165,7 @@ 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);
@@ -422,12 +425,16 @@ 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