request body filter last_buf

[Maxim Dounin]

Hello!

On Wed, Jan 26, 2022 at 04:55:52PM -0800, Dk Jack wrote:

> Hi,
> in my module I am inspecting the request body and making certain
> decisions such as sending a 403 based on the content in the body. I based
> my implementation based on the examples in the documentation and other
> nginx modules.
> 
> http://nginx.org/en/docs/dev/development_guide.html#http_request_body_filters
> 
> Sometimes, when my body_filter handler is invoked, I accumulate the body
> into a single buffer for processing in my module. To do this, I have to
> first get the length of the body. To get the length, I cycle through the
> body buffer chain. I also look for the last_buf to be set for the last
> buffer in the chain. The presence of the last_buf tells me that I have the
> complete body. However, sometimes I've noticed that the last_buf flag is
> not set (I log such requests), in which case I cannot process the body.
> 
> Under what conditions would the flag be not set when the body_filter
> handler is invoked? Does the body filter handler get invoked multiple times
> or only once? Is my assumption that the last_buf flag will always be set
> when the body-filter handler is  invoked correct? Any help is appreciated.

The last_buf flag is only guaranteed following 7913:185c86b830ef 
(http://hg.nginx.org/nginx/rev/185c86b830ef, nginx 1.21.2).  
Before this, it wasn't present, for example, in requests 
with empty request body (with "Content-Length: 0").

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