Index: Modules/gcmodule.c
===================================================================
--- Modules/gcmodule.c	(revision 86684)
+++ Modules/gcmodule.c	(working copy)
@@ -68,6 +68,9 @@
 /* Python string used to look for __del__ attribute. */
 static PyObject *delstr = NULL;
 
+/* a list of callbacks to be invoked when collection is performed */
+static PyObject *callbacks = NULL;
+
 /* This is the number of objects who survived the last full collection. It
    approximates the number of long lived objects tracked by the GC.
 
@@ -942,12 +945,29 @@
     return n+m;
 }
 
+static void
+invoke_gc_callback(int phase)
+{
+    int i;
+    for (i=0; i<PyList_GET_SIZE(callbacks); i++) {
+        PyObject *r, *cb = PyList_GET_ITEM(callbacks, i);
+        Py_INCREF(cb); //make sure cb doesn't go away
+        r = PyObject_CallFunction(cb, "i", phase);
+        Py_XDECREF(r);
+        if (r == NULL)
+            PyErr_WriteUnraisable(cb);
+        Py_DECREF(cb);
+    }
+}
+
 static Py_ssize_t
 collect_generations(void)
 {
     int i;
     Py_ssize_t n = 0;
 
+    invoke_gc_callback(0); /* start of gc run */
+
     /* Find the oldest generation (highest numbered) where the count
      * exceeds the threshold.  Objects in the that generation and
      * generations younger than it will be collected. */
@@ -964,6 +984,9 @@
             break;
         }
     }
+
+    invoke_gc_callback(1); /* end of gc run */
+
     return n;
 }
 
@@ -1324,6 +1347,15 @@
     if (PyModule_AddObject(m, "garbage", garbage) < 0)
         return NULL;
 
+    if (callbacks == NULL) {
+        callbacks = PyList_New(0);
+        if (callbacks == NULL)
+            return NULL;
+    }
+    Py_INCREF(callbacks);
+    if (PyModule_AddObject(m, "callbacks", callbacks) < 0)
+        return NULL;
+
     /* Importing can't be done in collect() because collect()
      * can be called via PyGC_Collect() in Py_Finalize().
      * This wouldn't be a problem, except that <initialized> is
@@ -1393,6 +1425,7 @@
             Py_XDECREF(bytes);
         }
     }
+    Py_CLEAR(callbacks);
 }
 
 /* for debugging */