bug: subrequest, addition_filter and not_modified

[Anton Luka Šijanec]

Hello!

I have just stumbled upon a nginx bug. When using add_before_body and/or 
add_after_body commands from ngx_http_addition_filter_module, the server 
will respond with "304 Not Modified" even if the added file has changed.

After investigation I figured out that this error could be applied to 
everything that uses ngx_http_subrequest to gather data. The request 
structure will remain with the same last_modified_time even if the 
subrequest contains data that was modified after this time. The solution 
to this problem would be to overwrite the request structure 
headers_out.last_modified_time with the last_modified_time in the 
structure, generated by ngx_http_subrequest, if it's in the future, but 
that would not be enough.

The problem is that ngx_http_subrequest does not populate the request 
struct, specified as it's fourth argument, with 
headers_out.last_modified_time, it stays at -1, but that wouldn't help 
very much in this particular case.

Even setting the headers_out.last_modified_time to -1 in 
addition_filter_module would not work and would not fix the issue, 
because the filter modules do not execute before not_modified module 
(nginx/auto/modules:142).

Well, at last resort, the not_modified module could load configuration 
of addition_filter module and, if addition is to be done, return to the 
next module. But that would not work either; the not_modified module 
source is included before the addition_filter so it's module struct and 
relying configurations are not available at the time of compilation.

Proof of bug: Run nginx with attached nginx.conf and create files 
/tmp/add.html and /tmp/index.html with some content. Load 
http://localhost:8080/ in a modern Firefox browser. Then change the file 
/tmp/add.html and nginx will proceed to report "304 Not Modified" and 
your browser's copy will after page refresh remain with the old 
/tmp/add.html file added at the beginning of the response.

Regards!

I submitted the same text as a ticket: 
https://trac.nginx.org/nginx/ticket/2055

-- 
Anton Luka Šijanec
https://šijanec.eu/
+38 6 64/176-345
anton at sijanec.eu (mail, xmpp & sip)
https://šijanec.eu/pgp-key.txt?F4C3E3A4DFB7254397A9F993E76135F49802CD14
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