resume EL sync at correct block on reconnect (#3957)

When connection to the EL is lost as part of EL deposits importing, the
targeted block range to sync would reset. This is changed to properly
remember import progress across reconnects.

beacon_chain/eth1/eth1_monitor.nim

@@ -1410,24 +1410,25 @@ proc startEth1Syncing(m: Eth1Monitor, delayBeforeStart: Duration) {.async.} =
 
   let shouldProcessDeposits = not m.depositContractAddress.isZeroMemory
   var eth1SyncedTo: Eth1BlockNumber
-  if shouldProcessDeposits and m.depositsChain.blocks.len == 0:
-    let startBlock = awaitWithRetries(
-      m.dataProvider.getBlockByHash(
-        m.depositsChain.finalizedBlockHash.asBlockHash))
+  if shouldProcessDeposits:
+    if m.depositsChain.blocks.len == 0:
+      let startBlock = awaitWithRetries(
+        m.dataProvider.getBlockByHash(
+          m.depositsChain.finalizedBlockHash.asBlockHash))
 
-    m.depositsChain.addBlock Eth1Block(
-      hash: m.depositsChain.finalizedBlockHash,
-      number: Eth1BlockNumber startBlock.number,
-      timestamp: Eth1BlockTimestamp startBlock.timestamp)
+      m.depositsChain.addBlock Eth1Block(
+        hash: m.depositsChain.finalizedBlockHash,
+        number: Eth1BlockNumber startBlock.number,
+        timestamp: Eth1BlockTimestamp startBlock.timestamp)
 
-    eth1SyncedTo = Eth1BlockNumber startBlock.number
+    eth1SyncedTo = Eth1BlockNumber m.depositsChain.blocks[^1].number
 
     eth1_synced_head.set eth1SyncedTo.toGaugeValue
     eth1_finalized_head.set eth1SyncedTo.toGaugeValue
     eth1_finalized_deposits.set(
       m.depositsChain.finalizedDepositsMerkleizer.getChunkCount.toGaugeValue)
 
-    debug "Starting Eth1 syncing", `from` = shortLog(m.depositsChain.blocks[0])
+    debug "Starting Eth1 syncing", `from` = shortLog(m.depositsChain.blocks[^1])
 
   while true:
     if bnStatus == BeaconNodeStatus.Stopping: