fix(rln-relay): reduce sync time (#2577)

* fix(rln-relay): reduce sync time

* fix: add batch handling of futures to prevent over utilization of cpu

* fix: need to handle the futures on last iteration when it isnt full

waku/waku_rln_relay/group_manager/on_chain/group_manager.nim

@@ -597,6 +597,19 @@ proc startListeningToEvents(
   let newBlockCallback = g.getNewBlockCallback()
   g.runInInterval(newBlockCallback, DefaultBlockPollRate)
 
+proc batchAwaitBlockHandlingFuture(
+    g: OnchainGroupManager, futs: seq[Future[bool]]
+): Future[void] {.async: (raises: [Exception]).} =
+  for fut in futs:
+    try:
+      var handleBlockRes: bool
+      g.retryWrapper(handleBlockRes, "Failed to handle block"):
+        await fut
+    except CatchableError:
+      raise newException(
+        CatchableError, "could not fetch events from block: " & getCurrentExceptionMsg()
+      )
+
 proc startOnchainSync(
     g: OnchainGroupManager
 ): Future[void] {.async: (raises: [Exception]).} =
@@ -606,6 +619,10 @@ proc startOnchainSync(
 
   # static block chunk size
   let blockChunkSize = 2_000
+  # delay between rpc calls to not overload the rate limit
+  let rpcDelay = 200.milliseconds
+  # max number of futures to run concurrently
+  let maxFutures = 10
 
   var fromBlock =
     if g.latestProcessedBlock > g.rlnContractDeployedBlockNumber:
@@ -616,25 +633,36 @@ proc startOnchainSync(
         blockNumber = g.rlnContractDeployedBlockNumber
       g.rlnContractDeployedBlockNumber
 
+  var futs = newSeq[Future[bool]]()
+  var currentLatestBlock: BlockNumber
+  g.retryWrapper(currentLatestBlock, "Failed to get the latest block number"):
+    cast[BlockNumber](await ethRpc.provider.eth_blockNumber())
+  
   try:
     # we always want to sync from last processed block => latest
     # chunk events
     while true:
-      var currentLatestBlock: BlockNumber
+      # if the fromBlock is less than 2k blocks behind the current block
-      g.retryWrapper(currentLatestBlock, "Failed to get the latest block number"):
+      # then fetch the new toBlock
-        cast[BlockNumber](await ethRpc.provider.eth_blockNumber())
       if fromBlock >= currentLatestBlock:
         break
       
+      if fromBlock + blockChunkSize.uint > currentLatestBlock.uint:
+        g.retryWrapper(currentLatestBlock, "Failed to get the latest block number"):
+          cast[BlockNumber](await ethRpc.provider.eth_blockNumber())
+      
+
       let toBlock = min(fromBlock + BlockNumber(blockChunkSize), currentLatestBlock)
       debug "fetching events", fromBlock = fromBlock, toBlock = toBlock
-      var handleBlockRes: bool
+      await sleepAsync(rpcDelay)
-      g.retryWrapper(handleBlockRes, "Failed to handle old blocks"):
+      futs.add(g.getAndHandleEvents(fromBlock, toBlock))
-        await g.getAndHandleEvents(fromBlock, toBlock)
+      if futs.len >= maxFutures or toBlock == currentLatestBlock:
+        await g.batchAwaitBlockHandlingFuture(futs)
+        futs = newSeq[Future[bool]]()
       fromBlock = toBlock + 1
   except CatchableError:
     raise newException(
-      ValueError,
+      CatchableError,
       "failed to get the history/reconcile missed blocks: " & getCurrentExceptionMsg(),
     )