bpo-30341: Improve _PyTrash_thread_destroy_chain() a little bit (#1545)

* add a comment about why we need to increase trash_delete_nesting
* move increase and decrese outside of the loop

Include/object.h

@@ -1029,7 +1029,7 @@ without deallocating anything (and so unbounded call-stack depth is avoided).
 When the call stack finishes unwinding again, code generated by the END macro
 notices this, and calls another routine to deallocate all the objects that
 may have been added to the list of deferred deallocations.  In effect, a
-chain of N deallocations is broken into N / PyTrash_UNWIND_LEVEL pieces,
+chain of N deallocations is broken into (N-1)/(PyTrash_UNWIND_LEVEL-1) pieces,
 with the call stack never exceeding a depth of PyTrash_UNWIND_LEVEL.
 */
 

Objects/object.c

@@ -2093,6 +2093,19 @@ void
 _PyTrash_thread_destroy_chain(void)
 {
     PyThreadState *tstate = PyThreadState_GET();
+    /* We need to increase trash_delete_nesting here, otherwise,
+       _PyTrash_thread_destroy_chain will be called recursively
+       and then possibly crash.  An example that may crash without
+       increase:
+           N = 500000  # need to be large enough
+           ob = object()
+           tups = [(ob,) for i in range(N)]
+           for i in range(49):
+               tups = [(tup,) for tup in tups]
+           del tups
+    */
+    assert(tstate->trash_delete_nesting == 0);
+    ++tstate->trash_delete_nesting;
     while (tstate->trash_delete_later) {
         PyObject *op = tstate->trash_delete_later;
         destructor dealloc = Py_TYPE(op)->tp_dealloc;
@@ -2107,10 +2120,10 @@ _PyTrash_thread_destroy_chain(void)
          * up distorting allocation statistics.
          */
         assert(op->ob_refcnt == 0);
-        ++tstate->trash_delete_nesting;
         (*dealloc)(op);
-        --tstate->trash_delete_nesting;
+        assert(tstate->trash_delete_nesting == 1);
     }
+    --tstate->trash_delete_nesting;
 }
 
 #ifndef Py_TRACE_REFS