Modules: Fix an integer sign bug in moduleTimerHandler (redis#8131)

bug was introduced in 1a91a27

src/module.c

@@ -5469,8 +5469,11 @@ int moduleTimerHandler(struct aeEventLoop *eventLoop, long long id, void *client
         } else {
             /* We call ustime() again instead of using the cached 'now' so that
              * 'next_period' isn't affected by the time it took to execute
-             * previous calls to 'callback. */
-            next_period = (expiretime-ustime())/1000; /* Scale to milliseconds. */
+             * previous calls to 'callback.
+             * We need to cast 'expiretime' so that the compiler will not treat
+             * the difference as unsigned (Causing next_period to be huge) in
+             * case expiretime < ustime() */
+            next_period = ((long long)expiretime-ustime())/1000; /* Scale to milliseconds. */
             break;
         }
     }
@@ -5519,7 +5522,8 @@ RedisModuleTimerID RM_CreateTimer(RedisModuleCtx *ctx, mstime_t period, RedisMod
 
     /* We need to install the main event loop timer if it's not already
      * installed, or we may need to refresh its period if we just installed
-     * a timer that will expire sooner than any other else. */
+     * a timer that will expire sooner than any other else (i.e. the timer
+     * we just installed is the first timer in the Timers rax). */
     if (aeTimer != -1) {
         raxIterator ri;
         raxStart(&ri,Timers);