lib: fix order of northbound callbacks

When ordering the NB callbacks according to their priorities, if the
operation is "destroy" we should reverse the order, to destroy the
dependants before the dependencies.

This fixes the crash, that can be reproduced with the following steps:
```
frr# conf term file-lock
frr(config)# affinity-map map bit-position 10
frr(config)# interface test
frr(config-if)# link-params
frr(config-link-params)# affinity map
frr(config-link-params)# exit
frr(config-if)# exit
frr(config)# mgmt commit apply
frr(config)# no affinity-map map
frr(config)# interface test
frr(config-if)# link-params
frr(config-link-params)# no affinity map
frr(config-link-params)# exit
frr(config-if)# exit
frr(config)# mgmt commit apply
```

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>

1 files changed, 6 insertions, 3 deletions

diff --git a/lib/northbound.c b/lib/northbound.c
index a3d91e56a..6b31b818c 100644
--- a/lib/northbound.c
+++ b/lib/northbound.c
@@ -391,11 +391,14 @@ void nb_config_replace(struct nb_config *config_dst,
 static inline int nb_config_cb_compare(const struct nb_config_cb *a,
 				       const struct nb_config_cb *b)
 {
-	/* Sort by priority first. */
+	/*
+	 * Sort by priority first. If the operation is "destroy", reverse the
+	 * order, so that the dependencies are destroyed before the dependants.
+	 */
 	if (a->nb_node->priority < b->nb_node->priority)
-		return -1;
+		return a->operation != NB_CB_DESTROY ? -1 : 1;
 	if (a->nb_node->priority > b->nb_node->priority)
-		return 1;
+		return a->operation != NB_CB_DESTROY ? 1 : -1;
 
 	/*
 	 * Preserve the order of the configuration changes as told by libyang.