Fix race condition on double RA during RRC reestablishment

Maxime Elkaelrequested to merge
rrc-race-condition into develop

Sometimes, especially in poor channel conditions, a UE can trigger multiple RA procedures during RRC reestablishment, causing reconfigCellGroup to be overwritten with a CellGroup that already has spCellConfig removed, leading to an assertion failure in update_cellgroup_for_reestablishment(). See logs below

[NR_MAC]  456.13 UE 991d: Received Ack of Msg4. CBRA procedure succeeded (UE Connected)
[NR_MAC] Adding new UE context with RNTI 0x991d
[NR_PHY] [RAPROC] 461.9 Initiating RA procedure with preamble 8, energy 48.0 dB (I0 274, thres 130), delay 21 start symbol 2 freq index 0
[NR_MAC] 461.9 UE RA-RNTI 0081 TC-RNTI ecd8: initiating RA procedure
[NR_MAC] UE ecd8: Sync beam index 0
[NR_MAC] UE ecd8: Msg3 scheduled at 462.3 (461.18 TDA 2) start 0 RBs 8
[NR_MAC] UE ecd8: 461.18 Generating RA-Msg2 DCI, RA RNTI 0x81, state 1, preamble_index(RAPID) 8, timing_offset = 21 (estimated distance 820.3 [m])
[NR_MAC]  462. 3 PUSCH with TC_RNTI 0xecd8 received correctly
[NR_MAC] Received a MAC CE for C-RNTI with 991d
[NR_MAC] Remove NR rnti 0xecd8
[NR_MAC]  462. 3 RA with C-RNTI 991d complete
[NR_PHY] [RAPROC] 471.19 Initiating RA procedure with preamble 6, energy 51.0 dB (I0 276, thres 130), delay 21 start symbol 2 freq index 0
[NR_MAC] 471.19 UE RA-RNTI 010d TC-RNTI 022d: initiating RA procedure
[NR_MAC] UE 022d: Sync beam index 0
[NR_MAC] UE 022d: Msg3 scheduled at 472.13 (472.8 TDA 2) start 0 RBs 8
[NR_MAC] UE 022d: 472.8 Generating RA-Msg2 DCI, RA RNTI 0x10d, state 1, preamble_index(RAPID) 6, timing_offset = 21 (estimated distance 820.3 [m])
[NR_MAC]  472.13 PUSCH with TC_RNTI 0x022d received correctly
[NR_MAC] Received a MAC CE for C-RNTI with 991d
[NR_MAC] Remove NR rnti 0x022d
[NR_MAC]  472.13 RA with C-RNTI 991d complete
[NR_RRC] [UL] (cellID 1, UE ID 1 RNTI 991d) Received RRCReestablishmentComplete
[NR_RRC] UE 1 Processing NR_RRCReestablishmentComplete from UE
[NR_MAC] UE 991d: gNB-DU Configuration Query received, will include CellGroupConfig in response

Assertion (new_CellGroup->spCellConfig) failed!
In update_cellgroup_for_reestablishment() /phy/openairinterface5g/openair2/LAYER2/NR_MAC_gNB/mac_rrc_dl_handler.c:666

This MR solves it by skipping the reconfigCellGroup overwrite when it already holds a valid config with spCellConfig, and by replacing the fatal assertion on missing spCellConfig with an early return.

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