feat: enforce jetstream retention and limits

src/NATS.Server/JetStream/Publish/JetStreamPublisher.cs

@@ -11,19 +11,14 @@ public sealed class JetStreamPublisher
 
     public bool TryCapture(string subject, ReadOnlyMemory<byte> payload, out PubAck ack)
     {
-        var stream = _streamManager.FindBySubject(subject);
-        if (stream == null)
+        var captured = _streamManager.Capture(subject, payload);
+        if (captured == null)
         {
             ack = new PubAck();
             return false;
         }
 
-        var seq = stream.Store.AppendAsync(subject, payload, default).GetAwaiter().GetResult();
-        ack = new PubAck
-        {
-            Stream = stream.Config.Name,
-            Seq = seq,
-        };
+        ack = captured;
 
         return true;
     }

src/NATS.Server/JetStream/Storage/FileStore.cs

@@ -62,6 +62,17 @@ public sealed class FileStore : IStreamStore, IAsyncDisposable
         });
     }
 
+    public void TrimToMaxMessages(ulong maxMessages)
+    {
+        while ((ulong)_messages.Count > maxMessages)
+        {
+            var first = _messages.Keys.Min();
+            _messages.Remove(first);
+        }
+
+        RewriteDataFile();
+    }
+
     public ValueTask DisposeAsync() => ValueTask.CompletedTask;
 
     private void LoadExisting()
@@ -91,6 +102,22 @@ public sealed class FileStore : IStreamStore, IAsyncDisposable
         }
     }
 
+    private void RewriteDataFile()
+    {
+        var lines = new List<string>(_messages.Count);
+        foreach (var message in _messages.OrderBy(kv => kv.Key).Select(kv => kv.Value))
+        {
+            lines.Add(JsonSerializer.Serialize(new FileRecord
+            {
+                Sequence = message.Sequence,
+                Subject = message.Subject,
+                PayloadBase64 = Convert.ToBase64String(message.Payload.ToArray()),
+            }));
+        }
+
+        File.WriteAllLines(_dataFilePath, lines);
+    }
+
     private sealed class FileRecord
     {
         public ulong Sequence { get; init; }

src/NATS.Server/JetStream/Storage/MemStore.cs

@@ -54,4 +54,16 @@ public sealed class MemStore : IStreamStore
             });
         }
     }
+
+    public void TrimToMaxMessages(ulong maxMessages)
+    {
+        lock (_gate)
+        {
+            while ((ulong)_messages.Count > maxMessages)
+            {
+                var first = _messages.Keys.Min();
+                _messages.Remove(first);
+            }
+        }
+    }
 }

src/NATS.Server/JetStream/StreamManager.cs

@@ -1,6 +1,7 @@
 using System.Collections.Concurrent;
 using NATS.Server.JetStream.Api;
 using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Publish;
 using NATS.Server.JetStream.Storage;
 using NATS.Server.Subscriptions;
 
@@ -37,6 +38,14 @@ public sealed class StreamManager
 
     public bool TryGet(string name, out StreamHandle handle) => _streams.TryGetValue(name, out handle!);
 
+    public ValueTask<StreamState> GetStateAsync(string name, CancellationToken ct)
+    {
+        if (_streams.TryGetValue(name, out var stream))
+            return stream.Store.GetStateAsync(ct);
+
+        return ValueTask.FromResult(new StreamState());
+    }
+
     public StreamHandle? FindBySubject(string subject)
     {
         foreach (var stream in _streams.Values)
@@ -48,6 +57,22 @@ public sealed class StreamManager
         return null;
     }
 
+    public PubAck? Capture(string subject, ReadOnlyMemory<byte> payload)
+    {
+        var stream = FindBySubject(subject);
+        if (stream == null)
+            return null;
+
+        var seq = stream.Store.AppendAsync(subject, payload, default).GetAwaiter().GetResult();
+        EnforceLimits(stream);
+
+        return new PubAck
+        {
+            Stream = stream.Config.Name,
+            Seq = seq,
+        };
+    }
+
     private static StreamConfig NormalizeConfig(StreamConfig config)
     {
         var copy = new StreamConfig
@@ -73,6 +98,22 @@ public sealed class StreamManager
             },
         };
     }
+
+    private static void EnforceLimits(StreamHandle stream)
+    {
+        if (stream.Config.MaxMsgs <= 0)
+            return;
+
+        var maxMessages = (ulong)stream.Config.MaxMsgs;
+        if (stream.Store is MemStore memStore)
+        {
+            memStore.TrimToMaxMessages(maxMessages);
+            return;
+        }
+
+        if (stream.Store is FileStore fileStore)
+            fileStore.TrimToMaxMessages(maxMessages);
+    }
 }
 
 public sealed record StreamHandle(StreamConfig Config, IStreamStore Store);

tests/NATS.Server.Tests/JetStreamApiFixture.cs

@@ -1,6 +1,7 @@
 using System.Text;
 using NATS.Server.JetStream;
 using NATS.Server.JetStream.Api;
+using NATS.Server.JetStream.Models;
 using NATS.Server.JetStream.Publish;
 
 namespace NATS.Server.Tests;
@@ -47,5 +48,10 @@ internal sealed class JetStreamApiFixture : IAsyncDisposable
         return Task.FromResult(_router.Route(subject, Encoding.UTF8.GetBytes(payload)));
     }
 
+    public Task<StreamState> GetStreamStateAsync(string streamName)
+    {
+        return _streamManager.GetStateAsync(streamName, default).AsTask();
+    }
+
     public ValueTask DisposeAsync() => ValueTask.CompletedTask;
 }

tests/NATS.Server.Tests/JetStreamRetentionPolicyTests.cs

@@ -0,0 +1,18 @@
+namespace NATS.Server.Tests;
+
+public class JetStreamRetentionPolicyTests
+{
+    [Fact]
+    public async Task MaxMsgs_limit_evicts_oldest_message()
+    {
+        await using var fixture = await JetStreamApiFixture.StartWithStreamAsync("L", "l.*", maxMsgs: 2);
+
+        await fixture.PublishAndGetAckAsync("l.1", "a");
+        await fixture.PublishAndGetAckAsync("l.2", "b");
+        await fixture.PublishAndGetAckAsync("l.3", "c");
+
+        var state = await fixture.GetStreamStateAsync("L");
+        state.Messages.ShouldBe((ulong)2);
+        state.FirstSeq.ShouldBe((ulong)2);
+    }
+}