trans_lcls_compose(): Set PLMN fron cell currently in use

The MS in general provides the Selected PLMN ID (IE) in the Complete
Layer 3 Information message. osmo-msc handles that message in
msc_a_ran_dec_from_msc_i() and stores the information of the PLMN in
msc_a->via_cell. If no PLMN information is provided in the message, then
at that same place the PLMN configured in the VTY is taken as an implicit
default.

This patch changes trans_lcls_compose() to use the PLMN stored in
msc_a->via_cell instead of the VTY configured one, meaning the PLMN
provided by the MS (through the RAN in use) is used if available
(otherwise the VTY-configure one is still used, as before).

With this patch the PLMN VTY config option use is relegated to a single
point of use in msc_a_ran_dec_from_msc_i() where the Complete Layer 3
Information is used. As a result, it becomes clear now that the VTY
config is only applied in the scenario where no PLMN is provided at that
time.

Related: SYS#6360
Change-Id: Ibad0005a1d7cef64dd8fefa3e554ba99a06c3666
---
M src/libmsc/transaction.c
1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/src/libmsc/transaction.c b/src/libmsc/transaction.c
index 89f77e5..3e458f1 100644
--- a/src/libmsc/transaction.c
+++ b/src/libmsc/transaction.c
@@ -153,7 +153,7 @@
 	lcls->gcr.node = ss7->cfg.primary_pc;
 
 	/* net id from Q.1902.3 3-5 bytes, this function gives 3 bytes exactly */
-	osmo_plmn_to_bcd(lcls->gcr.net, &trans->net->plmn);
+	osmo_plmn_to_bcd(lcls->gcr.net, &trans->msc_a->via_cell.lai.plmn);
 
 
 	/* TS 29.205 Table B.2.1.9.2 Call Reference ID

