[PATCH] nginx does not close the connection for 412 responses under extreme conditions

[Maxim Dounin]

Hello!

On Fri, May 11, 2012 at 08:59:48PM +0800, agentzh wrote:

> Hello!
> 
> I've just noticed that the "412 Precondition Failed page" for the
> If-Unmodified-Since request header could lead to connection hang. That
> is, when the 412 page cannot be sent out in a single run (seen EAGAIN
> for example), then ngx_http_finalize_request will never close the
> downstream connection due to the r->filter_finalize set by
> ngx_http_filter_finalize_request.
> 
> This issue can be reproduced with the standard ngx_http_static_module
> serving the sample index.html page.

Could you please clarify steps to reproduce you've used?  The only 
way I see is to redirect 412 error to an uncached static file with 
aio used, thus causing r->blocked to be set during request 
finalization.

> Here attaches the patch for both nginx 1.0.15 to fix this (it should
> also be applied to nginx 1.2.0, I think).
> 
> Comments welcome!
> 
> Thanks!
> -agentzh
> 
> --- nginx-1.0.15/src/http/ngx_http_request.c	2012-03-05 20:49:32.000000000 +0800
> +++ nginx-1.0.15-patched/src/http/ngx_http_request.c	2012-05-11
> 20:50:01.478111234 +0800
> @@ -1900,6 +1900,7 @@
> 
>      if (rc == NGX_OK && r->filter_finalize) {
>          c->error = 1;
> +        ngx_http_finalize_connection(r);
>          return;
>      }

While the patch looks ok for me, the whole filter_finalize thing 
is at best fragile (and known to cause at least one segfault as of 
now).  It really needs some attention (or even reimplementation).

Maxim Dounin