Pass request_id to tasks if available

We want to pass the `request_id` to Celery tasks if the task is called
from an HTTP request, so that we can add the `request_id` to the logs.
This change overwrites `apply_async` to add the `request_id` to the
kwargs if available. When we call the task, we then add the `request_id`
to g on Flask's application context.

Tasks called from `send_task` won't have a `request_id` for now, and
this change only affects tasks called from HTTP requests (not from other
tasks or from Celery Beat).

app/celery/celery.py

@@ -2,7 +2,8 @@ import time
 
 from celery import Celery, Task
 from celery.signals import worker_process_shutdown
-from flask import current_app
+from flask import current_app, g, request
+from flask.ctx import has_request_context
 
 
 @worker_process_shutdown.connect
@@ -32,8 +33,20 @@ def make_task(app):
             # ensure task has flask context to access config, logger, etc
             with app.app_context():
                 self.start = time.time()
+                # Remove 'request_id' from the kwargs (so the task doesn't get an unexpected kwarg), then add it to g
+                # so that it gets logged
+                g.request_id = kwargs.pop('request_id', None)
                 return super().__call__(*args, **kwargs)
 
+        def apply_async(self, args=None, kwargs=None, task_id=None, producer=None,
+                        link=None, link_error=None, **options):
+            kwargs = kwargs or {}
+
+            if has_request_context() and hasattr(request, 'request_id'):
+                kwargs['request_id'] = request.request_id
+
+            return super().apply_async(args, kwargs, task_id, producer, link, link_error, **options)
+
     return NotifyTask