Issue #13992: The trashcan mechanism is now thread-safe. This eliminates

sporadic crashes in multi-thread programs when several long deallocator
chains ran concurrently and involved subclasses of built-in container
types.

Because of this change, a couple extension modules compiled for 2.7.4
(those which use the trashcan mechanism, despite it being undocumented)
will not be loadable by 2.7.3 and earlier. However, extension modules
compiled for 2.7.3 and earlier will be loadable by 2.7.4.

Include/object.h

@@ -971,24 +971,33 @@ chain of N deallocations is broken into N / PyTrash_UNWIND_LEVEL pieces,
 with the call stack never exceeding a depth of PyTrash_UNWIND_LEVEL.
 */
 
+/* This is the old private API, invoked by the macros before 2.7.4.
+   Kept for binary compatibility of extensions. */
 PyAPI_FUNC(void) _PyTrash_deposit_object(PyObject*);
 PyAPI_FUNC(void) _PyTrash_destroy_chain(void);
 PyAPI_DATA(int) _PyTrash_delete_nesting;
 PyAPI_DATA(PyObject *) _PyTrash_delete_later;
 
+/* The new thread-safe private API, invoked by the macros below. */
+PyAPI_FUNC(void) _PyTrash_thread_deposit_object(PyObject*);
+PyAPI_FUNC(void) _PyTrash_thread_destroy_chain(void);
+
 #define PyTrash_UNWIND_LEVEL 50
 
 #define Py_TRASHCAN_SAFE_BEGIN(op) \
-    if (_PyTrash_delete_nesting < PyTrash_UNWIND_LEVEL) { \
-        ++_PyTrash_delete_nesting;
-        /* The body of the deallocator is here. */
+    do { \
+        PyThreadState *_tstate = PyThreadState_GET(); \
+        if (_tstate->trash_delete_nesting < PyTrash_UNWIND_LEVEL) { \
+            ++_tstate->trash_delete_nesting;
+            /* The body of the deallocator is here. */
 #define Py_TRASHCAN_SAFE_END(op) \
-        --_PyTrash_delete_nesting; \
-        if (_PyTrash_delete_later && _PyTrash_delete_nesting <= 0) \
-            _PyTrash_destroy_chain(); \
-    } \
-    else \
-        _PyTrash_deposit_object((PyObject*)op);
+            --_tstate->trash_delete_nesting; \
+            if (_tstate->trash_delete_later && _tstate->trash_delete_nesting <= 0) \
+                _PyTrash_thread_destroy_chain(); \
+        } \
+        else \
+            _PyTrash_thread_deposit_object((PyObject*)op); \
+    } while (0);
 
 #ifdef __cplusplus
 }

Include/pystate.h

@@ -95,6 +95,9 @@ typedef struct _ts {
     PyObject *async_exc; /* Asynchronous exception to raise */
     long thread_id; /* Thread id where this tstate was created */
 
+    int trash_delete_nesting;
+    PyObject *trash_delete_later;
+
     /* XXX signal handlers should also be here */
 
 } PyThreadState;

Lib/test/test_gc.py

@@ -1,9 +1,15 @@
 import unittest
 from test.test_support import verbose, run_unittest
 import sys
+import time
 import gc
 import weakref
 
+try:
+    import threading
+except ImportError:
+    threading = None
+
 ### Support code
 ###############################################################################
 
@@ -299,6 +305,69 @@ class GCTests(unittest.TestCase):
                 v = {1: v, 2: Ouch()}
         gc.disable()
 
+    @unittest.skipUnless(threading, "test meaningless on builds without threads")
+    def test_trashcan_threads(self):
+        # Issue #13992: trashcan mechanism should be thread-safe
+        NESTING = 60
+        N_THREADS = 2
+
+        def sleeper_gen():
+            """A generator that releases the GIL when closed or dealloc'ed."""
+            try:
+                yield
+            finally:
+                time.sleep(0.000001)
+
+        class C(list):
+            # Appending to a list is atomic, which avoids the use of a lock.
+            inits = []
+            dels = []
+            def __init__(self, alist):
+                self[:] = alist
+                C.inits.append(None)
+            def __del__(self):
+                # This __del__ is called by subtype_dealloc().
+                C.dels.append(None)
+                # `g` will release the GIL when garbage-collected.  This
+                # helps assert subtype_dealloc's behaviour when threads
+                # switch in the middle of it.
+                g = sleeper_gen()
+                next(g)
+                # Now that __del__ is finished, subtype_dealloc will proceed
+                # to call list_dealloc, which also uses the trashcan mechanism.
+
+        def make_nested():
+            """Create a sufficiently nested container object so that the
+            trashcan mechanism is invoked when deallocating it."""
+            x = C([])
+            for i in range(NESTING):
+                x = [C([x])]
+            del x
+
+        def run_thread():
+            """Exercise make_nested() in a loop."""
+            while not exit:
+                make_nested()
+
+        old_checkinterval = sys.getcheckinterval()
+        sys.setcheckinterval(3)
+        try:
+            exit = False
+            threads = []
+            for i in range(N_THREADS):
+                t = threading.Thread(target=run_thread)
+                threads.append(t)
+            for t in threads:
+                t.start()
+            time.sleep(1.0)
+            exit = True
+            for t in threads:
+                t.join()
+        finally:
+            sys.setcheckinterval(old_checkinterval)
+        gc.collect()
+        self.assertEqual(len(C.inits), len(C.dels))
+
     def test_boom(self):
         class Boom:
             def __getattr__(self, someattribute):

Misc/NEWS

@@ -9,6 +9,11 @@ What's New in Python 2.7.4
 Core and Builtins
 -----------------
 
+- Issue #13992: The trashcan mechanism is now thread-safe.  This eliminates
+  sporadic crashes in multi-thread programs when several long deallocator
+  chains ran concurrently and involved subclasses of built-in container
+  types.
+
 - Issue #15801: Make sure mappings passed to '%' formatting are actually
   subscriptable.
 

Objects/object.c

@@ -2428,6 +2428,18 @@ _PyTrash_deposit_object(PyObject *op)
     _PyTrash_delete_later = op;
 }
 
+/* The equivalent API, using per-thread state recursion info */
+void
+_PyTrash_thread_deposit_object(PyObject *op)
+{
+    PyThreadState *tstate = PyThreadState_GET();
+    assert(PyObject_IS_GC(op));
+    assert(_Py_AS_GC(op)->gc.gc_refs == _PyGC_REFS_UNTRACKED);
+    assert(op->ob_refcnt == 0);
+    _Py_AS_GC(op)->gc.gc_prev = (PyGC_Head *) tstate->trash_delete_later;
+    tstate->trash_delete_later = op;
+}
+
 /* Dealloccate all the objects in the _PyTrash_delete_later list.  Called when
  * the call-stack unwinds again.
  */
@@ -2454,6 +2466,31 @@ _PyTrash_destroy_chain(void)
     }
 }
 
+/* The equivalent API, using per-thread state recursion info */
+void
+_PyTrash_thread_destroy_chain(void)
+{
+    PyThreadState *tstate = PyThreadState_GET();
+    while (tstate->trash_delete_later) {
+        PyObject *op = tstate->trash_delete_later;
+        destructor dealloc = Py_TYPE(op)->tp_dealloc;
+
+        tstate->trash_delete_later =
+            (PyObject*) _Py_AS_GC(op)->gc.gc_prev;
+
+        /* Call the deallocator directly.  This used to try to
+         * fool Py_DECREF into calling it indirectly, but
+         * Py_DECREF was already called on this object, and in
+         * assorted non-release builds calling Py_DECREF again ends
+         * up distorting allocation statistics.
+         */
+        assert(op->ob_refcnt == 0);
+        ++tstate->trash_delete_nesting;
+        (*dealloc)(op);
+        --tstate->trash_delete_nesting;
+    }
+}
+
 #ifdef __cplusplus
 }
 #endif

Objects/typeobject.c

@@ -896,6 +896,7 @@ subtype_dealloc(PyObject *self)
 {
     PyTypeObject *type, *base;
     destructor basedealloc;
+    PyThreadState *tstate = PyThreadState_GET();
 
     /* Extract the type; we expect it to be a heap type */
     type = Py_TYPE(self);
@@ -945,8 +946,10 @@ subtype_dealloc(PyObject *self)
     /* See explanation at end of function for full disclosure */
     PyObject_GC_UnTrack(self);
     ++_PyTrash_delete_nesting;
+    ++ tstate->trash_delete_nesting;
     Py_TRASHCAN_SAFE_BEGIN(self);
     --_PyTrash_delete_nesting;
+    -- tstate->trash_delete_nesting;
     /* DO NOT restore GC tracking at this point.  weakref callbacks
      * (if any, and whether directly here or indirectly in something we
      * call) may trigger GC, and if self is tracked at that point, it
@@ -1025,8 +1028,10 @@ subtype_dealloc(PyObject *self)
 
   endlabel:
     ++_PyTrash_delete_nesting;
+    ++ tstate->trash_delete_nesting;
     Py_TRASHCAN_SAFE_END(self);
     --_PyTrash_delete_nesting;
+    -- tstate->trash_delete_nesting;
 
     /* Explanation of the weirdness around the trashcan macros:
 

Python/pystate.c

@@ -192,6 +192,9 @@ new_threadstate(PyInterpreterState *interp, int init)
         tstate->c_profileobj = NULL;
         tstate->c_traceobj = NULL;
 
+        tstate->trash_delete_nesting = 0;
+        tstate->trash_delete_later = NULL;
+
         if (init)
             _PyThreadState_Init(tstate);