Override getttext.find to cache result

This monkey patches gettext to cache the results of gettext.find. This
allows to drastically reduces system calls made over time for checking
the existence of mo files, which don't move.

Change-Id: I1464f131b90123aab67ef45ae2a2685a3ba111ef

oslo_i18n/_gettextutils.py

@@ -99,3 +99,29 @@ def get_available_languages(domain):
 
     _AVAILABLE_LANGUAGES[domain] = result
     return copy.copy(result)
+
+
+_original_find = gettext.find
+_FIND_CACHE = {}
+
+
+def cached_find(domain, localedir=None, languages=None, all=0):
+    """A version of gettext.find using a cache.
+
+    gettext.find looks for mo files on the disk using os.path.exists. Those
+    don't tend to change over time, but the system calls pile up with a
+    long-running service. This caches the result so that we return the same mo
+    files, and only call find once per domain.
+    """
+    key = (domain,
+           localedir,
+           tuple(languages) if languages is not None else None,
+           all)
+    if key in _FIND_CACHE:
+        return _FIND_CACHE[key]
+    result = _original_find(domain, localedir, languages, all)
+    _FIND_CACHE[key] = result
+    return result
+
+
+gettext.find = cached_find

oslo_i18n/tests/test_gettextutils.py

@@ -129,3 +129,13 @@ class GettextTest(test_base.BaseTestCase):
         unknown_domain_languages = _gettextutils.get_available_languages('huh')
         self.assertEqual(1, len(unknown_domain_languages))
         self.assertIn('en_US', unknown_domain_languages)
+
+    def test_cached_find(self):
+        domain = 'my-unique-domain'
+        key = (domain, None, None, 0)
+        self.assertNotIn(key, _gettextutils._FIND_CACHE)
+        gettext.find(domain)
+        self.assertIn(key, _gettextutils._FIND_CACHE)
+        _gettextutils._FIND_CACHE[key] = "spoof result"
+        self.assertEqual("spoof result", gettext.find(domain))
+        _gettextutils._FIND_CACHE.pop(key)