Question regarding lifetime of PRIV_TASK pointer

[Guillaume Quintard]

Hi Lee,

Looks like you had the right page, but missed the interesting part :-) In
you
https://varnish-cache.org/docs/trunk/reference/vmod.html#ref-vmod-private-pointers
have this bit:

> .fini will be called for a non-NULL .priv of the struct vmod_priv when
the scope ends with that .priv pointer as its second argument besides a
VRT_CTX.

i.e. if your vmod_priv has a methods->fini pointer, it will be called when
the vmod_priv is deleted.

Was this what you were after, or did I misunderstand your question?

-- 
Guillaume Quintard


On Wed, Mar 23, 2022 at 7:28 AM Lee Hambley <lee.hambley at gmail.com> wrote:

> Dear List,
>
> I inherited a project using PRIV_TASK [1] for which the documentation says:
>
> PRIV_TASK “per task” private pointers are useful for state that applies
> to calls for either a specific request or a backend request. For instance
> this can be the result of a parsed cookie specific to a client. Note that
> PRIV_TASK contexts are separate for the client side and the backend side,
> so use in vcl_backend_* will yield a different private pointer from the
> one used on the client side. These private pointers live only for the
> duration of their task.
>
> We do a form of reference counting in our internal data structures, and
> the PRIV_TASK pointer in parts is used to hold a (counted) reference to
> some data in the shared structure.
>
> We are struggling to find the latest possible safest place to hook where
> PRIV_TASK is about to be invalid (end of the request) so that we can
> safely, and finally decrement the reference count and clean-up.
>
> Writing this out now, I suspect that there's a safe exit from the state
> machine [2] where we could modify our VCL to include a call to a clean-up
> function in our vmod, however it's not clear to me if this would be "safe"
> (restarts, request coalescing, etc, etc)
>
> In short then, is there an obvious place into which we can hook which is
> the place where Varnish is already about to discard the "task" and it is
> unoquivically safe for us to decrement our reference counted pointer to the
> PRIV_TASK referenced data?
>
> Thanks so much, very much enjoying being in a role hacking on Varnish, and
> Varnish adjacent stuff in my job currently.
>
> [1]:
> https://varnish-cache.org/docs/trunk/reference/vmod.html#ref-vmod-private-pointers
> [2]:
> https://www.varnish-software.com/developers/tutorials/varnish-configuration-language-vcl/#finite-state-machine
>
>
> Lee Hambley
> http://lee.hambley.name/
> +49 (0) 170 298 5667
> _______________________________________________
> varnish-misc mailing list
> varnish-misc at varnish-cache.org
> https://www.varnish-cache.org/lists/mailman/listinfo/varnish-misc
>
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