MAC: fix ASSERT failure on D2D mode switching

Repro:

$ cd simulations/nr/d2d && simu5g_dbg -u Cmdenv -c SinglePair-modeSwitching-UDP

<!> Error: ASSERT: Condition 'connDescOut_.find(cid) != connDescOut_.end()' does not hold in function 'bufferizePacket' at simu5g/stack/mac/LteMacUe.cc:259 -- in module (simu5g::NrMacUe) SingleCell_Standalone_D2D.ueD2DTx[0].cellularNic.nrMac (id=309), at t=15.05952167965s, event #172116

Root cause: When D2D mode switches from DM->IM, macHandleD2DModeSwitch()
calls deleteOutgoingConnection() which permanently destroys the
MAC connection (connDescOut_ entry, queue, and virtual buffer).
However, the PDCP layer retains its LCID table entry and TX entity
for that CID.

When the mode switches back (IM->DM), PDCP considers the connection
"existing" and doesn't trigger binder_->establishUnidirectionalDataConnection().
The packet flows down to MAC which ASSERTs the connection exists -
but it was permanently deleted.

Fix:

LteMacBase: Added clearOutgoingConnectionBuffers(MacCid) that empties
queue and virtual buffer contents without destroying the connection
structure

LteMacUeD2D: Replaced deleteOutgoingConnection(cid) with
clearOutgoingConnectionBuffers(cid) in the mode switch handler,
and removed the now-unnecessary redundant lcgMap_ cleanup code
(which was dead code since deleteOutgoingConnection already cleaned it)

This ensures the MAC connection persists across mode switches with
empty buffers, ready to receive data when the mode switches back.
The idle connection with empty buffers has zero scheduling impact.

Author: avarga

src/simu5g/stack/mac/LteMacBase.cc

@@ -234,6 +234,23 @@ void LteMacBase::deleteOutgoingConnection(MacCid cid)
     connDescOut_.erase(it);
 }
 
+void LteMacBase::clearOutgoingConnectionBuffers(MacCid cid)
+{
+    auto it = connDescOut_.find(cid);
+    if (it == connDescOut_.end())
+        throw cRuntimeError("LteMacBase::clearOutgoingConnectionBuffers - Connection %s not found", cid.str().c_str());
+
+    OutgoingConnectionInfo& connInfo = it->second;
+
+    // Empty the real buffer (drop all packets)
+    while (!connInfo.queue->isEmpty())
+        delete connInfo.queue->popFront();
+
+    // Empty the virtual buffer
+    while (!connInfo.buffer->isEmpty())
+        connInfo.buffer->popFront();
+}
+
 void LteMacBase::createIncomingConnection(MacCid cid, const FlowDescriptor& connInfo)
 {
     Enter_Method("createIncomingConnection(%s)", cid.str().c_str());

src/simu5g/stack/mac/LteMacBase.h

@@ -428,6 +428,13 @@ class LteMacBase : public cSimpleModule
      */
     virtual void deleteOutgoingConnection(MacCid cid);
 
+    /**
+     * clearOutgoingConnectionBuffers() empties the MAC queues and virtual buffers
+     * for a given CID without destroying the connection. Used during D2D mode switch
+     * to preserve the connection for potential mode switch back.
+     */
+    virtual void clearOutgoingConnectionBuffers(MacCid cid);
+
     /**
      * createIncomingConnection() registers an incoming connection for a given CID
      * if it doesn't already exist

src/simu5g/stack/mac/LteMacUeD2D.cc

@@ -848,7 +848,8 @@ void LteMacUeD2D::macHandleD2DModeSwitch(cPacket *pktAux)
                 if (switchPkt->getClearRlcBuffer()) {
                     EV << NOW << " LteMacUeD2D::macHandleD2DModeSwitch - erasing buffered data" << endl;
 
-                    deleteOutgoingConnection(cid);
+                    // Clear buffers but keep the connection alive for potential mode switch back
+                    clearOutgoingConnectionBuffers(cid);
                 }
 
                 if (switchPkt->getInterruptHarq()) {
@@ -886,19 +887,6 @@ void LteMacUeD2D::macHandleD2DModeSwitch(cPacket *pktAux)
             *(pktDup->addTagIfAbsent<FlowControlInfo>()) = connInfo;
             sendUpperPackets(pktDup);
 
-            if (oldDirection != newDirection && switchPkt->getClearRlcBuffer()) {
-                EV << NOW << " LteMacUeD2D::macHandleD2DModeSwitch - clearing LCG map" << endl;
-
-                // Remove entry from lcgMap
-                for (auto lt = lcgMap_.begin(); lt != lcgMap_.end(); ) {
-                    if (lt->second.first == cid) {
-                        lt = lcgMap_.erase(lt);
-                    }
-                    else {
-                        ++lt;
-                    }
-                }
-            }
             EV << NOW << " LteMacUeD2D::macHandleD2DModeSwitch - send switch signal to the RLC TX entity corresponding to the old mode, cid " << cid << endl;
         }