improvements to rewrite cache invalidation

- stat the source path before it is read.
- Validate the source size in addition to mtime.

testing/test_assertrewrite.py

@@ -511,13 +511,13 @@ class TestAssertionRewriteHookDetails(object):
         state = AssertionState(config, "rewrite")
         source_path = tmpdir.ensure("source.py")
         pycpath = tmpdir.join("pyc").strpath
-        assert _write_pyc(state, [1], source_path, pycpath)
+        assert _write_pyc(state, [1], source_path.stat(), pycpath)
         def open(*args):
             e = IOError()
             e.errno = 10
             raise e
         monkeypatch.setattr(b, "open", open)
-        assert not _write_pyc(state, [1], source_path, pycpath)
+        assert not _write_pyc(state, [1], source_path.stat(), pycpath)
 
     def test_resources_provider_for_loader(self, testdir):
         """