nathelper: fix Contact concatenation when fix_nated_contact() runs after topology_hiding()

[NormB]

Summary

fix_nated_contact() called after topology_hiding() produces a malformed Contact header containing two concatenated SIP URIs when the Contact lacks angle-bracket enclosure (<>).

Details

Both topology_hiding() and fix_nated_contact() rewrite the Contact header using the lump mechanism. Each creates a LUMP_DEL entry (marking a region for deletion) with an after chain LUMP_ADD entry (inserting replacement text).

topology_hiding() via delete_existing_contact() deletes msg->contact->body (the full Contact body). fix_nated_contact_f() deletes from c->uri.s through the host:port portion.

The Contact parser (parser/contact/contact.c:217-222) strips angle brackets from contact_t->uri:

• With <>: body.s points to < (offset N), uri.s points to sip: (offset N+1) — offsets differ by 1
• Without <>: body.s and uri.s point to the same address — offsets are identical

In msg_translator.c, process_lumps() iterates lumps sorted by offset. It has a last_del exception that allows processing of multiple lumps anchored at the same deletion point:

if (t->u.offset < s_offset && t->u.offset != last_del) {
    continue;
}

When offsets differ by 1 (with brackets), the second lump falls inside the first's deleted region and is correctly skipped. When offsets are identical (no brackets), the last_del exception prevents the skip, and both replacement values are emitted — producing:

Contact: <sip:10.0.0.40;did=282.a0601bb2>sip:alice@10.0.0.40:15060

The reverse call order (fix_nated_contact() then topology_hiding()) is already handled: delete_existing_contact() in topo_hiding_logic.c:319-336 scans the lump list for existing Contact DEL lumps and neutralizes them. No equivalent guard exists in fix_nated_contact_f().

Solution

Add a helper function contact_already_rewritten() that scans msg->add_rm for a prior LUMP_DEL of type HDR_CONTACT_T whose offset and length fully encompass the current Contact URI. This detects when another function (such as topology_hiding()) has already scheduled the Contact for rewriting.

A new module parameter nated_contact_override controls the behavior when a conflict is detected:

• 0 (default): skip this Contact — the prior rewrite stands. fix_nated_contact() returns without creating lumps for this Contact.
• 1: override — neutralize the prior lump by setting op = LUMP_NOP and inserting COND_FALSE guards on its before/after chains (the same technique used by delete_existing_contact() in topo_hiding_logic.c:331-334). fix_nated_contact() then proceeds to create its own replacement.

Design notes:

• The check uses continue (not return) inside the get_contact_uri() loop, so only conflicting Contacts are affected; uncovered Contacts in sibling headers are processed normally.
• The lump scan exploits the sort-by-offset invariant maintained by del_lump() (data_lump.c:369-377) for early exit: once we see a DEL lump with offset past the URI start, no later lump can fully cover the URI.
• No locking is needed: msg->add_rm is per-process pkg memory owned by a single worker, processed sequentially in the routing script. This is the same concurrency model used by delete_existing_contact().
• The coverage test (crt->u.offset <= uri_off && crt->u.offset + crt->len >= uri_end) intentionally requires full containment. Partial-overlap lumps (e.g., from a prior fix_nated_contact() call) are not matched — that case is already handled by the existing "URI outside buffer" check at nathelper.c:703.

Compatibility

• Default nated_contact_override=0 preserves the current effective behavior: when topology_hiding() runs first, its Contact wins. The only change is that the concatenation bug is eliminated.
• No changes to core lump processing (msg_translator.c, data_lump.c). All changes are localized to the nathelper module.
• The detection is general: any HDR_CONTACT_T LUMP_DEL fully covering the URI will be detected, not only those from topology_hiding(). This is intentional and consistent with delete_existing_contact()'s general approach.
• Module documentation for the new parameter is included in nathelper_admin.xml.

Closing issues

closes #2172