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Re: Request Counter Clarification

Maxim Dounin
December 25, 2020 09:48AM

On Mon, Dec 21, 2020 at 08:54:54PM +0600, M L wrote:

> I am developing an NGINX module which would check the contents of the
> request and if the key components found, would block it. Currently, it
> seems to be working correctly, but I would like to clarify some parts and
> make sure that I am not hard-coding anything. So, the question is mainly
> about the request counter.
> During the execution of the request handler (which is registered on the
> HTTP_REWRITE_PHASE), the request counter is kept as it is. But once the
> handler finishes the request processing, the counter is changed to 1. But
> changing the counter to 1 does not seem like a right decision, as many
> other modules more often decrease it in the post_handler or call the
> "finalize request" function. However, the use of "finalize" cannot be
> implemented, as neither connection, nor request should not be finalized
> after the handler execution. Instead, the request needs to be handed over
> to the other phase handlers (return NGX_DECLINED). As for the decrementing
> in the post_handler of the ngx_http_read_client_request_body function, on
> the heavy loads, it results in the segfaults. Finally, leaving the counter
> unchanged throughout the process leads to memory leaks. Therefore, the
> above-described value assignment was implemented, but, perhaps, there are
> better ways of handling the request counter issue? And why the change in
> the request counter can cause a segfault in the first place?

In general, you shouldn't touch the request counter yourself
unless you really understand what you are doing. Instead, you
should correctly call ngx_http_finalize_request() to decrease it
(or make sure to return correct return code if the phase handler
does this for you, this will properly decrement). Increasing the
request counter in most cases is handled by nginx core as well.

In no cases you are expected to set the request counter to a
specific value. It is something to be done only during forced
request termination. Any attempt to do this in your own module is
certainly a bug.

Incorrectly adjusting request counter can lead to segfaults or to
connection/memory leaks, depending on the exact code path.

In the particular module you've described it looks like the
problem is that you are trying to read the request body from early
request processing phases (that is, before the content phase), and
do this incorrectly. For a correct example see the mirror module

In particular, to start reading the request body, do something
like this (note ngx_http_finalize_request(NGX_DONE) call to
decrement reference counter, and NGX_DONE return code to stop
further processing of the request with the phase handlers):

rc = ngx_http_read_client_request_body(r, ngx_http_mirror_body_handler);
return rc;

ngx_http_finalize_request(r, NGX_DONE);
return NGX_DONE;

And to continue processing with other phase handlers you should
do something like this in the body handler:

r->write_event_handler = ngx_http_core_run_phases;

This ensures that appropriate write event handler is set (as it is
removed by the request body reading code) and resumes phase
handling by calling ngx_http_core_run_phases().

Maxim Dounin
nginx-devel mailing list
Subject Author Views Posted

Request Counter Clarification

M L 129 December 21, 2020 09:56AM

Re: Request Counter Clarification

Maxim Dounin 19 December 25, 2020 09:48AM

Re: Request Counter Clarification

M L 8 January 16, 2021 12:14PM

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