Nginx silently drops response body of POST depending on its length

Server NGINX
1

I’ve come across an incredibly weird problem with POST responses using
nginx 1.0.1 and uWSGI 0.9.7.2 on CentOS 5.6.

uWSGI successfully sends the response body to nginx, but nginx seems to
ignore it if both (1) the request method is POST, and (2) the response
length is less than 4052 bytes. The issue doesn’t occur when serving
directly using uwsgi --http, so I’m pretty sure the problem is
something to do with nginx.

To explain things in code, this Python results in the desired response:

…def app_uwsgi(environ, start_response):
…start_response(‘200 OK’, [(‘Content-type’, ‘text/html’)])
…return [b’x’ * 4052]

while this results in zero bytes being sent to the client for POST
requests:

…def app_uwsgi(environ, start_response):
…start_response(‘200 OK’, [(‘Content-type’, ‘text/html’)])
…return [b’x’ * 4051]

I stripped down to a minimal nginx.conf, and the issue still occurs:

…events {
…worker_connections 1024;
…}

…http {
…upstream uwsgi_connect {
…server 127.0.0.1:39275;
…}

…server {
…listen 20101;
…server_name domain.net;

…location / {
…uwsgi_pass uwsgi_connect;
…include uwsgi_params;
…}
…}
…}

Does anyone have any clue what could be causing this behavior?

Posted at Nginx Forum:

2

Hello!

On Sun, May 08, 2011 at 03:15:28PM -0400, zildjohn01 wrote:

Does anyone have any clue what could be causing this behavior?

As it was already said - you have to read body of POST before
sending reply. Else you’ll get something like

… readv() failed (54: Connection reset by peer) while reading
upstream …

in nginx error log and possibly incorrect reply (depending on various
factors, including sizes and event method used).

Maxim D.

3

Thanks, that helped point me to the issue. It turns out the problem was
that I didn’t pass --pep3333-input to uWSGI, so it was converting all
CRLFs to LFs in the request body, so CONTENT_LENGTH didn’t match the
data length. Passing that flag to uWSGI made it pass the raw stream to
my app and solved the problem.

Posted at Nginx Forum:

4

Be sure to read POST body in your WSGI app if Content-Length > 0,
otherwise your socket queue will get clobbered (nginx will receive a
close
when there is still data available in the queue)

–
Roberto De Ioris
http://unbit.it