feat: enforce jetstream ack and redelivery semantics

src/NATS.Server/JetStream/Api/Handlers/ConsumerApiHandlers.cs

@@ -68,6 +68,16 @@ public static class ConsumerApiHandlers
             if (root.TryGetProperty("heartbeat_ms", out var hbEl) && hbEl.TryGetInt32(out var hbMs))
                 config.HeartbeatMs = hbMs;
 
+            if (root.TryGetProperty("ack_wait_ms", out var ackWaitEl) && ackWaitEl.TryGetInt32(out var ackWait))
+                config.AckWaitMs = ackWait;
+
+            if (root.TryGetProperty("ack_policy", out var ackPolicyEl))
+            {
+                var ackPolicy = ackPolicyEl.GetString();
+                if (string.Equals(ackPolicy, "explicit", StringComparison.OrdinalIgnoreCase))
+                    config.AckPolicy = AckPolicy.Explicit;
+            }
+
             return config;
         }
         catch (JsonException)

src/NATS.Server/JetStream/ConsumerManager.cs

@@ -66,7 +66,7 @@ public sealed class ConsumerManager
     public void OnPublished(string stream, StoredMessage message)
     {
         foreach (var handle in _consumers.Values.Where(c => c.Stream == stream && c.Config.Push))
-            _pushConsumerEngine.Enqueue(handle.PushFrames, message, handle.Config);
+            _pushConsumerEngine.Enqueue(handle, message);
     }
 
     public PushFrame? ReadPushFrame(string stream, string durableName)
@@ -86,4 +86,5 @@ public sealed record ConsumerHandle(string Stream, ConsumerConfig Config)
     public ulong NextSequence { get; set; } = 1;
     public Queue<StoredMessage> Pending { get; } = new();
     public Queue<PushFrame> PushFrames { get; } = new();
+    public AckProcessor AckProcessor { get; } = new();
 }

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

@@ -0,0 +1,24 @@
+namespace NATS.Server.JetStream.Consumers;
+
+public sealed class AckProcessor
+{
+    private readonly Dictionary<ulong, DateTime> _pending = new();
+
+    public void Register(ulong sequence, int ackWaitMs)
+    {
+        _pending[sequence] = DateTime.UtcNow.AddMilliseconds(Math.Max(ackWaitMs, 1));
+    }
+
+    public ulong? NextExpired()
+    {
+        foreach (var (seq, deadline) in _pending)
+        {
+            if (DateTime.UtcNow >= deadline)
+                return seq;
+        }
+
+        return null;
+    }
+
+    public bool HasPending => _pending.Count > 0;
+}

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

@@ -1,4 +1,5 @@
 using NATS.Server.JetStream.Storage;
+using NATS.Server.JetStream.Models;
 
 namespace NATS.Server.JetStream.Consumers;
 
@@ -7,6 +8,31 @@ public sealed class PullConsumerEngine
     public async ValueTask<PullFetchBatch> FetchAsync(StreamHandle stream, ConsumerHandle consumer, int batch, CancellationToken ct)
     {
         var messages = new List<StoredMessage>(batch);
+
+        if (consumer.Config.AckPolicy == AckPolicy.Explicit)
+        {
+            var expired = consumer.AckProcessor.NextExpired();
+            if (expired is { } expiredSequence)
+            {
+                var redelivery = await stream.Store.LoadAsync(expiredSequence, ct);
+                if (redelivery != null)
+                {
+                    messages.Add(new StoredMessage
+                    {
+                        Sequence = redelivery.Sequence,
+                        Subject = redelivery.Subject,
+                        Payload = redelivery.Payload,
+                        Redelivered = true,
+                    });
+                }
+
+                return new PullFetchBatch(messages);
+            }
+
+            if (consumer.AckProcessor.HasPending)
+                return new PullFetchBatch(messages);
+        }
+
         var sequence = consumer.NextSequence;
 
         for (var i = 0; i < batch; i++)
@@ -16,6 +42,8 @@ public sealed class PullConsumerEngine
                 break;
 
             messages.Add(message);
+            if (consumer.Config.AckPolicy == AckPolicy.Explicit)
+                consumer.AckProcessor.Register(message.Sequence, consumer.Config.AckWaitMs);
             sequence++;
         }
 

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

@@ -5,17 +5,20 @@ namespace NATS.Server.JetStream.Consumers;
 
 public sealed class PushConsumerEngine
 {
-    public void Enqueue(Queue<PushFrame> queue, StoredMessage message, ConsumerConfig config)
+    public void Enqueue(ConsumerHandle consumer, StoredMessage message)
     {
-        queue.Enqueue(new PushFrame
+        consumer.PushFrames.Enqueue(new PushFrame
         {
             IsData = true,
             Message = message,
         });
 
-        if (config.HeartbeatMs > 0)
+        if (consumer.Config.AckPolicy == AckPolicy.Explicit)
+            consumer.AckProcessor.Register(message.Sequence, consumer.Config.AckWaitMs);
+
+        if (consumer.Config.HeartbeatMs > 0)
         {
-            queue.Enqueue(new PushFrame
+            consumer.PushFrames.Enqueue(new PushFrame
             {
                 IsHeartbeat = true,
             });