prevent ARFCN+BSIC collisions in config

If multiple BTS are configured with identical ARFCN+BSIC, handover cannot
possibly work. Add warn_on_arfcn_bsic_collisions() and when collisions are
detected:

- exit right away on program startup.
- warn on vty bts node-exit (go_parent_cb()), i.e. when ARFCN+BSIC collide
  implicitly from default values.
- warn on vty bts/base_station_id_code cmd coming from telnet vty,
  i.e. when the user actively configures colliding BSIC.
- warn on vty trx/arfcn cmd coming from telnet vty,
  i.e. when the user actively configures colliding ARFCN.

When warning, warn on all of:
- DHO log,
- stderr,
- vty_out() in case a vty instance is passed (i.e. from interactive vty).
It's better to log this once too often than let the user miss it.

Rationale: when I started to implement ttcn3 tests for inter-bsc HO, I saw that
the new code based on ARFCN+BSIC would cause a handover request intended to go
from BTS 0 to BTS 1 to instead go from BTS 0 to back to BTS 0 -- because of
identical ARFCN+BSIC in the config of ttcn3-bsc-tests. Such a config badly
confuses handover: measurement reports interpreted to come from the wrong cell,
all handovers attempted to go to the first BTS (sometimes correctly, sometimes
wrongly), in short, non-obvious failure across the handover board. The network
would appear to work well, but occasionally a handover would kill a call
without apparent reason. It seems worthwhile to prevent such misconfiguration
as early as possible, and even allow previous ARFCN+BSIC misconfiguration that
used to work to instead break now (only upon reading config during program
startup, to not cause program exit just because a user introduces a temporary
collision in an interactive vty session).

Change-Id: Ia7c38188ccbad5d8b7398e3e5220015e62c08c8b

1 files changed, 2 insertions, 0 deletions

diff --git a/include/osmocom/bsc/vty.h b/include/osmocom/bsc/vty.h
index e63275546..bfea69d2a 100644
--- a/include/osmocom/bsc/vty.h
+++ b/include/osmocom/bsc/vty.h
@@ -35,4 +35,6 @@ int bsc_vty_init_extra(void);
 
 struct gsm_network *gsmnet_from_vty(struct vty *vty);
 
+int warn_on_arfcn_bsic_collisions(struct gsm_network *net, struct vty *vty);
+
 #endif