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.

Note that the trashcan functions are part of the stable ABI, therefore
they have to be kept around for binary compatibility of extensions.

2 files changed, 21 insertions, 9 deletions

diff --git a/Include/object.h b/Include/object.h
index 709a9ff..387cadb 100644
--- a/Include/object.h
+++ b/Include/object.h
@@ -961,24 +961,33 @@ 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 3.2.4.
+   Kept for binary compatibility of extensions using the stable ABI. */
 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);
 
 #ifndef Py_LIMITED_API
 PyAPI_FUNC(void)
diff --git a/Include/pystate.h b/Include/pystate.h
index 25c2faa..2017b02 100644
--- a/Include/pystate.h
+++ b/Include/pystate.h
@@ -114,6 +114,9 @@ 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;