Stop calling gc.collect in the Python incrgc.

@jimfulton and I have talked about it, and I'm (mostly :) convinced that
this shouldn't be an actual problem for any of the reasons described in
the previous comment.

If I use PyPy 5.4.1 to run the ZODB master test suite against
this (well, with #44 rolled back) I don't get any unexpected
failures. (I haven't run the ZEO test suite yet.) Which honestly amazes
me because I'm sure I used to get test failures---I guess the PyPy GC
has changed...which means we may see some failures on Travis. I'll try
to set up an older PyPy to verify.

CHANGES.rst

@@ -10,6 +10,12 @@
 4.2.2 (2016-11-29)
 ------------------
 
+- Stop calling ``gc.collect`` every time ``PickleCache.incrgc`` is called (every
+  transaction boundary) in pure-Python mode (PyPy). This means that
+  the reported size of the cache may be wrong (until the next GC), but
+  it is much faster. This should not have any observable effects for
+  user code.
+
 - Drop use of ``ctypes`` for determining maximum integer size, to increase
   pure-Python compatibility. See https://github.com/zopefoundation/persistent/pull/31
 

persistent/picklecache.py

@@ -13,7 +13,7 @@
 ##############################################################################
 import gc
 import weakref
-import sys
+
 
 from zope.interface import implementer
 
@@ -28,24 +28,6 @@ from persistent.persistence import _estimated_size_in_24_bits
 _CACHEABLE_TYPES = (type, Persistent)
 _SWEEPABLE_TYPES = (Persistent,)
 
-# The Python PickleCache implementation keeps track of the objects it
-# is caching in a WeakValueDictionary. The number of objects in the
-# cache (in this dictionary) is exposed as the len of the cache. Under
-# non-refcounted implementations like PyPy, the weak references in
-# this dictionary are only cleared when the garbage collector runs.
-# Thus, after an incrgc, the len of the cache is incorrect for some
-# period of time unless we ask the GC to run.
-# Furthermore, evicted objects can stay in the dictionary and be returned
-# from __getitem__ or possibly conflict with a new item in __setitem__.
-# We determine whether or not we need to do the GC based on the ability
-# to get a reference count: PyPy and Jython don't use refcounts and don't
-# expose this; this is safer than blacklisting specific platforms (e.g.,
-# what about IronPython?). On refcounted platforms, we don't want to
-# run a GC to avoid possible performance regressions (e.g., it may
-# block all threads).
-# Tests may modify this
-_SWEEP_NEEDS_GC = not hasattr(sys, 'getrefcount')
-
 # On Jython, we need to explicitly ask it to monitor
 # objects if we want a more deterministic GC
 if hasattr(gc, 'monitorObject'): # pragma: no cover
@@ -381,11 +363,7 @@ class PickleCache(object):
         ejected = len(to_eject)
         if ejected:
             self.ring.delete_all(to_eject)
-        del to_eject # Got to clear our local if we want the GC to get the weak refs
 
-        if ejected and _SWEEP_NEEDS_GC:
-            # See comments on _SWEEP_NEEDS_GC
-            gc.collect()
         return ejected
 
     @_sweeping_ring

persistent/tests/test_picklecache.py

@@ -30,13 +30,9 @@ class PickleCacheTests(unittest.TestCase):
         self.orig_types = persistent.picklecache._CACHEABLE_TYPES
         persistent.picklecache._CACHEABLE_TYPES += (DummyPersistent,)
 
-        self.orig_sweep_gc = persistent.picklecache._SWEEP_NEEDS_GC
-        persistent.picklecache._SWEEP_NEEDS_GC = True # coverage
-
     def tearDown(self):
         import persistent.picklecache
         persistent.picklecache._CACHEABLE_TYPES = self.orig_types
-        persistent.picklecache._SWEEP_NEEDS_GC = self.orig_sweep_gc
 
     def _getTargetClass(self):
         from persistent.picklecache import PickleCache
@@ -992,7 +988,7 @@ class PickleCacheTests(unittest.TestCase):
             return f
 
     @with_deterministic_gc
-    def test_cache_garbage_collection_bytes_also_deactivates_object(self, force_collect=False):
+    def test_cache_garbage_collection_bytes_also_deactivates_object(self, force_collect=_is_pypy or _is_jython):
         from persistent.interfaces import UPTODATE
         from persistent._compat import _b
         cache = self._makeOne()