Make sure we only compare ints in ZCacheable_getModTime

CHANGES.rst

@@ -11,7 +11,10 @@ https://github.com/zopefoundation/Zope/blob/4.0a6/CHANGES.rst
 4.0b5 (unreleased)
 ------------------
 
-- Nothing changed yet.
+Bugfixes
+++++++++
+
+- Fix comparison against non-ints in ZCacheable_getModTime.
 
 
 4.0b4 (2018-04-23)

src/OFS/Cache.py

@@ -226,10 +226,12 @@ def ZCacheable_getModTime(self, mtime_func=None):
             # Allow mtime_func to influence the mod time.
             mtime = mtime_func()
         base = aq_base(self)
-        mtime = max(getattr(base, '_p_mtime', mtime), mtime)
+        mtime = max(getattr(base, '_p_mtime', mtime) or 0, mtime)
         klass = getattr(base, '__class__', None)
         if klass:
-            mtime = max(getattr(klass, '_p_mtime', mtime), mtime)
+            klass_mtime = getattr(klass, '_p_mtime', mtime)
+            if isinstance(klass_mtime, int):
+                mtime = max(klass_mtime, mtime)
         return mtime
 
     security.declareProtected(ViewManagementScreensPermission,

src/OFS/tests/testCache.py

@@ -32,3 +32,17 @@ def test_managersExist(self):
 
         # The parent_cache should still trigger managersExist
         self.assertTrue(managersExist(root.child.child_content))
+
+
+class CacheableTests(unittest.TestCase):
+
+    def _getTargetClass(self):
+        from OFS.Cache import Cacheable
+        return Cacheable
+
+    def _makeOne(self):
+        return self._getTargetClass()()
+
+    def test_ZCacheable_getModTime(self):
+        ob = self._makeOne()
+        self.assertEqual(0, ob.ZCacheable_getModTime())

tox.ini

@@ -9,7 +9,7 @@ commands =
 skip_install = true
 deps =
     coverage
-    setuptools==33.1.1
+    setuptools==39.0.1
     zc.buildout
 setenv =
     COVERAGE_FILE=.coverage.{envname}