[Aggregator] Use semaphores to lock APs' ring

The APs' ring is accessed concurrently by listen_for_aps() and
monitor_aps(), so we had to secure that.

TODO

@@ -30,9 +30,6 @@
 
 * Aggregator
 
-- Use locks to read/write the AP list
-   Currently, several threads can access the list simultaneously, and
-   that's not cool!
 - inet_ntoa() is not secure with threads
    (But it is currently used by only one thread.) Use inet_ntop()
    instead?

owlps-aggregator/owlps-aggregatord.c

@@ -27,7 +27,8 @@ request_list *requests = NULL ; // Computed data list
 sem_t lock_requests ; // Semaphore to get access to the requests
 
 ap_list *token_aps = NULL ; // Token ring of the APs
-uint_fast16_t nb_aps = 0 ; // Number of APs in the AP ring
+uint_fast16_t nb_aps = 0 ; // Number of APs in the APs' ring
+sem_t lock_aps ; // Semaphore to get access to the APs' ring
 
 
 
@@ -55,6 +56,7 @@ int main(int argc, char **argv)
 
   /* Set up semaphores */
   sem_init(&lock_requests, 0, 1) ;
+  sem_init(&lock_aps, 0, 1) ;
 
   /* Create UDP socket */
   listening_port = cfg_getint(cfg, "listening_port") ;
@@ -139,6 +141,7 @@ int main(int argc, char **argv)
   cfg_free(cfg) ; // Clean configuration
   // Destroy semaphores:
   sem_destroy(&lock_requests) ;
+  sem_destroy(&lock_aps) ;
 
   fprintf(stderr, "%s: end.\n", program_name) ;
   return ret ;
@@ -894,7 +897,9 @@ void* listen_for_aps(void *NULL_value)
               ap_ip_addr) ;
 #endif // DEBUG
 
+      sem_wait(&lock_aps) ;
       update_ap(message.ap_mac_addr_bytes, ap_ip_addr) ;
+      sem_post(&lock_aps) ;
     }
 
   /* Close the socket */
@@ -1026,13 +1031,21 @@ void* monitor_aps(void *NULL_value)
 
   while (owl_run)
     {
+      sem_wait(&lock_aps) ;
       delete_old_aps() ;
+      sem_post(&lock_aps) ;
 
+      // Here we're not in a hurry, so we released the semaphore to
+      // allow listen_for_aps() to process a received hello packet,
+      // if needed.
+
+      sem_wait(&lock_aps) ;
       if (nb_aps > 1)
         {
           order_send(token_aps) ;
           token_aps = token_aps->next ;
         }
+      sem_post(&lock_aps) ;
 
       usleep(cfg_getint(cfg, "ap_check_interval") * 1000) ;
     }