swig-py: Fix doubly destroying memory pool because weakref's callback is not
invoked when it is finalized by cyclic garbage collector.

* subversion/bindings/swig/include/proxy_apr.swg
  (apr_pool_t.valid): Check whether parent pool is valid.

* subversion/bindings/swig/python/tests/pool.py
  (PoolTestCase): Add tests for pools referred from circular reference.

Index: subversion/bindings/swig/include/proxy_apr.swg
===================================================================
--- subversion/bindings/swig/include/proxy_apr.swg	(revision 1889441)
+++ subversion/bindings/swig/include/proxy_apr.swg	(working copy)
@@ -142,9 +142,15 @@
         are still valid"""
         try:
           self._is_valid
-          return True
         except AttributeError:
           return False
+        # We must check whether the parent pool is valid even if
+        # the pool is valid because weakref's callback is not
+        # invoked when it is finalized by cyclic garbage collector
+        if self._parent_pool:
+          return self._parent_pool.valid()
+        else:
+          return True
 
       def assert_valid(self):
         """Assert that this memory_pool is still valid."""
Index: subversion/bindings/swig/python/tests/pool.py
===================================================================
--- subversion/bindings/swig/python/tests/pool.py	(revision 1889441)
+++ subversion/bindings/swig/python/tests/pool.py	(working copy)
@@ -19,10 +19,11 @@
 #
 #
 import unittest, weakref, setup_path
-import os, tempfile
+import os, tempfile, gc
 import svn.core, svn.client, libsvn.core
 from svn.core import *
 from libsvn.core import application_pool, GenericSWIGWrapper
+import utils
 
 # Test case for the new automatic pool management infrastructure
 
@@ -208,6 +209,51 @@
     # We can still destroy and create pools at will
     svn_pool_destroy(svn_pool_create())
 
+  def _test_pools_in_circular_reference(self, finalizer=False):
+
+    class Circular(object):
+
+      def __init__(self, pool):
+        self.pool = pool
+        self.loop = None
+
+      if finalizer:
+        def __del__(self):
+          self.pool = self.loop = None
+
+    def create_circularl():
+      pool = Pool(libsvn.core.application_pool)
+      subpool1 = Pool(pool)
+      subpool2 = Pool(pool)
+      circularly1 = Circular(pool)
+      circularly2 = Circular(subpool2)
+      circularly3 = Circular(subpool1)
+      circularly1.loop = circularly3
+      circularly2.loop = circularly1
+      circularly3.loop = circularly2
+      refs = weakref.WeakValueDictionary()
+      refs['pool'] = pool
+      refs['subpool1'] = subpool1
+      refs['subpool2'] = subpool2
+      return refs
+
+    refs = create_circularl()
+    self.assertEqual({'pool', 'subpool1', 'subpool2'},
+                     set(name for name, pool in refs.items()
+                              if pool is not None))
+    gc.collect()
+    self.assertEqual(set(), set(name for name, pool in refs.items()
+                                     if pool is not None))
+
+  def test_pools_in_circular_reference_without_finalizer(self):
+    self._test_pools_in_circular_reference(finalizer=False)
+
+  @unittest.skipIf(not utils.IS_PY3,
+                   "Python 2 cannot collect garbage which involves circular "
+                   "references with finalizer")
+  def test_pools_in_circular_reference_with_finalizer(self):
+    self._test_pools_in_circular_reference(finalizer=True)
+
 def suite():
   return unittest.defaultTestLoader.loadTestsFromTestCase(PoolTestCase)