Search: ExistingFilter: Invalidate old cache

Fixes #70

src/python/klausurarchiv/search/exams.py

@@ -83,6 +83,7 @@ class ExistingFilter:
   # No additional arguments are required by this generator.
   def __init__(s, enumerator):
     s._cache=None
+    s._cache_epoch=caching.get_epoch()
     s._enumerator=enumerator
 
   ## \brief Clears internal cache in this filter and all predecessors 
@@ -93,10 +94,14 @@ class ExistingFilter:
   ## \brief Request method as specified in
   # \ref _fn_99-meta_doc_concepts_search-engines_md.
   def request(s, **kwargs):
+    if caching.get_epoch()>s._cache_epoch:
+      s.invalidateCache()
+
     allItems=s._enumerator.request(**kwargs)
 
     if s._cache==None:
       s._cache=set()
+      s._cache_epoch=caching.get_epoch()
       for item in allItems:
         if item.folder.archived or item.folder.lost: continue
         s._cache.add(item)