id-pool: Allocate the lowest available ids.

This simple change makes id-pool to always allocate the
lowest possible id from the pool. No any other code affected
because, actually, there is no users of 'id_pool_free_id' in
OVS.

This behaviour of id-pool will be used in the next patch.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

lib/id-pool.c

@@ -148,6 +148,9 @@ id_pool_free_id(struct id_pool *pool, uint32_t id)
         id_node = id_pool_find(pool, id);
         if (id_node) {
             hmap_remove(&pool->map, &id_node->node);
+            if (id < pool->next_free_id) {
+                pool->next_free_id = id;
+            }
             free(id_node);
         }
     }

lib/id-pool.h

@@ -35,7 +35,7 @@ void id_pool_add(struct id_pool *, uint32_t id);
  * ========
  *
  * Pool of unique 32bit ids.
- *
+ * Allocation always returns the lowest available id.
  *
  * Thread-safety
  * =============