chore: better approch for busy waiting and async task

waku/waku_rln_relay/rln_relay.nim

@@ -89,6 +89,7 @@ type WakuRLNRelay* = ref object of RootObj
   groupManager*: GroupManager
   onFatalErrorAction*: OnFatalErrorHandler
   nonceManager*: NonceManager
+  epochMonitorFuture*: Future[void]
 
 proc calcEpoch*(rlnPeer: WakuRLNRelay, t: float64): Epoch =
   ## gets time `t` as `flaot64` with subseconds resolution in the fractional part
@@ -392,22 +393,29 @@ proc generateRlnValidator*(
 
   return validator
 
-proc monitorEpochs(wakuRlnRelay: WakuRLNRelay) {.async.} =
+proc monitorEpochs(wakuRlnRelay: WakuRLNRelay): Future[void] {.async.} =
+  var nextEpochTime = epochTime()
   var lastEpoch = wakuRlnRelay.calcEpoch(epochTime())
+  var firstChangeDetected = false
 
   while true:
     try:
       let currentEpoch = wakuRlnRelay.calcEpoch(epochTime())
 
       if currentEpoch != lastEpoch:
+        nextEpochTime = epochTime() + wakuRlnRelay.rlnEpochSizeSec
         waku_rln_proof_remining.set(
           wakuRlnRelay.groupManager.userMessageLimit.get().float64
         )
         lastEpoch = currentEpoch
+        firstChangeDetected = true
 
-      await sleepAsync(500)
-    except CatchableError as e:
-      error "Error in epoch monitoring", err = e.msg
+      if firstChangeDetected:
+        await sleepAsync(nextEpochTime - epochTime())
+      else:
+        await sleepAsync(1000) # 1 second
+    except CatchableError:
+      error "Error in epoch monitoring", error = getCurrentExceptionMsg()
 
 proc mount(
     conf: WakuRlnConfig, registrationHandler = none(RegistrationHandler)
@@ -472,7 +480,7 @@ proc mount(
   )
 
   # Start epoch monitoring in the background
-  asyncSpawn monitorEpochs(wakuRlnRelay)
+  wakuRlnRelay.epochMonitorFuture = monitorEpochs(wakuRlnRelay)
 
   return ok(wakuRlnRelay)