From fe293c4bf3cc6d08df06a057c27b3df485d1161f Mon Sep 17 00:00:00 2001
From: Ivaylo Badinov <632282+venimus@users.noreply.github.com>
Date: Wed, 9 Sep 2026 00:41:00 +0300
Subject: [PATCH] fix: don't crash exec_all_mem_mutations on an empty batch

List.last(blocks_mutations) returns nil when the batch has zero blocks
(e.g. a resume/extension check that turns out to have nothing new past
the already-synced height), and matching {{height, _mbi}, _block, _}
against nil raises MatchError, crashing the sync task.

Confirmed live on mainnet mdw1 (157.180.104.172): crashed 48 times in a
tight burst, hit @max_restarts, and AeMdw.Sync.Server went idle (no sync
progress, mdw_gens_per_minute: 0.0) until manually hot-patched. Live
patch verified sync resumed immediately after (512+ gens/min, no new
crashes).

Fix: treat an empty batch as a no-op (return state unchanged) instead of
crashing.
---
 lib/ae_mdw/sync/server.ex | 33 +++++++++++++++++++++------------
 1 file changed, 21 insertions(+), 12 deletions(-)

diff --git a/lib/ae_mdw/sync/server.ex b/lib/ae_mdw/sync/server.ex
index 748d6cbf4..3cd9d5570 100644
--- a/lib/ae_mdw/sync/server.ex
+++ b/lib/ae_mdw/sync/server.ex
@@ -491,22 +491,31 @@ defmodule AeMdw.Sync.Server do
       |> List.flatten()
       |> maybe_add_accounts_balance_mutations()
 
-    {{height, _mbi}, _block, _mutations} = List.last(blocks_mutations)
-    Log.info("[sync_mem] exec until height=#{height}")
+    # An empty batch (no blocks to apply - e.g. a resume/extension check that
+    # turns out to have nothing new past the already-synced height) is a
+    # legitimate no-op, not an error - List.last/1 would return nil and crash
+    # the match below if this weren't guarded.
+    case List.last(blocks_mutations) do
+      nil ->
+        state
+
+      {{height, _mbi}, _block, _mutations} ->
+        Log.info("[sync_mem] exec until height=#{height}")
+
+        {ts, new_state} =
+          :timer.tc(fn ->
+            all_mutations =
+              Enum.map(blocks_mutations, fn {_height, _block, mutations} -> mutations end)
 
-    {ts, new_state} =
-      :timer.tc(fn ->
-        all_mutations =
-          Enum.map(blocks_mutations, fn {_height, _block, mutations} -> mutations end)
+            State.commit_mem(state, all_mutations)
+          end)
 
-        State.commit_mem(state, all_mutations)
-      end)
+        :ok = profile_sync("sync_mem", height, ts, blocks_mutations, gens_in_batch)
 
-    :ok = profile_sync("sync_mem", height, ts, blocks_mutations, gens_in_batch)
+        broadcast_blocks(gens_mutations)
 
-    broadcast_blocks(gens_mutations)
-
-    new_state
+        new_state
+    end
   end
 
   defp spawn_task(fun) do