faster slot finding in nimbus import (#2491)

* faster slot finding in nimbus import

* feat: blocknumber based slot finding

* fix: formatting

* added comments

* fix: added is_execution_block

* added comment

nimbus/nimbus_import.nim

@@ -220,26 +220,36 @@ proc importBlocks*(conf: NimbusConf, com: CommonRef) =
         warn "Could not write csv", err = exc.msg
     blocks.setLen(0)
 
+  # Finds the slot number to resume the import process
+  # First it sets the initial lower bound to `firstSlotAfterMerge` + number of blocks after Era1
+  # Then it iterates over the slots to find the current slot number, along with reducing the
+  # search space by calculating the difference between the `blockNumber` and the `block_number` from the executionPayload
+  # of the slot, then adding the difference to the importedSlot. This pushes the lower bound more, 
+  # making the search way smaller
   template updateLastImportedSlot(
       era: EraDB,
       historical_roots: openArray[Eth2Digest],
       historical_summaries: openArray[HistoricalSummary],
   ) =
     if blockNumber > 1:
-      importedSlot = blockNumber
+      # Setting the initial lower bound 
+      importedSlot = (blockNumber - lastEra1Block) + firstSlotAfterMerge
       notice "Finding slot number after resuming import", importedSlot
-      var parentHash: common.Hash256
-      let currentHash = com.db.getHeadBlockHash()
-      while currentHash != parentHash:
+
+      # BlockNumber based slot finding
+      var clNum = 0'u64
+
+      while clNum < blockNumber:
         let clBlock = getBlockFromEra(
           era, historical_roots, historical_summaries, Slot(importedSlot), clConfig.cfg
-        )
-        if clBlock.isSome:
-          let ethBlock = getEth1Block(clBlock.get())
-          parentHash = ethBlock.header.parentHash
-
+        ).valueOr:
           importedSlot += 1
-      importedSlot -= 1
+          continue
+
+        clNum = getEth1BlockNumber(clBlock)
+        # decreasing the lower bound with each iteration
+        importedSlot += blockNumber - clNum
+
       notice "Found the slot to start with", importedSlot
 
   if isDir(conf.era1Dir.string) or isDir(conf.eraDir.string):

nimbus/utils/era_helpers.nim

@@ -134,6 +134,16 @@ proc getWithdrawals*(x: seq[capella.Withdrawal]): seq[common.Withdrawal] =
     )
   return withdrawals
 
+# This is a helper function to get the eth1 block number 
+# from a beacon block in slot finding process
+proc getEth1BlockNumber*(blck: ForkedTrustedSignedBeaconBlock): uint64 =
+  ## Get the eth1 block number from a beacon block.
+  ## This does not check for pre/post merge, despite having only 
+  ## post merge meaning
+  withBlck(blck):
+    when consensusFork >= ConsensusFork.Bellatrix:
+      return forkyBlck.message.body.execution_payload.block_number
+
 proc getEth1Block*(blck: ForkedTrustedSignedBeaconBlock): EthBlock =
   ## Convert a beacon block to an eth1 block.
   withBlck(blck):