NGINX, uWSGI with CGI plugin, Python scripts - 502 Bad Gateway

Question

I want to use uWSGI with CGI plugin to process all requests to .py files passed from NGINX.

NGINX server config:

location ~ \.py$ {
    include         uwsgi_params;
    uwsgi_modifier1 9;
    uwsgi_pass      unix:/var/www/html/cgi-bin/wsgi.sock;
}

uWSGI config:

[uwsgi]
plugin-dir = /usr/lib/uwsgi/plugins
plugins = cgi
cgi = /var/www/html/cgi-bin
cgi-allowed-ext = .py
cgi-helper = .py=python
master = true
processes = 5
socket = wsgi.sock
chmod-socket = 664
vacuum = true
die-on-term = true

uWSGI starting output:

$ sudo -u www-data uwsgi --ini uwsgi.ini 
[uWSGI] getting INI configuration from uwsgi.ini
*** Starting uWSGI 2.0.18 (64bit) on [Fri Oct 18 15:27:33 2019] ***
compiled with version: 7.4.0 on 18 October 2019 12:14:16
os: Linux-4.15.0-65-generic #74-Ubuntu SMP Tue Sep 17 17:06:04 UTC 2019
nodename: some-host
machine: x86_64
clock source: unix
pcre jit disabled
detected number of CPU cores: 4
current working directory: /var/www/html/cgi-bin
detected binary path: /usr/bin/uwsgi
your processes number limit is 15501
your memory page size is 4096 bytes
detected max file descriptor number: 1024
lock engine: pthread robust mutexes
thunder lock: disabled (you can enable it with --thunder-lock)
uwsgi socket 0 bound to UNIX address wsgi.sock fd 3
your server socket listen backlog is limited to 100 connections
your mercy for graceful operations on workers is 60 seconds
mapped 437424 bytes (427 KB) for 5 cores
*** Operational MODE: preforking ***
initialized CGI path: /var/www/html/cgi-bin
*** no app loaded. going in full dynamic mode ***
*** uWSGI is running in multiple interpreter mode ***
spawned uWSGI master process (pid: 6868)
spawned uWSGI worker 1 (pid: 6869, cores: 1)
spawned uWSGI worker 2 (pid: 6870, cores: 1)
spawned uWSGI worker 3 (pid: 6871, cores: 1)
spawned uWSGI worker 4 (pid: 6872, cores: 1)
spawned uWSGI worker 5 (pid: 6873, cores: 1)

There are 2 files in /var/www/html/cgi-bin/:

• info.py
• some.py

If I open an URL with any other file name, I get 404 error from uWSGI - that is good. But if I open an URL with either of actual script names, then I get 500 from uWSGI (hence 502 Bad Gateway from NGINX).

URLs:

1. http://localhost/another.py
2. http://localhost/info.py
3. http://localhost/some.py

Output:

[pid: 9606|app: -1|req: -1/1] 127.0.0.1 () {42 vars in 624 bytes} [Fri Oct 18 15:40:50 2019] GET /another.py => generated 9 bytes in 0 msecs (HTTP/1.1 404) 2 headers in 71 bytes (0 switches on core 0)
[pid: 9604|app: -1|req: -1/2] 127.0.0.1 () {42 vars in 618 bytes} [Fri Oct 18 15:41:01 2019] GET /info.py => generated 0 bytes in 15 msecs (HTTP/1.1 500) 0 headers in 0 bytes (0 switches on core 0)
[pid: 9606|app: -1|req: -1/3] 127.0.0.1 () {42 vars in 618 bytes} [Fri Oct 18 15:41:05 2019] GET /some.py => generated 0 bytes in 15 msecs (HTTP/1.1 500) 0 headers in 0 bytes (0 switches on core 0)

So obviously it finds the files (info.py and some.py), but what is the reason of the 500 error from uWSGI? Do I have something wrong in either of configurations?

Here's one of the Python scripts:

def application(environ, start_response):
    start_response('200 OK', [('Content-Type', 'text/html')])
    return [ "Ololo".encode("utf-8") ]

uWSGI with CGI plugin is installed using instructions from documentaion:

curl http://uwsgi.it/install | bash -s cgi /tmp/uwsgi
sudo mv /tmp/uwsgi /usr/bin

I wrote a blog post with more details on the matter.

Answer 1

You're getting 500's because the uWSGI CGI plugin doesn't use WSGI, it uses CGI (took me a few hours to figure that difference out). The python script you're using implements the WSG Interface (with an "application" method) but isn't actually CGI-compatible.

To make it compatible just output the HTTP response headers, a new line, and then the HTTP body to standard out (https://en.wikipedia.org/wiki/Common_Gateway_Interface#Example).

So for your example this would become:

print("Content-Type: text/html")
print("Status: 200 OK")
print()

print("Ololo")