From 56177a70996777587afd3caec21de2413f917155 Mon Sep 17 00:00:00 2001
From: Joseph Doherty <dohejw01@gmail.com>
Date: Mon, 23 Feb 2026 14:51:30 -0500
Subject: [PATCH] feat: enforce filestore durability and recovery invariants

---
 .../JetStream/Storage/FileStore.cs            |  5 +++
 .../JetStreamFileStoreInvariantTests.cs       | 35 ++++++++++++++++++
 ...treamFileStoreRecoveryStrictParityTests.cs | 37 +++++++++++++++++++
 3 files changed, 77 insertions(+)
 create mode 100644 tests/NATS.Server.Tests/JetStream/JetStreamFileStoreInvariantTests.cs
 create mode 100644 tests/NATS.Server.Tests/JetStream/JetStreamFileStoreRecoveryStrictParityTests.cs

diff --git a/src/NATS.Server/JetStream/Storage/FileStore.cs b/src/NATS.Server/JetStream/Storage/FileStore.cs
index 7c12199..8252350 100644
--- a/src/NATS.Server/JetStream/Storage/FileStore.cs
+++ b/src/NATS.Server/JetStream/Storage/FileStore.cs
@@ -91,7 +91,11 @@ public sealed class FileStore : IStreamStore, IAsyncDisposable
     {
         var removed = _messages.Remove(sequence);
         if (removed)
+        {
+            if (sequence == _last)
+                _last = _messages.Count == 0 ? 0UL : _messages.Keys.Max();
             RewriteDataFile();
+        }
         return ValueTask.FromResult(removed);
     }
 
@@ -227,6 +231,7 @@ public sealed class FileStore : IStreamStore, IAsyncDisposable
         _blockCount = 0;
         _activeBlockBytes = 0;
         _writeOffset = 0;
+        _last = _messages.Count == 0 ? 0UL : _messages.Keys.Max();
 
         using var stream = new FileStream(_dataFilePath, FileMode.Create, FileAccess.Write, FileShare.Read);
         using var writer = new StreamWriter(stream, Encoding.UTF8);
diff --git a/tests/NATS.Server.Tests/JetStream/JetStreamFileStoreInvariantTests.cs b/tests/NATS.Server.Tests/JetStream/JetStreamFileStoreInvariantTests.cs
new file mode 100644
index 0000000..d4fb4d7
--- /dev/null
+++ b/tests/NATS.Server.Tests/JetStream/JetStreamFileStoreInvariantTests.cs
@@ -0,0 +1,35 @@
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.Tests.JetStream;
+
+public class JetStreamFileStoreInvariantTests
+{
+    [Fact]
+    public async Task Filestore_recovery_preserves_sequence_subject_index_and_integrity_after_prune_and_restart_cycles()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), $"nats-js-fs-invariants-{Guid.NewGuid():N}");
+        var options = new FileStoreOptions { Directory = dir };
+
+        try
+        {
+            await using var store = new FileStore(options);
+            var seq1 = await store.AppendAsync("orders.created", "1"u8.ToArray(), default);
+            var seq2 = await store.AppendAsync("orders.updated", "2"u8.ToArray(), default);
+            seq1.ShouldBe((ulong)1);
+            seq2.ShouldBe((ulong)2);
+
+            (await store.RemoveAsync(seq2, default)).ShouldBeTrue();
+            var state = await store.GetStateAsync(default);
+
+            state.Messages.ShouldBe((ulong)1);
+            state.LastSeq.ShouldBe((ulong)1);
+            state.FirstSeq.ShouldBe((ulong)1);
+            (await store.LoadLastBySubjectAsync("orders.updated", default)).ShouldBeNull();
+        }
+        finally
+        {
+            if (Directory.Exists(dir))
+                Directory.Delete(dir, recursive: true);
+        }
+    }
+}
diff --git a/tests/NATS.Server.Tests/JetStream/JetStreamFileStoreRecoveryStrictParityTests.cs b/tests/NATS.Server.Tests/JetStream/JetStreamFileStoreRecoveryStrictParityTests.cs
new file mode 100644
index 0000000..6829bdc
--- /dev/null
+++ b/tests/NATS.Server.Tests/JetStream/JetStreamFileStoreRecoveryStrictParityTests.cs
@@ -0,0 +1,37 @@
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.Tests.JetStream;
+
+public class JetStreamFileStoreRecoveryStrictParityTests
+{
+    [Fact]
+    public async Task Filestore_recovery_preserves_sequence_subject_index_and_integrity_after_prune_and_restart_cycles()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), $"nats-js-fs-recovery-{Guid.NewGuid():N}");
+        var options = new FileStoreOptions { Directory = dir };
+
+        try
+        {
+            await using (var store = new FileStore(options))
+            {
+                await store.AppendAsync("orders.created", "a"u8.ToArray(), default);
+                await store.AppendAsync("orders.created", "b"u8.ToArray(), default);
+                await store.RemoveAsync(2, default);
+            }
+
+            await using var reopened = new FileStore(options);
+            var state = await reopened.GetStateAsync(default);
+            state.Messages.ShouldBe((ulong)1);
+            state.LastSeq.ShouldBe((ulong)1);
+
+            var msg1 = await reopened.LoadAsync(1, default);
+            msg1.ShouldNotBeNull();
+            msg1.Subject.ShouldBe("orders.created");
+        }
+        finally
+        {
+            if (Directory.Exists(dir))
+                Directory.Delete(dir, recursive: true);
+        }
+    }
+}