Transform RqlIOSchemaHolder into a singleton

Its two class methods get_schema and get_schema_hash will only compute
the schema once. They where computed every time the schema was
requested.

Closes #1.

cubicweb_rqlcontroller/rql_schema_holder.py

@@ -28,6 +28,8 @@ class RqlIOSchemaHolder(object):
        so it can decide to update the locally stored schema if
        the integrity is not valid.
     """
+    _schema = None
+    _hash = None
 
     @staticmethod
     def format_schema(typeNames,
@@ -70,10 +72,13 @@ class RqlIOSchemaHolder(object):
             } for row in relationsFrom]
         }
 
-    @cachedproperty
-    def get_schema(self):
+    @classmethod
+    def get_schema(cls, cnx):
+        if cls._schema is not None:
+            return cls._schema
+
         entities = []
-        etypes = self._cw.execute('Any T WHERE T is CWEType')
+        etypes = cnx.execute('Any T WHERE T is CWEType')
         for etype in etypes:
             eid = etype[0]
             rql_requests = [
@@ -122,12 +127,16 @@ class RqlIOSchemaHolder(object):
                     ATTR ordernum ON"""
                 )
             ]
-            rsets = [self._cw.execute(request) for request in rql_requests]
+            rsets = [cnx.execute(request) for request in rql_requests]
             entities.append(RqlIOSchemaHolder.format_schema(*rsets))
-        return {"entities": entities}
+        cls._schema = {"entities": entities}
+        return cls._schema
 
-    @cachedproperty
-    def get_schema_hash(self):
-        return hashlib.md5(
-            json_dumps(self.get_schema["entities"]).encode(self._cw.encoding)
+    @classmethod
+    def get_schema_hash(cls, cnx):
+        if cls._hash is not None:
+            return cls._hash
+        cls._hash = hashlib.md5(
+            json_dumps(cls.get_schema(cnx)["entities"]).encode(cnx.encoding)
         ).hexdigest()
+        return cls._hash

cubicweb_rqlcontroller/views.py

@@ -75,15 +75,15 @@ class match_request_content_type(ExpectedValuePredicate):
         return header
 
 
-class RqlIOSchemaController(Controller, RqlIOSchemaHolder):
+class RqlIOSchemaController(Controller):
     __regid__ = 'rqlio_schema'
     __select__ = match_http_method('GET', 'HEAD')
 
     def publish(self, rset=None):
         self._cw.set_content_type('application/json')
-        self._cw.add_header('Etag', self.get_schema_hash)
+        self._cw.add_header('Etag', RqlIOSchemaHolder.get_schema_hash(self._cw))
         return json.dumps(
-            self.get_schema
+            RqlIOSchemaHolder.get_schema(self._cw)
         ).encode(self._cw.encoding)