[3.13] gh-142048: Fix lost gc allocations count on thread cleanup (GH… (#142506)

[3.13] gh-142048: Fix lost gc allocations count on thread cleanup (GH-142233)
(cherry picked from commit 49b1fb4)

Co-authored-by: Kevin Wang <kevmo314@gmail.com>

Author: kumaraditya303

Python/pystate.c

@@ -1743,6 +1743,14 @@ PyThreadState_Clear(PyThreadState *tstate)
 
     // Remove ourself from the biased reference counting table of threads.
     _Py_brc_remove_thread(tstate);
+
+    // Flush the thread's local GC allocation count to the global count
+    // before the thread state is cleared, otherwise the count is lost.
+    _PyThreadStateImpl *tstate_impl = (_PyThreadStateImpl *)tstate;
+    _Py_atomic_add_int(&tstate->interp->gc.generations[0].count,
+                       (int)tstate_impl->gc.alloc_count);
+    tstate_impl->gc.alloc_count = 0;
+
 #endif
 
     // Merge our queue of pointers to be freed into the interpreter queue.