Thumbor On Heroku causes heroku time out when fetching URL returns 404
This issue on the tc_aws library we are using. It wasn't executing callback functions when 404 is returned from S3. We were at version  2.0.10. After upgrade the library, the problem is fixed.
We are hosting our thumbor image resizing service on Heroku, with image store in aws s3. The Thumbor service access s3 via the aws plugin.
Recently we observe a behavior on our thumbor service which I don't understand.
Use case: our client application was sending a resize request to resize an image that does not exist in aws S3.
expected behavior:

the thumbor service fetch image in s3.

s3 returns 404 not found

thumbor return 404 to heroku, the router returns 404 to client app.


what we observe:
under certain situation(I cannot reproduce this consistently).
s3 returns 404 but thumbor does not let the heroku router know. As a result the heroku wait for 30s and return the request as 503 time out.
The flowing are the logs

2016-05-31T19:38:15.094468+00:00 app[web.1]: 2016-05-31 19:38:15 thumbor:WARNING ERROR retrieving image from S3 [bucket]/3C6A3A84-F249-458B-9EFA-BF9BC863874B: {'ResponseMetadata': {'HTTPStatusCode': 404, 'RequestId': '3D61A8CBB187D846', 'HostId': 'C1qYC9Au42J0Salt1SVlCkcvcrKcQv4dltwOCdwGNF1TUFScWpkHb1qC++ZBJ0JzVqQlXW0xONU='}, 'Error': {'Key': '[bucket]/3C6A3A84-F249-458B-9EFA-BF9BC863874B', 'Code': 'NoSuchKey', 'Message': 'The specified key does not exist.'}}
2016-05-31T19:38:14.777549+00:00 heroku[router]: at=error code=H12 desc="Request timeout" method=GET path="/bALh_vgGXd7e_J7kZ0GhyE_lhZ0=/150x150/[bucket]/3C6A3A84-F249-458B-9EFA-BF9BC863874B" host=heroku.app.com request_id=67d87ea3-8010-4fbe-8c29-b2b7298f1cbc fwd="54.162.233.176,54.240.144.43" dyno=web.1 connect=5ms service=30000ms status=503 bytes=0

I am wondering if anyone can help to understand why thumbor hangs?
Many thanks in advance!