[asterisk-dev] Adding headers to NOTIFY responses

[Joshua C. Colp]

On Tue, Nov 15, 2022 at 7:12 PM <asterisk at phreaknet.org> wrote:

> On 11/15/2022 9:56 AM, Joshua C. Colp wrote:
> > On Tue, Nov 15, 2022 at 10:50 AM <asterisk at phreaknet.org
> > <mailto:asterisk at phreaknet.org>> wrote:
> >
> >
> >     If res_pjsip_pubsub would need to be extended to support this,
> >     would it
> >     reasonable to add a callback to a pubsub module that allows it
> >     access to
> >     the pjsip_tx_data, so it can do whatever it needs to with it,
> >     before the
> >     response gets sent? Or another preferred method of allowing
> >     modules to
> >     add headers?
> >
> >
> > At a surface it is probably fine.
>
> Thanks, doing that allowed just what I needed to do.
>
> Next limitation... the new_subscribe callback is supposed to return 200
> (or some other code) to accept or reject the subscription. The only
> arguments are the endpoint name and resource name.
> This is not really always sufficient; it may be necessary to approve or
> reject the subscription using some information present in the
> subscription itself (for example, a header). I think this is all
> consequent of the very narrow range of scenarios that res_pjsip_pubsub
> was written for originally.
>
> The subscription_established callback is actually perfectly set up for
> this. We have a handle on the ast_sip_subscription, and can call
> ast_sip_subscription_get_header if needed to get the header.
> However, this requires approving all subscriptions with a 200 in the
> new_subscribe callback, only to potentially realize it should be
> rejected in the subscription_established callback. This is too late
> because the 200 OK already gets sent to the endpoint before
> subscription_established gets called.
>
> So, the only good solution is to extend new_subscribe to accept a third
> argument: rdata, since a subscription hasn't yet been created at that
> point so we could not use ast_sip_subscription_get_header to fetch
> headers. Yuck, since it's a public API... there could also be a
> new_subscribe_with_rdata callback that gets executed instead if a module
> defines one. Or maybe we can break ABI and go master only here if that
> would be too inelegant.
>

Even adding a new callback will break the ABI. I think fundamentally this
work should only occur in master where changing ABI is fine. I have
concerns that it will cause unintended consequences in some way and I've
had enough of those in the past year in release branches.

-- 
Joshua C. Colp
Asterisk Project Lead
Sangoma Technologies
Check us out at www.sangoma.com and www.asterisk.org
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