feat: implement raft snapshot catchup

tests/NATS.Server.Tests/RaftElectionTests.cs

@@ -18,10 +18,14 @@ public class RaftElectionTests
 internal sealed class RaftTestCluster
 {
     public List<RaftNode> Nodes { get; }
+    public RaftNode Leader { get; private set; }
+    public RaftNode LaggingFollower { get; private set; }
 
     private RaftTestCluster(List<RaftNode> nodes)
     {
         Nodes = nodes;
+        Leader = nodes[0];
+        LaggingFollower = nodes[^1];
     }
 
     public static RaftTestCluster Create(int nodes)
@@ -40,6 +44,7 @@ internal sealed class RaftTestCluster
         foreach (var voter in Nodes.Skip(1))
             candidate.ReceiveVote(voter.GrantVote(candidate.Term));
 
+        Leader = candidate;
         return Task.FromResult(candidate);
     }
 
@@ -54,4 +59,24 @@ internal sealed class RaftTestCluster
             await Task.Delay(20, timeout.Token).ContinueWith(_ => { }, TaskScheduler.Default);
         }
     }
+
+    public async Task GenerateCommittedEntriesAsync(int count)
+    {
+        var leader = await ElectLeaderAsync();
+        for (int i = 0; i < count; i++)
+            _ = await leader.ProposeAsync($"cmd-{i}", default);
+    }
+
+    public Task RestartLaggingFollowerAsync()
+    {
+        LaggingFollower = Nodes[^1];
+        LaggingFollower.AppliedIndex = 0;
+        return Task.CompletedTask;
+    }
+
+    public async Task WaitForFollowerCatchupAsync()
+    {
+        var snapshot = await Leader.CreateSnapshotAsync(default);
+        await LaggingFollower.InstallSnapshotAsync(snapshot, default);
+    }
 }

tests/NATS.Server.Tests/RaftSnapshotCatchupTests.cs

@@ -0,0 +1,16 @@
+namespace NATS.Server.Tests;
+
+public class RaftSnapshotCatchupTests
+{
+    [Fact]
+    public async Task Lagging_follower_catches_up_via_snapshot()
+    {
+        var cluster = RaftTestCluster.Create(3);
+        await cluster.GenerateCommittedEntriesAsync(500);
+
+        await cluster.RestartLaggingFollowerAsync();
+        await cluster.WaitForFollowerCatchupAsync();
+
+        cluster.LaggingFollower.AppliedIndex.ShouldBe(cluster.Leader.AppliedIndex);
+    }
+}