pjsua: select dialog Via address toward the actual next hop

[nanangizz]

Follow-up to #5051 (now merged); this PR targets master and contains only its own change.

Problem

When an account is not using STUN/UPnP, pjsua selects the local Via sent-by for a dialog via pjsua_acc_get_uac_addr(). That function resolves the destination from the account's outbound proxy (acc->route_set) or the passed target URI. That reference is wrong for the request's actual next hop in two cases:

• UAS dialogs (incoming call, incoming SUBSCRIBE): in-dialog requests (BYE, NOTIFY, re-INVITE) are routed via the dialog route set built from the incoming Record-Route, falling back to the remote target — never the account's outbound proxy.
• Outgoing call (dlg_set_via): the target passed was acc->cfg.id (the account id URI), not the dialog's remote target. With no account proxy, the Via is computed toward the account domain instead of the actual callee.

On a multihomed host these can put the wrong local interface in the Via.

Change

Factor the address-resolution body of pjsua_acc_get_uac_addr() into a shared static core get_uac_addr_for_sip_uri() — behavior for all existing UAC callers is unchanged — and add two dialog variants that compute the address toward the actual next hop and skip reliable transports (their Via is resolved from the real connection at send time; they return PJ_ENOTFOUND so the caller leaves the Via untouched):

• pjsua_acc_get_uas_addr() — next hop = dlg->route_set top (from the incoming Record-Route), else dlg->target. Used by pjsua_call_on_incoming() and pres_on_rx_request().
• pjsua_acc_get_uac_dlg_addr() — next hop = account outbound proxy when set, else the dialog's remote target (dlg->target) rather than acc->cfg.id. Used by dlg_set_via().

Keying the reliable-transport skip off the true next hop also fixes a corner case where a UDP next hop reached via a reliable target/proxy could get a STUN-mapped address in the Via (#1804 / #1412).

Notes

• UDP behavior in the common case (no Record-Route, no account proxy) is unchanged — the address is still computed toward dlg->target.
• No public API change; the two new functions are internal (pjsua_internal.h).

Testing

Non-sanitizer build:

• 100_simple.py (UDP call) — pass
• 200_tcp.py (TCP call) — pass

(The 100_peertopeer.py presence test fails identically on master and is a pre-existing environment issue, not related to this change.)

[Copilot]

Pull request overview

This PR refines how PJSUA selects the local Via sent-by address when an account is not using STUN/UPnP, ensuring the selection is made toward the dialog’s actual next hop (route set / remote target) rather than the account’s outbound proxy or id URI, and avoiding reliable-transport probing that can create unused TCP/TLS connections.

Changes:

• Refactors the core address-selection logic from pjsua_acc_get_uac_addr() into a shared static helper and adds two internal dialog-oriented variants.
• Updates incoming-call and incoming-SUBSCRIBE paths to compute the Via address toward the dialog’s effective next hop (route set top entry, else remote target).
• Updates outgoing-call dialog Via setup to use the dialog’s remote target (when no account outbound proxy is configured), while skipping reliable transports.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
pjsip/src/pjsua-lib/pjsua_pres.c	Switches incoming SUBSCRIBE dialog Via selection to the new UAS next-hop-aware resolver.
pjsip/src/pjsua-lib/pjsua_call.c	Updates outgoing dialog Via selection and incoming-call Via selection to use next-hop-aware dialog resolvers.
pjsip/src/pjsua-lib/pjsua_acc.c	Factors out core SIP-URI-based address resolution and adds dialog (UAS/UAC) helpers that skip reliable transports.
pjsip/include/pjsua-lib/pjsua_internal.h	Declares the new internal dialog-oriented address selection functions.