fix/refactor neighbor config

The neighbor configuration storage is fundamentally broken: it requires
all local cells to be configured before being able to list them as
neighbors of each other. Upon config write-back, the neighbor config
however is placed back inline with the other config, and hence a
written-out neighbor config no longer works on program restart.

The cause of this problem is that the config is stored as explicit
pointers between local cells (struct gsm_bts), which of course requires
the pointer to exist before being able to reference it.

Instead, store the actual configuration that the user entered as-is,
without pointers or references to objects that need to be ready. Resolve
the neighbors every time a neighbor is needed.

Hence the user may enter any config at any place in the config file,
even non-working config (like a BTS number that doesn't exist), and the
relation to actual local or remote neighbor cells is made at runtime.

Abort program startup if the initial neighbor configuration contains
errors.

Related: OS#5018
Change-Id: I9ed992f8bfff888b3933733c0576f92d50f2625b

1 files changed, 7 insertions, 4 deletions

diff --git a/include/osmocom/bsc/bts.h b/include/osmocom/bsc/bts.h
index efa413411..f40aa3ec2 100644
--- a/include/osmocom/bsc/bts.h
+++ b/include/osmocom/bsc/bts.h
@@ -496,10 +496,13 @@ struct gsm_bts {
 
 	struct handover_cfg *ho;
 
-	/* A list of struct gsm_bts_ref, indicating neighbors of this BTS.
-	 * When the si_common neigh_list is in automatic mode, it is populated from this list as well as
-	 * gsm_network->neighbor_bss_cells. */
-	struct llist_head local_neighbors;
+	/* Local and remote neighbor configuration: a list of neighbors as written in the VTY config, not resolved to
+	 * actual cells. Entries may point at non-existing BTS numbers, or yet unconfigured ARFCN+BSIC. The point of
+	 * this list is to keep the config as the user entered it: a) to write it back exactly as entered, and b) to
+	 * allow adding neighbor cells that will only be configured further down in the config file.
+	 * An actual neighbor cell object (local or remote-BSS) is resolved "at runtime" whenever a neighbor is being
+	 * looked up. */
+	struct llist_head neighbors;
 
 	/* BTS-specific overrides for timer values from struct gsm_network. */
 	uint8_t T3122;	/* ASSIGNMENT REJECT wait indication */