feat: implement jetstream pull consumer fetch

src/NATS.Server/JetStream/ConsumerManager.cs

@@ -1,5 +1,6 @@
 using System.Collections.Concurrent;
 using NATS.Server.JetStream.Api;
+using NATS.Server.JetStream.Consumers;
 using NATS.Server.JetStream.Models;
 using NATS.Server.JetStream.Storage;
 
@@ -8,6 +9,7 @@ namespace NATS.Server.JetStream;
 public sealed class ConsumerManager
 {
     private readonly ConcurrentDictionary<(string Stream, string Name), ConsumerHandle> _consumers = new();
+    private readonly PullConsumerEngine _pullConsumerEngine = new();
 
     public int ConsumerCount => _consumers.Count;
 
@@ -48,6 +50,17 @@ public sealed class ConsumerManager
 
     public bool TryGet(string stream, string durableName, out ConsumerHandle handle)
         => _consumers.TryGetValue((stream, durableName), out handle!);
+
+    public async ValueTask<PullFetchBatch> FetchAsync(string stream, string durableName, int batch, StreamManager streamManager, CancellationToken ct)
+    {
+        if (!_consumers.TryGetValue((stream, durableName), out var consumer))
+            return new PullFetchBatch([]);
+
+        if (!streamManager.TryGet(stream, out var streamHandle))
+            return new PullFetchBatch([]);
+
+        return await _pullConsumerEngine.FetchAsync(streamHandle, consumer, batch, ct);
+    }
 }
 
 public sealed record ConsumerHandle(string Stream, ConsumerConfig Config)

src/NATS.Server/JetStream/Consumers/PullConsumerEngine.cs

@@ -0,0 +1,35 @@
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.JetStream.Consumers;
+
+public sealed class PullConsumerEngine
+{
+    public async ValueTask<PullFetchBatch> FetchAsync(StreamHandle stream, ConsumerHandle consumer, int batch, CancellationToken ct)
+    {
+        var messages = new List<StoredMessage>(batch);
+        var sequence = consumer.NextSequence;
+
+        for (var i = 0; i < batch; i++)
+        {
+            var message = await stream.Store.LoadAsync(sequence, ct);
+            if (message == null)
+                break;
+
+            messages.Add(message);
+            sequence++;
+        }
+
+        consumer.NextSequence = sequence;
+        return new PullFetchBatch(messages);
+    }
+}
+
+public sealed class PullFetchBatch
+{
+    public IReadOnlyList<StoredMessage> Messages { get; }
+
+    public PullFetchBatch(IReadOnlyList<StoredMessage> messages)
+    {
+        Messages = messages;
+    }
+}