bpo-39877: Fix PyEval_RestoreThread() for daemon threads (GH-18811)

* exit_thread_if_finalizing() does now access directly _PyRuntime
  variable, rather than using tstate->interp->runtime since tstate
  can be a dangling pointer after Py_Finalize() has been called.
* exit_thread_if_finalizing() is now called *before* calling
  take_gil(). _PyRuntime.finalizing is an atomic variable,
  we don't need to hold the GIL to access it.
* Add ensure_tstate_not_null() function to check that tstate is not
  NULL at runtime. Check tstate earlier. take_gil() does not longer
  check if tstate is NULL.

Cleanup:

* PyEval_RestoreThread() no longer saves/restores errno: it's already
  done inside take_gil().
* PyEval_AcquireLock(), PyEval_AcquireThread(),
  PyEval_RestoreThread() and _PyEval_EvalFrameDefault() now check if
  tstate is valid with the new is_tstate_valid() function which uses
  _PyMem_IsPtrFreed().

1 files changed, 2 insertions, 2 deletions

diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index 9e3b257..eaae5fd 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -1364,8 +1364,8 @@ Py_FinalizeEx(void)
     int malloc_stats = interp->config.malloc_stats;
 #endif
 
-    /* Remaining threads (e.g. daemon threads) will automatically exit
-       after taking the GIL (in PyEval_RestoreThread()). */
+    /* Remaining daemon threads will automatically exit
+       when they attempt to take the GIL (ex: PyEval_RestoreThread()). */
     _PyRuntimeState_SetFinalizing(runtime, tstate);
     runtime->initialized = 0;
     runtime->core_initialized = 0;