bpo-19466: Py_Finalize() clears daemon threads earlier (GH-18848)

Clear the frames of daemon threads earlier during the Python shutdown to
call objects destructors. So "unclosed file" resource warnings are now
emitted for daemon threads in a more reliable way.

Cleanup _PyThreadState_DeleteExcept() code: rename "garbage" to
"list".

2 files changed, 22 insertions, 7 deletions

diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index c99c367..d00bf82 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -1373,6 +1373,16 @@ Py_FinalizeEx(void)
     runtime->initialized = 0;
     runtime->core_initialized = 0;
 
+    /* Destroy the state of all threads of the interpreter, except of the
+       current thread. In practice, only daemon threads should still be alive,
+       except if wait_for_thread_shutdown() has been cancelled by CTRL+C.
+       Clear frames of other threads to call objects destructors. Destructors
+       will be called in the current Python thread. Since
+       _PyRuntimeState_SetFinalizing() has been called, no other Python thread
+       can take the GIL at this point: if they try, they will exit
+       immediately. */
+    _PyThreadState_DeleteExcept(runtime, tstate);
+
     /* Flush sys.stdout and sys.stderr */
     if (flush_std_files() < 0) {
         status = -1;
diff --git a/Python/pystate.c b/Python/pystate.c
index a1eb523..f907fc1 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -895,25 +895,30 @@ void
 _PyThreadState_DeleteExcept(_PyRuntimeState *runtime, PyThreadState *tstate)
 {
     PyInterpreterState *interp = tstate->interp;
-    PyThreadState *p, *next, *garbage;
+
     HEAD_LOCK(runtime);
     /* Remove all thread states, except tstate, from the linked list of
        thread states.  This will allow calling PyThreadState_Clear()
        without holding the lock. */
-    garbage = interp->tstate_head;
-    if (garbage == tstate)
-        garbage = tstate->next;
-    if (tstate->prev)
+    PyThreadState *list = interp->tstate_head;
+    if (list == tstate) {
+        list = tstate->next;
+    }
+    if (tstate->prev) {
         tstate->prev->next = tstate->next;
-    if (tstate->next)
+    }
+    if (tstate->next) {
         tstate->next->prev = tstate->prev;
+    }
     tstate->prev = tstate->next = NULL;
     interp->tstate_head = tstate;
     HEAD_UNLOCK(runtime);
+
     /* Clear and deallocate all stale thread states.  Even if this
        executes Python code, we should be safe since it executes
        in the current thread, not one of the stale threads. */
-    for (p = garbage; p; p = next) {
+    PyThreadState *p, *next;
+    for (p = list; p; p = next) {
         next = p->next;
         PyThreadState_Clear(p);
         PyMem_RawFree(p);