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 3.2.4
(those which use the trashcan mechanism, despite it being undocumented)
will not be loadable by 3.2.3 and earlier. However, extension modules
compiled for 3.2.3 and earlier will be loadable by 3.2.4.

1 files changed, 69 insertions, 0 deletions

diff --git a/Lib/test/test_gc.py b/Lib/test/test_gc.py
index d35f9ed..c59b72e 100644
--- a/Lib/test/test_gc.py
+++ b/Lib/test/test_gc.py
@@ -2,9 +2,15 @@ import unittest
 from test.support import (verbose, refcount_test, run_unittest,
                             strip_python_stderr)
 import sys
+import time
 import gc
 import weakref
 
+try:
+    import threading
+except ImportError:
+    threading = None
+
 ### Support code
 ###############################################################################
 
@@ -328,6 +334,69 @@ 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_switchinterval = sys.getswitchinterval()
+        sys.setswitchinterval(1e-5)
+        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.setswitchinterval(old_switchinterval)
+        gc.collect()
+        self.assertEqual(len(C.inits), len(C.dels))
+
     def test_boom(self):
         class Boom:
             def __getattr__(self, someattribute):