Introduce mg_timer_expired(). Adopt DHCP and link status to use it

src/timer.c

@@ -5,8 +5,8 @@
 
 void mg_timer_init(struct mg_timer **head, struct mg_timer *t, uint64_t ms,
                    unsigned flags, void (*fn)(void *), void *arg) {
-  struct mg_timer tmp = {0U, ms, 0U, 0U, flags, fn, arg, *head};
-  *t = tmp;
+  t->id = 0, t->period_ms = ms, t->expire = 0;
+  t->flags = flags, t->fn = fn, t->arg = arg, t->next = *head;
   *head = t;
 }
 
@@ -15,27 +15,26 @@ void mg_timer_free(struct mg_timer **head, struct mg_timer *t) {
   if (*head) *head = t->next;
 }
 
+// t: expiration time, prd: period, now: current time. Return true if expired
+bool mg_timer_expired(uint64_t *t, uint64_t prd, uint64_t now) {
+  if (now + prd < *t) *t = 0;                    // Time wrapped? Reset timer
+  if (*t == 0) *t = now + prd;                   // Firt poll? Set expiration
+  if (*t > now) return false;                    // Not expired yet, return
+  *t = (now - *t) > prd ? now + prd : *t + prd;  // Next expiration time
+  return true;                                   // Expired, return true
+}
+
 void mg_timer_poll(struct mg_timer **head, uint64_t now_ms) {
-  // If time goes back (wrapped around), reset timers
   struct mg_timer *t, *tmp;
   for (t = *head; t != NULL; t = tmp) {
+    bool once = t->expire == 0 && (t->flags & MG_TIMER_RUN_NOW) &&
+                !(t->flags & MG_TIMER_CALLED);  // Handle MG_TIMER_NOW only once
+    bool expired = mg_timer_expired(&t->expire, t->period_ms, now_ms);
     tmp = t->next;
-    if (t->prev_ms > now_ms) t->expire = 0;  // Handle time wrap
-    t->prev_ms = now_ms;
-    if (t->expire == 0 && (t->flags & MG_TIMER_RUN_NOW) &&
-        !(t->flags & MG_TIMER_CALLED)) {
-      // Handle MG_TIMER_NOW only once
-    } else if (t->expire == 0) {
-      t->expire = now_ms + t->period_ms;
-    }
-    if (t->expire > now_ms) continue;
+    if (!once && !expired) continue;
     if ((t->flags & MG_TIMER_REPEAT) || !(t->flags & MG_TIMER_CALLED)) {
       t->fn(t->arg);
     }
     t->flags |= MG_TIMER_CALLED;
-    // Try to tick timers with the given period as accurate as possible,
-    // even if this polling function is called with some random period.
-    t->expire = now_ms - t->expire > t->period_ms ? now_ms + t->period_ms
-                                                  : t->expire + t->period_ms;
   }
 }