caches: use delete_multi for clearing out cache keys.

- This is a safer and more efficient way of purging many keys.
- For file based cache it only opens the DB once

rhodecode/lib/rc_cache/utils.py

@@ -102,6 +102,5 b' def get_or_create_region(region_name, re'
 def clear_cache_namespace(cache_region, cache_namespace_uid):
     region = get_or_create_region(cache_region, cache_namespace_uid)
     cache_keys = region.backend.list_keys(prefix=cache_namespace_uid)
-    for k in cache_keys:
-        region.delete(k)
+    region.delete_multi(cache_keys)
     return len(cache_keys)

rhodecode/tests/lib/test_caches.py

@@ -74,8 +74,7 b' class TestCaches(object):'
     def test_store_value_in_cache(self):
         cache_region = rc_cache.get_or_create_region('cache_perms')
         # make sure we empty the cache now
-        for key in cache_region.backend.list_keys():
-            cache_region.delete(key)
+        cache_region.delete_multi(cache_region.backend.list_keys())
 
         assert cache_region.backend.list_keys() == []