fix(alerts, thumbnails): celery tasks using flask-sqlalchemy sessions (#11950)

* fix(alerts, thumbnails): celery tasks using flask-sqlalchemy sessions

* selenium user is case sensitive

superset/security/manager.py

@@ -27,6 +27,7 @@ from flask_appbuilder.security.sqla.models import (
     assoc_permissionview_role,
     assoc_user_role,
     PermissionView,
+    User,
 )
 from flask_appbuilder.security.views import (
     PermissionModelView,
@@ -38,6 +39,7 @@ from flask_appbuilder.security.views import (
 from flask_appbuilder.widgets import ListWidget
 from sqlalchemy import and_, or_
 from sqlalchemy.engine.base import Connection
+from sqlalchemy.orm import Session
 from sqlalchemy.orm.mapper import Mapper
 from sqlalchemy.orm.query import Query as SqlaQuery
 
@@ -986,6 +988,21 @@ class SupersetSecurityManager(  # pylint: disable=too-many-public-methods
                     self.get_datasource_access_error_object(datasource)
                 )
 
+    def get_user_by_username(
+        self, username: str, session: Session = None
+    ) -> Optional[User]:
+        """
+        Retrieves a user by it's username case sensitive. Optional session parameter
+        utility method normally useful for celery tasks where the session
+        need to be scoped
+        """
+        session = session or self.get_session
+        return (
+            session.query(self.user_model)
+            .filter(self.user_model.username == username)
+            .one_or_none()
+        )
+
     def get_rls_filters(self, table: "BaseDatasource") -> List[SqlaQuery]:
         """
         Retrieves the appropriate row level security filters for the current user and