[Aggregator] Use semaphores to lock requests' list

The requests' list might be accessed by the main thread and the monitor
thread simultaneously, so we need to secure the concurrent accesses.

owlps-aggregator/owlps-aggregatord.c

@@ -12,6 +12,7 @@
 #include <errno.h>
 #include <signal.h>
 #include <pthread.h>
+#include <semaphore.h>
 
 #include <confuse.h>
 
@@ -21,7 +22,10 @@
 
 char *program_name = NULL ;
 cfg_t *cfg = NULL ; // Configuration structure
+
 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
 
@@ -49,6 +53,9 @@ int main(int argc, char **argv)
   action.sa_handler = owl_sigterm_handler ;
   sigaction(SIGTERM, &action, NULL) ;
 
+  /* Set up semaphores */
+  sem_init(&lock_requests, 0, 1) ;
+
   /* Create UDP socket */
   listening_port = cfg_getint(cfg, "listening_port") ;
   if ((sockfd = owl_create_udp_listening_socket(listening_port)) < 0)
@@ -130,6 +137,8 @@ int main(int argc, char **argv)
   free_request_list() ;
   free_ap_list() ;
   cfg_free(cfg) ; // Clean configuration
+  // Destroy semaphores:
+  sem_destroy(&lock_requests) ;
 
   fprintf(stderr, "%s: end.\n", program_name) ;
   return ret ;
@@ -531,10 +540,13 @@ void* monitor_requests(void *NULL_value)
 
   while (owl_run)
     {
-      request_ptr = requests ;
       request_prev = NULL ;
       request_info_ptr = NULL ;
+
+      sem_wait(&lock_requests) ;
+
       owl_timestamp_now(&current_time) ;
+      request_ptr = requests ;
 
       while (request_ptr != NULL) // Parsing list
         {
@@ -666,6 +678,8 @@ void* monitor_requests(void *NULL_value)
           request_ptr = request_ptr->next ;
         }
 
+      sem_post(&lock_requests) ;
+
       fflush(NULL) ;
       usleep(cfg_getint(cfg, "check_interval")) ; // Wait to check again
     }
@@ -698,6 +712,8 @@ void got_request(owl_captured_request request)
   tmp_info->next = NULL ;
 
   /* Add it in the list */
+  sem_wait(&lock_requests) ;
+
   tmp_request = requests ;
   if (requests == NULL) // If the request list does not exist,
     {
@@ -798,6 +814,8 @@ void got_request(owl_captured_request request)
             }
         }
     }
+
+  sem_post(&lock_requests) ;
 }
 
 
@@ -810,6 +828,8 @@ void free_request_list()
   request_list *next_request ;
   request_info_list *next_request_info ;
 
+  sem_wait(&lock_requests) ;
+
   while (requests != NULL)
     {
       while (requests->info != NULL)
@@ -822,6 +842,8 @@ void free_request_list()
       free(requests) ;
       requests = next_request ;
     }
+
+  sem_post(&lock_requests) ;
 }
 
 
@@ -1152,19 +1174,22 @@ void free_ap_list()
  */
 void print_request_list()
 {
-  request_list *request_ptr = requests ;
+  request_list *request_ptr = NULL ;
   request_info_list *info_ptr = NULL ;
   char mobile_mac_str[OWL_ETHER_ADDR_STRLEN] ;
   char
     request_time_str[OWL_TIMESTAMP_STR_LEN],
     start_time_str[OWL_TIMESTAMP_STR_LEN] ;
 
+  sem_wait(&lock_requests) ;
+
   if (requests == NULL) // Empty list
     {
       fprintf(stderr, "No request.\n") ;
       return ;
     }
 
+  request_ptr = requests ;
   while (request_ptr != NULL)
     {
       info_ptr = request_ptr->info ; // Get the sub-list pointer
@@ -1199,6 +1224,8 @@ void print_request_list()
 
       request_ptr = request_ptr->next ;
     }
+
+  sem_post(&lock_requests) ;
 }