HO Preparation: allocate HO Context in the F1 trigger rather than nr_initiate_handover

During the N2 HO review has been found that the HO context
was not allocated at trigger time but later in the flow. This
might lead to issues in some scenarios (e.g. in N2 HO)
and it was fixed in this commit.

openair2/RRC/NR/rrc_gNB_mobility.c

@@ -128,11 +128,8 @@ static void nr_initiate_handover(const gNB_RRC_INST *rrc,
   DevAssert(target_du != NULL);
   // source_du might be NULL -> inter-CU handover
   DevAssert(ho_prep_info->buf != NULL && ho_prep_info->len > 0);
+  DevAssert(ue->ho_context);
 
-  if (ue->ho_context != NULL) {
-    LOG_E(NR_RRC, "handover for UE %u ongoing, cannot trigger new\n", ue->rrc_ue_id);
-    return;
-  }
   // if any reconfiguration is ongoing, abort handover request
   for (int i = 0; i < NR_RRC_TRANSACTION_IDENTIFIER_NUMBER; ++i) {
     if (ue->xids[i] != RRC_ACTION_NONE) {
@@ -141,10 +138,7 @@ static void nr_initiate_handover(const gNB_RRC_INST *rrc,
     }
   }
 
-  // if we know the source DU (F1 handover), allocate the context for it,
-  // otherwise only for the target DU (N2, Xn)
-  ho_ctx_type_t ctx_type = source_du != NULL ? HO_CTX_BOTH : HO_CTX_TARGET;
-  nr_handover_context_t *ho_ctx = alloc_ho_ctx(ctx_type);
+  nr_handover_context_t *ho_ctx = ue->ho_context;
   ho_ctx->target->du = target_du;
   // we will know target->{du_ue_id,new_rnti} once we have UE ctxt setup
   // response
@@ -164,7 +158,7 @@ static void nr_initiate_handover(const gNB_RRC_INST *rrc,
     int result = asn_copy(&asn_DEF_NR_CellGroupConfig, (void **)&ho_ctx->source->old_cellGroupConfig, ue->masterCellGroup);
     AssertFatal(result == 0, "error during asn_copy() of CellGroupConfig\n");
   }
-  ue->ho_context = ho_ctx;
+
   LOG_A(NR_RRC,
         "Handover triggered for UE %u/RNTI %04x towards DU %ld/assoc_id %d/PCI %d\n",
         ue->rrc_ue_id,
@@ -355,6 +349,13 @@ void nr_rrc_trigger_f1_ho(gNB_RRC_INST *rrc, gNB_RRC_UE_t *ue, nr_rrc_du_contain
   uint8_t buf[NR_RRC_BUF_SIZE];
   int size = do_NR_HandoverPreparationInformation(ue->ue_cap_buffer.buf, ue->ue_cap_buffer.len, buf, sizeof buf);
 
+  // Allocate handover context (both CU and DU)
+  if (ue->ho_context != NULL) {
+    LOG_E(NR_RRC, "Ongoing handover for UE %d, cannot trigger new\n", ue->rrc_ue_id);
+    return;
+  }
+  ue->ho_context = alloc_ho_ctx(HO_CTX_BOTH);
+
   // corresponds to a "handover request", 38.300 Sec 9.3.2.3
   // see also 38.413 Sec 9.3.1.29 for information on source-CU to target-CU
   // information (Source NG-RAN Node to Target NG-RAN Node Transparent Container)