Move logging rate-limiting code

The new file can't be called logging.py because then it would be
annoying to import the system logging module within it.

(imported from commit 71d116e4be98d45b09dda049a43142a82647b727)

zephyr/lib/logging_util.py

@@ -0,0 +1,47 @@
+import logging
+import traceback
+from hashlib import sha256
+from datetime import datetime, timedelta
+
+# Adapted http://djangosnippets.org/snippets/2242/ by user s29 (October 25, 2010)
+
+class _RateLimitFilter(object):
+    last_error = datetime.min
+
+    def filter(self, record):
+        from django.conf import settings
+        from django.core.cache import cache
+
+        # Track duplicate errors
+        duplicate = False
+        rate = getattr(settings, '%s_LIMIT' %  self.__class__.__name__.upper(),
+               600)  # seconds
+        if rate > 0:
+            # Test if the cache works
+            try:
+                cache.set('RLF_TEST_KEY', 1, 1)
+                use_cache = cache.get('RLF_TEST_KEY') == 1
+            except:
+                use_cache = False
+
+            if use_cache:
+                key = self.__class__.__name__.upper()
+                duplicate = cache.get(key) == 1
+                cache.set(key, 1, rate)
+            else:
+                min_date = datetime.now() - timedelta(seconds=rate)
+                duplicate = (self.last_error >= min_date)
+                if not duplicate:
+                    self.last_error = datetime.now()
+
+        return not duplicate
+
+class HumbugLimiter(_RateLimitFilter):
+    pass
+
+class EmailLimiter(_RateLimitFilter):
+    pass
+
+class ReturnTrue(logging.Filter):
+    def filter(self, record):
+        return True