[entropy.db] EntropyRepository: validate Live cache against mtime()

Scenario: Process A is writing to EntropyRepository, adding new
package entries. Process B is reading from EntropyRepository,
querying for the same package entries, for example by using
retrieveKeySlot(). This method uses a dict-based cache to speed
up things, but this should be invalidated also when the mtime()
value changes.

lib/entropy/db/__init__.py

@@ -538,6 +538,10 @@ class EntropyRepository(EntropyRepositoryBase):
         if self._db_path is None:
             raise AttributeError("valid database path needed")
 
+        # tracking mtime to validate repository Live cache as
+        # well.
+        self._cur_mtime = self.mtime()
+
         # setup service interface
         self.__skip_checks = skipChecks
 
@@ -738,6 +742,10 @@ class EntropyRepository(EntropyRepositoryBase):
         self._live_cacher.set(self._getLiveCacheKey() + key, value)
 
     def _getLiveCache(self, key):
+        mtime = self.mtime()
+        if self._cur_mtime != mtime:
+            self._cur_mtime = mtime
+            self._discardLiveCache()
         return self._live_cacher.get(self._getLiveCacheKey() + key)
 
     def __del__(self):