Image scales cache does not work in tests

[rafaelbco]

I recently updated from 2.0.9 to 3.0.11 in my buildout and noticed some test failures. I was able to identify the problem: when running tests, the image scales cache does not work.

The reason is in the following lines:

plone.namedfile/plone/namedfile/scaling.py

Lines 387 to 393 in 7fc0e67

	def modified(self):
	"""Provide a callable to return the modification time of content
	items, so stored image scales can be invalidated.
	"""
	context = aq_base(self.context)
	date = DateTime(context._p_mtime)
	return date.millis()

During tests, a content item may have the _p_mtime attribute set to None. So the code above is actually doing DateTime(None), which returns the current date/time. The result is every time a scale is requested it considers the cached value invalid, because the object is considered modified.

In the old version context.modified() was used instead of _p_mtime, and this method always returned a non-None value.

I wonder how to solve this problem. Maybe modified should return a default date in the past?

[rafaelbco]

Note: this bug still happens in version 5.6.0 which is used by Plone 5.2.