Nginx converts 409 from upstream to 502

[Maxim Dounin]

Hello!

On Sat, Dec 21, 2019 at 07:49:06AM -0500, sachin.shetty at gmail.com wrote:

> Our request flow looks like this:
> 
> client --> nginx --> haproxy --> tomcat
> 
> Put requests with large bodies are used to upload files. Sometimes due to
> some application logic, tomcat may reject an upload early and return 409,
> tomcat does not drain the input stream, we do not want to read the input
> stream and rather reject early.
> 
> When tomcat rejects a PUT request early with 409, tomcat access logs show
> 409. The haproxy fronting tomcats also log a 409 in access logs, but nginx
> logs a 502 and sends a 502 to the clients. It looks like nginx does not like
> that request is rejected and a 409 is sent without reading the full request
> by the upstream. 
> 
> What can we do to make sure client receives the correct response code i.e.
> 409

The 502 error suggests that nginx do not receive HTTP response 
from the upstream server.  Most likely reason is that haproxy 
closes connection with nginx without trying to read data which is 
in socket buffers and/or in transit, resulting in connection 
reset.  See 
http://mailman.nginx.org/pipermail/nginx/2019-December/058812.html 
for details.

-- 
Maxim Dounin
http://mdounin.ru/