Avoid depending on app context when templating failure reports (#4231)

* don't need to depend on context when templating failure reports

* extract a render_template function with some docs

* CodeClimate has really outdone itself this time. Removed a whitespace character in order to fix 2 CodeClimate errors

* apparently whitespace doesn't count as a character

redash/tasks/failure_report.py

@@ -1,11 +1,10 @@
 import datetime
 import re
 from collections import Counter
-from flask import render_template
 from redash.tasks.general import send_mail
 from redash.worker import celery
 from redash import redis_connection, settings, models
-from redash.utils import json_dumps, json_loads, base_url
+from redash.utils import json_dumps, json_loads, base_url, render_template
 
 
 def key(user_id):
@@ -50,9 +49,9 @@ def send_failure_report(user_id):
             'base_url': base_url(user.org)
         }
 
-        html = render_template('emails/failures.html', **context)
-        text = render_template('emails/failures.txt', **context)
         subject = "Redash failed to execute {} of your scheduled queries".format(len(unique_errors.keys()))
+        html, text = [render_template('emails/failures.{}'.format(f), context) for f in ['html', 'txt']]
+
         send_mail.delay([user.email], subject, html, text)
 
     redis_connection.delete(key(user_id))

redash/utils/__init__.py

@@ -15,6 +15,7 @@ from six import string_types
 import pystache
 import pytz
 import simplejson
+from flask import current_app
 from funcy import select_values
 from redash import settings
 from sqlalchemy.orm.query import Query
@@ -205,3 +206,11 @@ def deprecated():
         return K
 
     return wrapper
+
+
+def render_template(path, context):
+    """ Render a template with context, without loading the entire app context.
+    Using Flask's `render_template` function requires the entire app context to load, which in turn triggers any
+    function decorated with the `context_processor` decorator, which is not explicitly required for rendering purposes.
+    """
+    current_app.jinja_env.get_template(path).render(**context)

tests/tasks/test_failure_report.py

@@ -26,7 +26,7 @@ class TestSendAggregatedErrorsTask(BaseTestCase):
     def send_email(self, user, render_template):
         send_failure_report(user.id)
 
-        _, context = render_template.call_args
+        _, context = render_template.call_args[0]
         return context['failures']
 
     def test_schedules_email_if_failure_count_is_beneath_limit(self):