feat: checkpoint-sync-url now can be a list of checkpoint nodes (#1411)

Author: LeanSerra

README.md

@@ -106,6 +106,14 @@ You can specify a URL to fetch it from with the "--checkpoint-sync-url" flag:
 iex -S mix run -- --checkpoint-sync-url <your_url_here>
 ```
 
+or you can specify mulitple urls by passing a comma separated list of urls:
+
+```shell
+iex -S mix run -- --checkpoint-sync-url "<url1>, <url2>, ..."
+```
+
+If multiple urls are provided the downloaded state will be compared for all urls and fail if even one of them differs from the rest
+
 Some public endpoints can be found in [eth-clients.github.io/checkpoint-sync-endpoints](https://eth-clients.github.io/checkpoint-sync-endpoints/).
 
 > [!IMPORTANT]

config/runtime.exs

@@ -103,7 +103,13 @@ config :lambda_ethereum_consensus, LambdaEthereumConsensus.Store.Db, dir: datadi
 # We use put_env here as we need this immediately after to read the state.
 Application.put_env(:lambda_ethereum_consensus, ChainSpec, config: chain_config)
 
-strategy = StoreSetup.make_strategy!(testnet_dir, checkpoint_sync_url)
+checkpoint_urls =
+  case checkpoint_sync_url do
+    urls when is_binary(urls) -> urls |> String.split(",") |> Enum.map(&String.trim/1)
+    nil -> nil
+  end
+
+strategy = StoreSetup.make_strategy!(testnet_dir, checkpoint_urls)
 
 genesis_validators_root =
   case strategy do

lib/lambda_ethereum_consensus/beacon/store_setup.ex

@@ -20,15 +20,15 @@ defmodule LambdaEthereumConsensus.Beacon.StoreSetup do
   @doc """
   Args: at least one can be nil.
   - testnet_dir: directory of a testnet configuration, including ssz and yaml config.
-  - checkpoint_sync_url: a url where checkpoint sync can be performed.
+  - checkpoint_sync_url: list of urls where checkpoint sync can be performed.
 
   Return value: a store setup strategy, which is one of the following:
   - {:file, anchor_state}: path of an ssz file to get the genesis state from.
-  - {:checkpoint_sync_url, url}: url to get the genesis state from if performing checkpoint sync.
+  - {:checkpoint_sync_url, url}: list of urls to get the genesis state from if performing checkpoint sync.
   - :db : the genesis state and store can only be recovered from the db.
   """
   def make_strategy!(nil, nil), do: :db
-  def make_strategy!(nil, url) when is_binary(url), do: {:checkpoint_sync_url, url}
+  def make_strategy!(nil, urls) when is_list(urls), do: {:checkpoint_sync_url, urls}
 
   def make_strategy!(dir, nil) when is_binary(dir) do
     Path.join(dir, "genesis.ssz")
@@ -55,14 +55,14 @@ defmodule LambdaEthereumConsensus.Beacon.StoreSetup do
     store
   end
 
-  def setup!({:checkpoint_sync_url, checkpoint_url}) do
+  def setup!({:checkpoint_sync_url, checkpoint_urls}) do
     case restore_state_from_db() do
       {:ok, store} ->
         Logger.warning("[Checkpoint sync] Recent state found. Ignoring the checkpoint URL.")
         store
 
       _ ->
-        fetch_state_from_url(checkpoint_url)
+        fetch_and_compare_state_from_urls(checkpoint_urls)
     end
   end
 
@@ -87,8 +87,12 @@ defmodule LambdaEthereumConsensus.Beacon.StoreSetup do
   @spec get_deposit_snapshot!() :: DepositTreeSnapshot.t() | nil
   def get_deposit_snapshot!(), do: get_deposit_snapshot!(get_strategy!())
 
+  # The endpoint for deposit snapshots is deprecated in electra and will be removed in Fulu
+  # https://github.com/ethereum/beacon-APIs/pull/494
+  # For this reason we don't compare the deposits from the urls as most checkpoints are returning error 500
   @spec get_deposit_snapshot!(store_setup_strategy()) :: DepositTreeSnapshot.t() | nil
-  def get_deposit_snapshot!({:checkpoint_sync_url, url}), do: fetch_deposit_snapshot(url)
+  def get_deposit_snapshot!({:checkpoint_sync_url, urls}),
+    do: fetch_deposit_snapshot(List.first(urls))
 
   def get_deposit_snapshot!(:db) do
     case StoreDb.fetch_deposits_snapshot() do
@@ -129,28 +133,65 @@ defmodule LambdaEthereumConsensus.Beacon.StoreSetup do
     end
   end
 
-  defp fetch_state_from_url(url) do
+  defp fetch_and_compare_state_from_urls(urls) do
     Logger.info("[Checkpoint sync] Initiating checkpoint sync")
 
+    # Fetch last finalized block for all urls
+    blocks = for {:ok, res} <- Enum.map(urls, &CheckpointSync.get_block/1), do: res
+
+    case Enum.uniq(blocks) do
+      [_] ->
+        Logger.info(
+          "[Checkpoin sync] Received the same last finalized block from #{length(blocks)} checkpoint nodes"
+        )
+
+      _ ->
+        Logger.error(
+          "[Checkpoint sync] Received inconsistent last finalized block from #{length(blocks)} checkpoint nodes"
+        )
+
+        Logger.flush()
+        System.halt(1)
+    end
+
     genesis_validators_root = ChainSpec.get_genesis_validators_root()
 
+    # All urls returned the same last finalized block, we will trust the first to get the state
+    {anchor_state, anchor_block} = fetch_state_from_url(genesis_validators_root, List.first(urls))
+
+    first_block = List.first(blocks)
+
+    if anchor_state.latest_block_header.parent_root == first_block.message.parent_root do
+      {:ok, store} = Store.get_forkchoice_store(anchor_state, anchor_block)
+
+      # Save store in DB
+      StoreDb.persist_store(store)
+
+      store
+    else
+      Logger.error(
+        "[Checkpoint sync] Root mismatch when comparing latest finalized block with downloaded state"
+      )
+
+      Logger.flush()
+      System.halt(1)
+    end
+  end
+
+  defp fetch_state_from_url(genesis_validators_root, url) do
     case CheckpointSync.get_finalized_block_and_state(url, genesis_validators_root) do
       {:ok, {anchor_state, anchor_block}} ->
         Logger.info(
-          "[Checkpoint sync] Received beacon state and block",
+          "[Checkpoint sync] Received beacon state and block from URL #{url}",
           slot: anchor_state.slot
         )
 
-        # We already checked block and state match
-        {:ok, store} = Store.get_forkchoice_store(anchor_state, anchor_block)
-
-        # Save store in DB
-        StoreDb.persist_store(store)
-
-        store
+        {anchor_state, anchor_block}
 
       _ ->
-        Logger.error("[Checkpoint sync] Failed to fetch the latest finalized state and block")
+        Logger.error(
+          "[Checkpoint sync] Failed to fetch the latest finalized state and block for URL: #{url}"
+        )
 
         Logger.flush()
         System.halt(1)