Fix celery#5734 Celery does not consider authMechanism on mongodb bac…

…kend URLs

celery/backends/mongodb.py

@@ -157,6 +157,10 @@ def _get_connection(self):
             # don't change self.options
             conf = dict(self.options)
             conf['host'] = host
+            if self.user:
+                conf['username'] = self.user
+            if self.password:
+                conf['password'] = self.password
 
             self._connection = MongoClient(**conf)
 
@@ -268,27 +272,13 @@ def __reduce__(self, args=(), kwargs=None):
         return super(MongoBackend, self).__reduce__(
             args, dict(kwargs, expires=self.expires, url=self.url))
 
-    def _get_database(self):
+    @cached_property
+    def database(self):
+        """Get database from MongoDB connection."""
         conn = self._get_connection()
         db = conn[self.database_name]
-        if self.user and self.password:
-            source = self.options.get(
-                'authsource',
-                self.database_name or 'admin'
-            )
-            if not db.authenticate(self.user, self.password, source=source):
-                raise ImproperlyConfigured(
-                    'Invalid MongoDB username or password.')
         return db
 
-    @cached_property
-    def database(self):
-        """Get database from MongoDB connection.
-
-        performs authentication if necessary.
-        """
-        return self._get_database()
-
     @cached_property
     def collection(self):
         """Get the meta-data task collection."""