From f97ff25ae5ebbc15bd78136654d6131de59e811f Mon Sep 17 00:00:00 2001
From: Donat Zenichev <dzenichev@sipwise.com>
Date: Tue, 3 Dec 2024 04:03:32 +0100
Subject: [PATCH] MT#61574 DSM: re-use previous SDP if new given empty

Re-use previously offered SDP body, when processing
a call session with an involvement of DSM, and getting
newer SIP invite having no SDP offer.

This affects applications using the B2B `connectCollee()`
functionality and doesn't actually touch the B2B core itself.
(Which would be wrong if changed in a usual core SDP
processing, whereas original request can be empty and
an offer/answer is postponed to 200OK/ACK exchange)

(Re)invites towards DSM applications technically must
not be empty, because DSMs don't support late offering.
In order to overcome that just pretend that empty
re-INVITEs are using previous body version.

Change-Id: I252eb9f32e7dc073454a2258b176f77bf15d35c9
---
 apps/dsm/DSMCall.cpp  | 12 ++++++++++--
 core/AmB2BSession.cpp | 16 ++++++++++++++++
 core/AmB2BSession.h   |  1 +
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/apps/dsm/DSMCall.cpp b/apps/dsm/DSMCall.cpp
index 1b7730af..f7d2151b 100644
--- a/apps/dsm/DSMCall.cpp
+++ b/apps/dsm/DSMCall.cpp
@@ -98,11 +98,19 @@ void DSMCall::onStart()
 
 void DSMCall::onInvite(const AmSipRequest& req) {
   // make B2B dialogs work in onInvite as well
+
+  /* save SDP body to re-use if newer request has no SDP */
+  AmMimeBody previous_body(invite_req.body);
   invite_req = req;
 
+  if (invite_req.body.empty() && !previous_body.empty()) {
+    invite_req.body = previous_body;
+    DBG("Currently processed INVITE has no SDP body, use the one from previous offer.\n");
+  }
+
   if (!process_invite) {
     // re-INVITEs
-    AmB2BCallerSession::onInvite(req);
+    AmB2BCallerSession::onInviteKeepSDP(req);
     return;
   }
   process_invite = false;
@@ -126,7 +134,7 @@ void DSMCall::onInvite(const AmSipRequest& req) {
   }    
 
   if (run_session_invite) 
-    AmB2BCallerSession::onInvite(req);
+    AmB2BCallerSession::onInviteKeepSDP(req);
 }
 
 void DSMCall::onInvite2xx(const AmSipReply& reply) {
diff --git a/core/AmB2BSession.cpp b/core/AmB2BSession.cpp
index 1f1e1ec3..b991fdc2 100644
--- a/core/AmB2BSession.cpp
+++ b/core/AmB2BSession.cpp
@@ -1419,6 +1419,22 @@ void AmB2BCallerSession::onInvite(const AmSipRequest& req)
   AmB2BSession::onInvite(req);
 }
 
+void AmB2BCallerSession::onInviteKeepSDP(const AmSipRequest& req)
+{
+  /* save SDP body to re-use if newer request has no SDP */
+  AmMimeBody previous_body(invite_req.body);
+  invite_req = req;
+
+  if (invite_req.body.empty() && !previous_body.empty()) {
+     invite_req.body = previous_body;
+     DBG("Currently processed INVITE has no SDP body, use the one from previous offer.\n");
+  }
+
+  est_invite_cseq = req.cseq;
+
+  AmB2BSession::onInvite(req);
+}
+
 void AmB2BCallerSession::onInvite2xx(const AmSipReply& reply)
 {
   invite_req.cseq = reply.cseq;
diff --git a/core/AmB2BSession.h b/core/AmB2BSession.h
index 22a53a06..c68d91bc 100644
--- a/core/AmB2BSession.h
+++ b/core/AmB2BSession.h
@@ -434,6 +434,7 @@ class AmB2BCallerSession: public AmB2BSession
 
   // @see AmSession
   void onInvite(const AmSipRequest& req);
+  void onInviteKeepSDP(const AmSipRequest& req); /* keeps previous SDP if given INVITE has empty SDP */
   void onInvite2xx(const AmSipReply& reply);
   void onCancel(const AmSipRequest& req);
   void onBye(const AmSipRequest& req);