MAC: guard against unsigned underflow in macSduRequest when grant < MAC_HEADER

LteMacEnb::macSduRequest() computed the SDU size as
allocatedBytes - MAC_HEADER in unsigned arithmetic. When the
scheduler allocates a grant smaller than MAC_HEADER (2 bytes),
this underflows to a huge value, so the MAC would request an absurdly
large SDU from RLC, tripping a misleading "configured queueSize too low"
error.

Reproducible with 3 UEs x 4-5 DRBs under QOS_PF contention.

Workaround: Clamp the requested size to 0 instead.

Also added TODO comments.

Author: MohamedSeliem

src/simu5g/stack/mac/LteMacEnb.cc

@@ -223,12 +223,15 @@ void LteMacEnb::macSduRequest()
 
             // send the request message to the upper layer
             auto pkt = new Packet("LteMacSduRequest");
-            auto macSduRequest = makeShared<LteMacSduRequest>();
+            auto macSduRequest = makeShared<LteMacSduRequest>(); //TODO this should be a tag on a Message, not a packet
             macSduRequest->setUeId(destId);
             macSduRequest->setChunkLength(b(1)); // TODO: should be 0
             macSduRequest->setUeId(destId);
             macSduRequest->setLcid(destCid.getLcid());
-            macSduRequest->setSduSize(allocatedBytes - MAC_HEADER);    // do not consider MAC header size
+            // discount MAC header size from grant; clamp to zero if grant is smaller than MAC header
+            //TODO maybe simply skip sending if grant<=MAC_HEADER (i.e. no room for SDU)
+            unsigned int sduSize = (allocatedBytes > MAC_HEADER) ? (allocatedBytes - MAC_HEADER) : 0;
+            macSduRequest->setSduSize(sduSize);
             pkt->insertAtFront(macSduRequest);
             if (queueSize_ != 0 && queueSize_ < macSduRequest->getSduSize()) {
                 throw cRuntimeError("LteMacEnb::macSduRequest: configured queueSize too low - requested SDU will not fit in queue!"