SF bug 839548: Bug in type's GC handling causes segfaults.

Also SF patch 843455.

This is a critical bugfix.
I'll backport to 2.3 maint, but not beyond that.  The bugs this fixes
have been there since weakrefs were introduced.

2 files changed, 236 insertions, 13 deletions

diff --git a/Modules/gc_weakref.txt b/Modules/gc_weakref.txt
new file mode 100644
index 0000000..b07903b
--- /dev/null
+++ b/Modules/gc_weakref.txt
@@ -0,0 +1,107 @@
+Before 2.3.3, Python's cyclic gc didn't pay any attention to weakrefs.
+Segfaults in Zope3 resulted.
+
+weakrefs in Python are designed to, at worst, let *other* objects learn
+that a given object has died, via a callback function.  The weakly
+referenced object itself is not passed to the callback, and the presumption
+is that the weakly referenced object is unreachable trash at the time the
+callback is invoked.
+
+That's usually true, but not always.  Suppose a weakly referenced object
+becomes part of a clump of cyclic trash.  When enough cycles are broken by
+cyclic gc that the object is reclaimed, the callback is invoked.  If it's
+possible for the callback to get at objects in the cycle(s), then it may be
+possible for those objects to access (via strong references in the cycle)
+the weakly referenced object being torn down, or other objects in the cycle
+that have already suffered a tp_clear() call.  There's no guarantee that an
+object is in a sane state after tp_clear().  Bad things (including
+segfaults) can happen right then, during the callback's execution, or can
+happen at any later time if the callback manages to resurrect an insane
+object.
+
+Note that if it's possible for the callback to get at objects in the trash
+cycles, it must also be the case that the callback itself is part of the
+trash cycles.  Else the callback would have acted as an external root to
+the current collection, and nothing reachable from it would be in cyclic
+trash either.
+
+More, if the callback itself is in cyclic trash, then the weakref to which
+the callback is attached must also be trash, and for the same kind of
+reason:  if the weakref acted as an external root, then the callback could
+not have been cyclic trash.
+
+So a problem here requires that a weakref, that weakref's callback, and the
+weakly referenced object, all be in cyclic trash at the same time.  This
+isn't easy to stumble into by accident while Python is running, and, indeed,
+it took quite a while to dream up failing test cases.  Zope3 saw segfaults
+during shutdown, during the second call of gc in Py_Finalize, after most
+modules had been torn down.  That creates many trash cycles (esp. those
+involving new-style classes), making the problem much more likely.  Once you
+know what's required to provoke the problem, though, it's easy to create
+tests that segfault before shutdown.
+
+In 2.3.3, before breaking cycles, we first clear all the weakrefs with
+callbacks in cyclic trash.  Since the weakrefs *are* trash, and there's no
+defined-- or even predictable --order in which tp_clear() gets called on
+cyclic trash, it's defensible to first clear weakrefs with callbacks.  It's
+a feature of Python's weakrefs too that when a weakref goes away, the
+callback (if any) associated with it is thrown away too, unexecuted.
+
+Just that much is almost enough to prevent problems, by throwing away
+*almost* all the weakref callbacks that could get triggered by gc.  The
+problem remaining is that clearing a weakref with a callback decrefs the
+callback object, and the callback object may *itself* be weakly referenced,
+via another weakref with another callback.  So the process of clearing
+weakrefs can trigger callbacks attached to other weakrefs, and those
+latter weakrefs may or may not be part of cyclic trash.
+
+So, to prevent any Python code from running while gc is invoking tp_clear()
+on all the objects in cyclic trash, it's not quite enough just to invoke
+tp_clear() on weakrefs with callbacks first.  Instead the weakref module
+grew a new private function (_PyWeakref_ClearRef) that does only part of
+tp_clear():  it removes the weakref from the weakly-referenced object's list
+of weakrefs, but does not decref the callback object.  So calling
+_PyWeakref_ClearRef(wr) ensures that wr's callback object will never
+trigger, and (unlike weakref's tp_clear()) also prevents any callback
+associated *with* wr's callback object from triggering.
+
+Then we can call tp_clear on all the cyclic objects and never trigger
+Python code.
+
+After we do that, the callback objects still need to be decref'ed.  Callbacks
+(if any) *on* the callback objects that were also part of cyclic trash won't
+get invoked, because we cleared all trash weakrefs with callbacks at the
+start.  Callbacks on the callback objects that were not part of cyclic trash
+acted as external roots to everything reachable from them, so nothing
+reachable from them was part of cyclic trash, so gc didn't do any damage to
+objects reachable from them, and it's safe to call them at the end of gc.
+
+An alternative would have been to treat objects with callbacks like objects
+with __del__ methods, refusing to collect them, appending them to gc.garbage
+instead.  That would have been much easier.  Jim Fulton gave a strong
+argument against that (on Python-Dev):
+
+    There's a big difference between __del__ and weakref callbacks.
+    The __del__ method is "internal" to a design.  When you design a
+    class with a del method, you know you have to avoid including the
+    class in cycles.
+
+    Now, suppose you have a design that makes has no __del__ methods but
+    that does use cyclic data structures.  You reason about the design,
+    run tests, and convince yourself you don't have a leak.
+
+    Now, suppose some external code creates a weakref to one of your
+    objects.  All of a sudden, you start leaking.  You can look at your
+    code all you want and you won't find a reason for the leak.
+
+IOW, a class designer can out-think __del__ problems, but has no control
+over who creates weakrefs to his classes or class instances.  The class
+user has little chance either of predicting when the weakrefs he creates
+may end up in cycles.
+
+Callbacks on weakref callbacks are executed in an arbitrary order, and
+that's not good (a primary reason not to collect cycles with objects with
+__del__ methods is to avoid running finalizers in an arbitrary order).
+However, a weakref callback on a weakref callback has got to be rare.
+It's possible to do such a thing, so gc has to be robust against it, but
+I doubt anyone has done it outside the test case I wrote for it.
diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index e6aabe4..7976b40 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -396,13 +396,17 @@ has_finalizer(PyObject *op)
 		return 0;
 }
 
-/* Move the objects in unreachable with __del__ methods into finalizers.
- * The objects remaining in unreachable do not have __del__ methods, and
- * gc_refs remains GC_TENTATIVELY_UNREACHABLE for them.  The objects
- * moved into finalizers have gc_refs changed to GC_REACHABLE.
+/* Move the objects in unreachable with __del__ methods into finalizers,
+ * and weakrefs with callbacks into wr_callbacks.
+ * The objects remaining in unreachable do not have __del__ methods, and are
+ * not weakrefs with callbacks.
+ * The objects moved have gc_refs changed to GC_REACHABLE; the objects
+ * remaining in unreachable are left at GC_TENTATIVELY_UNREACHABLE.
  */
 static void
-move_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)
+move_troublemakers(PyGC_Head *unreachable,
+		   PyGC_Head *finalizers,
+		   PyGC_Head *wr_callbacks)
 {
 	PyGC_Head *gc = unreachable->gc.gc_next;
 
@@ -417,6 +421,12 @@ move_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)
 			gc_list_append(gc, finalizers);
 			gc->gc.gc_refs = GC_REACHABLE;
 		}
+		else if (PyWeakref_Check(op) &&
+			 ((PyWeakReference *)op)->wr_callback) {
+			gc_list_remove(gc);
+			gc_list_append(gc, wr_callbacks);
+			gc->gc.gc_refs = GC_REACHABLE;
+		}
 		gc = next;
 	}
 }
@@ -453,6 +463,93 @@ move_finalizer_reachable(PyGC_Head *finalizers)
 	}
 }
 
+/* Clear all trash weakrefs with callbacks.  This clears weakrefs first,
+ * which has the happy result of disabling the callbacks without executing
+ * them.  A nasty technical complication:  a weakref callback can itself be
+ * the target of a weakref, in which case decrefing the callback can cause
+ * another callback to trigger.  But we can't allow arbitrary Python code to
+ * get executed at this point (the callback on the callback may try to muck
+ * with other cyclic trash we're trying to collect, even resurrecting it
+ * while we're in the middle of doing tp_clear() on the trash).
+ *
+ * The private _PyWeakref_ClearRef() function exists so that we can clear
+ * the reference in a weakref without triggering a callback on the callback.
+ *
+ * We have to save the callback objects and decref them later.  But we can't
+ * allocate new memory to save them (if we can't get new memory, we're dead).
+ * So we grab a new reference on the clear'ed weakref, which prevents the
+ * rest of gc from reclaiming it.  _PyWeakref_ClearRef() leaves the
+ * weakref's wr_callback member intact.
+ *
+ * In the end, then, wr_callbacks consists of cleared weakrefs that are
+ * immune from collection.  Near the end of gc, after collecting all the
+ * cyclic trash, we call release_weakrefs().  That releases our references
+ * to the cleared weakrefs, which in turn may trigger callbacks on their
+ * callbacks.
+ */
+static void
+clear_weakrefs(PyGC_Head *wr_callbacks)
+{
+	PyGC_Head *gc = wr_callbacks->gc.gc_next;
+
+	for (; gc != wr_callbacks; gc = gc->gc.gc_next) {
+		PyObject *op = FROM_GC(gc);
+		PyWeakReference *wr;
+
+		assert(IS_REACHABLE(op));
+		assert(PyWeakref_Check(op));
+		wr = (PyWeakReference *)op;
+		assert(wr->wr_callback != NULL);
+		Py_INCREF(op);
+		_PyWeakref_ClearRef(wr);
+	}
+}
+
+/* Called near the end of gc.  This gives up the references we own to
+ * cleared weakrefs, allowing them to get collected, and in turn decref'ing
+ * their callbacks.
+ *
+ * If a callback object is itself the target of a weakref callback,
+ * decref'ing the callback object may trigger that other callback.  If
+ * that other callback was part of the cyclic trash in this generation,
+ * that won't happen, since we cleared *all* trash-weakref callbacks near
+ * the start of gc.  If that other callback was not part of the cyclic trash
+ * in this generation, then it acted like an external root to this round
+ * of gc, so all the objects reachable from that callback are still alive.
+ *
+ * Giving up the references to the weakref objects will probably make
+ * them go away too.  However, if a weakref is reachable from finalizers,
+ * it won't go away.  We move it to the old generation then.  Since a
+ * weakref object doesn't have a finalizer, that's the right thing to do (it
+ * doesn't belong in gc.garbage).
+ *
+ * We return the number of weakref objects freed (those not appended to old).
+ */
+static int
+release_weakrefs(PyGC_Head *wr_callbacks, PyGC_Head *old)
+{
+	int num_freed = 0;
+
+	while (! gc_list_is_empty(wr_callbacks)) {
+		PyGC_Head *gc = wr_callbacks->gc.gc_next;
+		PyObject *op = FROM_GC(gc);
+		PyWeakReference *wr = (PyWeakReference *)op;
+
+		assert(IS_REACHABLE(op));
+		assert(PyWeakref_Check(op));
+		assert(wr->wr_callback != NULL);
+		Py_DECREF(op);
+		if (wr_callbacks->gc.gc_next == gc) {
+			/* object is still alive -- move it */
+			gc_list_remove(gc);
+			gc_list_append(gc, old);
+		}
+		else
+			++num_freed;
+	}
+	return num_freed;
+}
+
 static void
 debug_instance(char *msg, PyInstanceObject *inst)
 {
@@ -554,8 +651,9 @@ collect(int generation)
 	long n = 0;	/* # unreachable objects that couldn't be collected */
 	PyGC_Head *young; /* the generation we are examining */
 	PyGC_Head *old; /* next older generation */
-	PyGC_Head unreachable;
-	PyGC_Head finalizers;
+	PyGC_Head unreachable; /* non-problematic unreachable trash */
+	PyGC_Head finalizers;  /* objects with, & reachable from, __del__ */
+	PyGC_Head wr_callbacks;  /* weakrefs with callbacks */
 	PyGC_Head *gc;
 
 	if (delstr == NULL) {
@@ -616,20 +714,33 @@ collect(int generation)
 	/* All objects in unreachable are trash, but objects reachable from
 	 * finalizers can't safely be deleted.  Python programmers should take
 	 * care not to create such things.  For Python, finalizers means
-	 * instance objects with __del__ methods.
+	 * instance objects with __del__ methods.  Weakrefs with callbacks
+	 * can call arbitrary Python code, so those are special-cased too.
 	 *
-	 * Move unreachable objects with finalizers into a different list.
+	 * Move unreachable objects with finalizers, and weakrefs with
+	 * callbacks, into different lists.
  	 */
 	gc_list_init(&finalizers);
-	move_finalizers(&unreachable, &finalizers);
+	gc_list_init(&wr_callbacks);
+	move_troublemakers(&unreachable, &finalizers, &wr_callbacks);
+	/* Clear the trash weakrefs with callbacks.  This prevents their
+	 * callbacks from getting invoked (when a weakref goes away, so does
+	 * its callback).
+	 * We do this even if the weakrefs are reachable from finalizers.
+	 * If we didn't, breaking cycles in unreachable later could trigger
+	 * deallocation of objects in finalizers, which could in turn
+	 * cause callbacks to trigger.  This may not be ideal behavior.
+	 */
+	clear_weakrefs(&wr_callbacks);
 	/* finalizers contains the unreachable objects with a finalizer;
-	 * unreachable objects reachable only *from* those are also
-	 * uncollectable, and we move those into the finalizers list too.
+	 * unreachable objects reachable *from* those are also uncollectable,
+	 * and we move those into the finalizers list too.
 	 */
 	move_finalizer_reachable(&finalizers);
 
 	/* Collect statistics on collectable objects found and print
-	 * debugging information. */
+	 * debugging information.
+	 */
 	for (gc = unreachable.gc.gc_next; gc != &unreachable;
 			gc = gc->gc.gc_next) {
 		m++;
@@ -643,6 +754,11 @@ collect(int generation)
 	 */
 	delete_garbage(&unreachable, old);
 
+        /* Now that we're done analyzing stuff and breaking cycles, let
+         * delayed weakref callbacks run.
+         */
+	m += release_weakrefs(&wr_callbacks, old);
+
 	/* Collect statistics on uncollectable objects found and print
 	 * debugging information. */
 	for (gc = finalizers.gc.gc_next;