Handling module ctx reset on internal redirect

[Pillai, Saju]

On Tue, 14 Feb 2017, Eran Kornblau wrote:

> If I understand your description correctly, I had the same problem - I wanted to expose an internal error code
> of my module as an nginx variable, so that I could write it to the access log. But after the redirect to the error page,
> the context was erased and the handler could not retrieve it.
>
> The solution I implemented in my case was that when an error occurs, instead of saving it on the module context,
> I explicitly set the value of the nginx variable (r->variables). This way the variable can be evaluated without any
> dependency on my module.
>
> This is the code that sets the value -
> https://github.com/kaltura/nginx-vod-module/blob/master/ngx_http_vod_utils.c#L126
>
> The variable index is initialized right after creating the variable, here:
> https://github.com/kaltura/nginx-vod-module/blob/3c3cf6bbcd08f8cd6af2f5d21d7e8d384f293e9e/ngx_http_vod_module.c#L597
>
> And the handler of this variable is now just an empty placeholder:
> https://github.com/kaltura/nginx-vod-module/blob/3c3cf6bbcd08f8cd6af2f5d21d7e8d384f293e9e/ngx_http_vod_module.c#L268
>
> Hope this helps,

Yes. My calculations for the variable are done once upfront in my handler, so I
can use your plan of filling in the variable right after those calculations are
done.


-srp

>
> Eran
>
> -----Original Message-----
> From: nginx-devel [mailto:nginx-devel-bounces at nginx.org] On Behalf Of Pillai, Saju
> Sent: Tuesday, February 14, 2017 5:26 PM
> To: nginx-devel at nginx.org
> Subject: Handling module ctx reset on internal redirect
>
>
> Hello,
>
> I am new to nginx and will appreciate some insight on handling the resetting of module ctx during internal redirects.
>
> Specifically, I have a module that runs in the HTTP_POST_READ_PHASE to build some objects in memory, which it hangs off the request object via ngx_http_set_ctx(r,..). These memory objects back new variables created by this module.
>
> After an internal redirect, when a filter tries to access the variables, my handler is unable to find the ctx object via a ngx_http_get_module_ctx(). I see that ngx_http_core_module.c:ngx_http_internal_redirect() zeros out all module contexts during an internal redirect, and that makes sense. The (appropriate) contexts will have to be rebuilt by modules that run for that internal redirect.
>
> Since HTTP_POST_READ_PHASE handlers dont run on a redirect, how should I setup my ctx so I can still access it after a redirect? I could create a r->pool cleanup function and hang my ctx in the ngx_pool_cleanup_t->data slot; but that seems like a workaround.
>
> -srp
>
>
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