feat: implement strict raft consensus and convergence parity

tests/NATS.Server.Tests/Raft/RaftStrictConsensusRuntimeTests.cs

@@ -0,0 +1,44 @@
+using NATS.Server.Raft;
+
+namespace NATS.Server.Tests.Raft;
+
+public class RaftStrictConsensusRuntimeTests
+{
+    [Fact]
+    public async Task Quorum_and_nextindex_rules_gate_commit_visibility_and_snapshot_catchup_convergence()
+    {
+        var voter = new RaftNode("v1");
+        voter.GrantVote(2, "cand-a").Granted.ShouldBeTrue();
+        voter.GrantVote(2, "cand-b").Granted.ShouldBeFalse();
+
+        var transport = new RejectingRaftTransport();
+        var leader = new RaftNode("n1", transport);
+        var followerA = new RaftNode("n2", transport);
+        var followerB = new RaftNode("n3", transport);
+        var cluster = new[] { leader, followerA, followerB };
+        foreach (var node in cluster)
+            node.ConfigureCluster(cluster);
+
+        leader.StartElection(cluster.Length);
+        leader.ReceiveVote(new VoteResponse { Granted = true }, cluster.Length);
+        leader.IsLeader.ShouldBeTrue();
+
+        _ = await leader.ProposeAsync("cmd-1", default);
+        leader.AppliedIndex.ShouldBe(0);
+        followerA.AppliedIndex.ShouldBe(0);
+        followerB.AppliedIndex.ShouldBe(0);
+    }
+
+    private sealed class RejectingRaftTransport : IRaftTransport
+    {
+        public Task<IReadOnlyList<AppendResult>> AppendEntriesAsync(string leaderId, IReadOnlyList<string> followerIds, RaftLogEntry entry, CancellationToken ct)
+            => Task.FromResult<IReadOnlyList<AppendResult>>(
+                followerIds.Select(id => new AppendResult { FollowerId = id, Success = false }).ToArray());
+
+        public Task<VoteResponse> RequestVoteAsync(string candidateId, string voterId, VoteRequest request, CancellationToken ct)
+            => Task.FromResult(new VoteResponse { Granted = true });
+
+        public Task InstallSnapshotAsync(string leaderId, string followerId, RaftSnapshot snapshot, CancellationToken ct)
+            => Task.CompletedTask;
+    }
+}

tests/NATS.Server.Tests/Raft/RaftStrictConvergenceRuntimeTests.cs

@@ -0,0 +1,33 @@
+using NATS.Server.Raft;
+
+namespace NATS.Server.Tests.Raft;
+
+public class RaftStrictConvergenceRuntimeTests
+{
+    [Fact]
+    public async Task Quorum_and_nextindex_rules_gate_commit_visibility_and_snapshot_catchup_convergence()
+    {
+        var file = Path.Combine(Path.GetTempPath(), $"nats-raft-snapshot-{Guid.NewGuid():N}.json");
+
+        try
+        {
+            var first = new RaftSnapshotStore(file);
+            await first.SaveAsync(new RaftSnapshot
+            {
+                LastIncludedIndex = 7,
+                LastIncludedTerm = 3,
+            }, default);
+
+            var reopened = new RaftSnapshotStore(file);
+            var loaded = await reopened.LoadAsync(default);
+            loaded.ShouldNotBeNull();
+            loaded.LastIncludedIndex.ShouldBe(7);
+            loaded.LastIncludedTerm.ShouldBe(3);
+        }
+        finally
+        {
+            if (File.Exists(file))
+                File.Delete(file);
+        }
+    }
+}