Cleanup/Deduplicate blockdownloader code

src/Nethermind/Nethermind.Merge.Plugin/Synchronization/MergeSynchronizer.cs

@@ -78,12 +78,8 @@ public class MergeSynchronizerModule : Module
     protected override void Load(ContainerBuilder builder)
     {
         builder
-            .RegisterType<MergeBlockDownloader>()
-            .As<BlockDownloader>()
-            .As<ISyncDownloader<BlocksRequest>>()
-            .InstancePerLifetimeScope();
-
-        builder
+            .Add<IPosTransitionHook, PosTransitionHook>()
+            .AddDecorator<BlockDownloader, MergeBlockDownloader>()
             .AddSingleton<ISynchronizer, MergeSynchronizer>()
             .AddSingleton<IChainLevelHelper, ChainLevelHelper>()
             .AddScoped<IPeerAllocationStrategyFactory<BlocksRequest>, MergeBlocksSyncPeerAllocationStrategyFactory>()

src/Nethermind/Nethermind.Merge.Plugin/Synchronization/PosTransitionHook.cs

@@ -0,0 +1,50 @@
+// SPDX-FileCopyrightText: 2025 Demerzel Solutions Limited
+// SPDX-License-Identifier: LGPL-3.0-only
+
+using System.Linq;
+using Nethermind.Blockchain;
+using Nethermind.Consensus;
+using Nethermind.Core;
+using Nethermind.Core.Collections;
+using Nethermind.Core.Extensions;
+using Nethermind.Logging;
+using Nethermind.Synchronization.Blocks;
+using Nethermind.Synchronization.Peers;
+
+namespace Nethermind.Merge.Plugin.Synchronization;
+
+public class PosTransitionHook(IBlockTree blockTree, IPoSSwitcher poSSwitcher, ILogManager logManager) : IPosTransitionHook
+{
+    private readonly ILogger _logger = logManager.GetClassLogger<PosTransitionHook>();
+
+    public void TryUpdateTerminalBlock(BlockHeader currentHeader, bool shouldProcess)
+    {
+        if (shouldProcess == false) // if we're processing the block we will find TerminalBlock after processing
+            poSSwitcher.TryUpdateTerminalBlock(currentHeader);
+    }
+
+    public bool ImprovementRequirementSatisfied(PeerInfo? bestPeer)
+    {
+        return bestPeer!.TotalDifficulty > (blockTree.BestSuggestedHeader?.TotalDifficulty ?? 0) &&
+               poSSwitcher.HasEverReachedTerminalBlock() == false;
+    }
+
+    public IOwnedReadOnlyList<BlockHeader> FilterPosHeader(IOwnedReadOnlyList<BlockHeader> response)
+    {
+        // Override PoW's RequestHeaders so that it won't request beyond PoW.
+        // This fixes `Incremental Sync` hive test.
+        if (response.Count > 0)
+        {
+            BlockHeader lastBlockHeader = response[^1];
+            bool lastBlockIsPostMerge = poSSwitcher.GetBlockConsensusInfo(response[^1]).IsPostMerge;
+            if (lastBlockIsPostMerge) // Initial check to prevent creating new array every time
+            {
+                response = response
+                    .TakeWhile(header => !poSSwitcher.GetBlockConsensusInfo(header).IsPostMerge)
+                    .ToPooledList(response.Count);
+                if (_logger.IsInfo) _logger.Info($"Last block is post merge. {lastBlockHeader.Hash}. Trimming to {response.Count} sized batch.");
+            }
+        }
+        return response;
+    }
+}

src/Nethermind/Nethermind.Synchronization.Test/BlockDownloaderTests.Merge.cs

@@ -25,7 +25,6 @@
 using Nethermind.Synchronization.Blocks;
 using Nethermind.Synchronization.Peers;
 using Nethermind.Synchronization.Peers.AllocationStrategies;
-using Nethermind.TxPool;
 using NSubstitute;
 using NSubstitute.ClearExtensions;
 using NUnit.Framework;

src/Nethermind/Nethermind.Synchronization/Blocks/BlockDownloadContext.cs

@@ -71,6 +71,8 @@ public BlockDownloadContext(ISpecProvider specProvider, PeerInfo syncPeer, IRead
 
         public List<Hash256> NonEmptyBlockHashes { get; }
 
+        public bool DownloadReceipts => _downloadReceipts;
+
         public IReadOnlyList<Hash256> GetHashesByOffset(int offset, int maxLength)
         {
             var hashesToRequest =

src/Nethermind/Nethermind.Synchronization/Blocks/IPosTransitionHook.cs

@@ -0,0 +1,15 @@
+// SPDX-FileCopyrightText: 2025 Demerzel Solutions Limited
+// SPDX-License-Identifier: LGPL-3.0-only
+
+using Nethermind.Core;
+using Nethermind.Core.Collections;
+using Nethermind.Synchronization.Peers;
+
+namespace Nethermind.Synchronization.Blocks;
+
+public interface IPosTransitionHook
+{
+    void TryUpdateTerminalBlock(BlockHeader currentHeader, bool shouldProcess);
+    bool ImprovementRequirementSatisfied(PeerInfo? peerInfo);
+    IOwnedReadOnlyList<BlockHeader> FilterPosHeader(IOwnedReadOnlyList<BlockHeader> headers);
+}

src/Nethermind/Nethermind.Synchronization/Blocks/NoPosTransition.cs

@@ -0,0 +1,26 @@
+// SPDX-FileCopyrightText: 2025 Demerzel Solutions Limited
+// SPDX-License-Identifier: LGPL-3.0-only
+
+using Nethermind.Blockchain;
+using Nethermind.Core;
+using Nethermind.Core.Collections;
+using Nethermind.Synchronization.Peers;
+
+namespace Nethermind.Synchronization.Blocks;
+
+public class NoPosTransition(IBlockTree blockTree) : IPosTransitionHook
+{
+    public void TryUpdateTerminalBlock(BlockHeader currentHeader, bool shouldProcess)
+    {
+    }
+
+    public bool ImprovementRequirementSatisfied(PeerInfo? bestPeer)
+    {
+        return bestPeer!.TotalDifficulty > (blockTree.BestSuggestedHeader?.TotalDifficulty ?? 0);
+    }
+
+    public IOwnedReadOnlyList<BlockHeader> FilterPosHeader(IOwnedReadOnlyList<BlockHeader> headers)
+    {
+        return headers;
+    }
+}

src/Nethermind/Nethermind.Synchronization/Synchronizer.cs

@@ -302,7 +302,13 @@ protected override void Load(ContainerBuilder builder)
 
             // For blocks. There are two block scope, Fast and Full
             .AddScoped<SyncFeedComponent<BlocksRequest>>()
-            .AddScoped<ISyncDownloader<BlocksRequest>, BlockDownloader>()
+
+            // The direct implementation is decorated by merge plugin (not the interface)
+            // so its  declared on its own and other use is binded.
+            .AddScoped<BlockDownloader>()
+            .Add<IPosTransitionHook, NoPosTransition>()
+            .Bind<ISyncDownloader<BlocksRequest>, BlockDownloader>()
+
             .AddScoped<IPeerAllocationStrategyFactory<BlocksRequest>, BlocksSyncPeerAllocationStrategyFactory>()
             .AddScoped<SyncDispatcher<BlocksRequest>>()