To implement group member query API in persistent mode, add group member query message protocol.

* Add a mechanism to respond with binary payload from service.
    * Move some util method in `ServiceProtocol` to a separate util class `MessagePackUtils`, so that they could be reused by other classes.
    * Deprecate `string message` member in `AckMessage` and add `Payload` member. The `string message` member is never used.
 * Introduce `IMessagePackSerializable` interface, so that we could put the (de)serialization methods of model classes inside themselves.
 * Refactor `AckHandler` to allow acking with binary payload
* Add `GroupMemberQueryMessage` in service protocol.
* Add models `GroupMember` and `GroupMemberQueryResponsePayload`.

Author: Y-Sindo

src/Microsoft.Azure.SignalR.Common/ServiceConnections/ServiceConnectionContainerBase.cs

@@ -221,7 +221,7 @@ public virtual Task HandlePingAsync(PingMessage pingMessage)
 
     public void HandleAck(AckMessage ackMessage)
     {
-        _ackHandler.TriggerAck(ackMessage.AckId, (AckStatus)ackMessage.Status);
+        _ackHandler.TriggerAck(ackMessage.AckId, (AckStatus)ackMessage.Status, ackMessage.Payload);
     }
 
     public virtual Task WriteAsync(ServiceMessage serviceMessage)
@@ -249,6 +249,29 @@ public async Task<bool> WriteAckableMessageAsync(ServiceMessage serviceMessage,
         return AckHandler.HandleAckStatus(ackableMessage, status);
     }
 
+    /// <summary>
+    /// <see cref="WriteAckableMessageAsync(ServiceMessage, CancellationToken)"/> only checks <see cref="AckMessage.Status"/> as the response, 
+    /// while this method checks <see cref="AckMessage.Payload"/> and deserialize it to <typeparamref name="T"/>.
+    /// </summary>
+    public async Task<T> InvokeAsync<T>(ServiceMessage serviceMessage, CancellationToken cancellationToken = default) where T : IMessagePackSerializable, new()
+    {
+        if (serviceMessage is not IAckableMessage ackableMessage)
+        {
+            throw new ArgumentException($"{nameof(serviceMessage)} is not {nameof(IAckableMessage)}");
+        }
+
+        var task = _ackHandler.CreateSingleAck<T>(out var id, null, cancellationToken);
+        ackableMessage.AckId = id;
+
+        // Sending regular messages completes as soon as the data leaves the outbound pipe,
+        // whereas ackable ones complete upon full roundtrip of the message and the ack (or timeout).
+        // Therefore sending them over different connections creates a possibility for processing them out of original order.
+        // By sending both message types over the same connection we ensure that they are sent (and processed) in their original order.
+        await WriteMessageAsync(serviceMessage);
+
+        return await task;
+    }
+
     public virtual Task OfflineAsync(GracefulShutdownMode mode, CancellationToken token)
     {
         _terminated = true;

src/Microsoft.Azure.SignalR.Common/Utilities/AckHandler.cs

@@ -1,7 +1,12 @@
-using System;
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+using System.Buffers;
 using System.Collections.Concurrent;
 using System.Threading;
 using System.Threading.Tasks;
+using MessagePack;
 using Microsoft.Azure.SignalR.Common;
 using Microsoft.Azure.SignalR.Protocol;
 
@@ -35,15 +40,27 @@ public Task<AckStatus> CreateSingleAck(out int id, TimeSpan? ackTimeout = defaul
             {
                 return Task.FromResult(AckStatus.Ok);
             }
-            var info = (IAckInfo<AckStatus>)_acks.GetOrAdd(id, _ => new SingleAckInfo(ackTimeout ?? _defaultAckTimeout));
-            if (info is MultiAckInfo)
+            var info = (IAckInfo<AckStatus>)_acks.GetOrAdd(id, _ => new SingleAckWithStatusInfo(ackTimeout ?? _defaultAckTimeout));
+            if (info is MultiAckWithStatusInfo)
             {
                 throw new InvalidOperationException();
             }
             cancellationToken.Register(() => info.Cancel());
             return info.Task;
         }
 
+        public Task<T> CreateSingleAck<T>(out int id, TimeSpan? ackTimeout = default, CancellationToken cancellationToken = default) where T : IMessagePackSerializable, new()
+        {
+            id = NextId();
+            if (_disposed)
+            {
+                return Task.FromResult(new T());
+            }
+            var info = (IAckInfo<IMessagePackSerializable>)_acks.GetOrAdd(id, _ => new SingleAckWithMessagePackPayloadInfo<T>(ackTimeout ?? _defaultAckTimeout));
+            cancellationToken.Register(info.Cancel);
+            return info.Task.ContinueWith(task => (T)task.Result);
+        }
+
         public static bool HandleAckStatus(IAckableMessage message, AckStatus status)
         {
             return status switch
@@ -62,29 +79,19 @@ public Task<AckStatus> CreateMultiAck(out int id, TimeSpan? ackTimeout = default
             {
                 return Task.FromResult(AckStatus.Ok);
             }
-            var info = (IAckInfo<AckStatus>)_acks.GetOrAdd(id, _ => new MultiAckInfo(ackTimeout ?? _defaultAckTimeout));
-            if (info is SingleAckInfo)
+            var info = (IAckInfo<AckStatus>)_acks.GetOrAdd(id, _ => new MultiAckWithStatusInfo(ackTimeout ?? _defaultAckTimeout));
+            if (info is SingleAckInfo<AckStatus>)
             {
                 throw new InvalidOperationException();
             }
             return info.Task;
         }
 
-        public void TriggerAck(int id, AckStatus status = AckStatus.Ok)
+        public void TriggerAck(int id, AckStatus status = AckStatus.Ok, ReadOnlySequence<byte>? payload = default)
         {
-            if (_acks.TryGetValue(id, out var info))
+            if (_acks.TryGetValue(id, out var info) && info.Ack(status, payload))
             {
-                switch (info)
-                {
-                    case IAckInfo<AckStatus> ackInfo:
-                        if (ackInfo.Ack(status))
-                        {
-                            _acks.TryRemove(id, out _);
-                        }
-                        break;
-                    default:
-                        throw new InvalidCastException($"Expected: IAckInfo<{typeof(IAckInfo<AckStatus>).Name}>, actual type: {info.GetType().Name}");
-                }
+                _acks.TryRemove(id, out _);
             }
         }
 
@@ -125,11 +132,11 @@ private void CheckAcks()
                 {
                     if (_acks.TryRemove(id, out _))
                     {
-                        if (ack is SingleAckInfo singleAckInfo)
+                        if (ack is SingleAckInfo<AckStatus> singleAckInfo)
                         {
                             singleAckInfo.Ack(AckStatus.Timeout);
                         }
-                        else if (ack is MultiAckInfo multipleAckInfo)
+                        else if (ack is MultiAckWithStatusInfo multipleAckInfo)
                         {
                             multipleAckInfo.ForceAck(AckStatus.Timeout);
                         }
@@ -170,39 +177,57 @@ private interface IAckInfo
         {
             DateTime TimeoutAt { get; }
             void Cancel();
+            bool Ack(AckStatus status, ReadOnlySequence<byte>? payload = null);
         }
 
         private interface IAckInfo<T> : IAckInfo
         {
             Task<T> Task { get; }
-            bool Ack(T status);
         }
 
         public interface IMultiAckInfo
         {
             bool SetExpectedCount(int expectedCount);
         }
 
-        private sealed class SingleAckInfo : IAckInfo<AckStatus>
+        private abstract class SingleAckInfo<T> : IAckInfo<T>
         {
-            public readonly TaskCompletionSource<AckStatus> _tcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
-
+            public readonly TaskCompletionSource<T> _tcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
             public DateTime TimeoutAt { get; }
-
             public SingleAckInfo(TimeSpan timeout)
             {
                 TimeoutAt = DateTime.UtcNow + timeout;
             }
+            public abstract bool Ack(AckStatus status, ReadOnlySequence<byte>? payload = null);
+            public Task<T> Task => _tcs.Task;
+            public void Cancel() => _tcs.TrySetCanceled();
+        }
 
-            public bool Ack(AckStatus status = AckStatus.Ok) =>
-                _tcs.TrySetResult(status);
+        private class SingleAckWithStatusInfo : SingleAckInfo<AckStatus>
+        {
 
-            public Task<AckStatus> Task => _tcs.Task;
+            public SingleAckWithStatusInfo(TimeSpan timeout) : base(timeout) { }
 
-            public void Cancel() => _tcs.TrySetCanceled();
+            public override bool Ack(AckStatus status, ReadOnlySequence<byte>? payload = null) =>
+                _tcs.TrySetResult(status);
+        }
+
+        private sealed class SingleAckWithMessagePackPayloadInfo<T> : SingleAckInfo<IMessagePackSerializable> where T : IMessagePackSerializable, new()
+        {
+            public SingleAckWithMessagePackPayloadInfo(TimeSpan timeout) : base(timeout) { }
+            public override bool Ack(AckStatus status, ReadOnlySequence<byte>? payload = null)
+            {
+                if (payload == null)
+                {
+                    throw new ArgumentNullException(nameof(payload));
+                }
+                var reader = new MessagePackReader(payload.Value);
+                var result = reader.Deserialize<T>(string.Empty);
+                return _tcs.TrySetResult(result);
+            }
         }
 
-        private sealed class MultiAckInfo : IAckInfo<AckStatus>, IMultiAckInfo
+        private sealed class MultiAckWithStatusInfo : IAckInfo<AckStatus>, IMultiAckInfo
         {
             public readonly TaskCompletionSource<AckStatus> _tcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
 
@@ -211,7 +236,7 @@ private sealed class MultiAckInfo : IAckInfo<AckStatus>, IMultiAckInfo
 
             public DateTime TimeoutAt { get; }
 
-            public MultiAckInfo(TimeSpan timeout)
+            public MultiAckWithStatusInfo(TimeSpan timeout)
             {
                 TimeoutAt = DateTime.UtcNow + timeout;
             }
@@ -239,7 +264,7 @@ public bool SetExpectedCount(int expectedCount)
                 return result;
             }
 
-            public bool Ack(AckStatus status = AckStatus.Ok)
+            public bool Ack(AckStatus status = AckStatus.Ok, ReadOnlySequence<byte>? payload = null)
             {
                 bool result;
                 lock (_tcs)

src/Microsoft.Azure.SignalR.Management/HubContext/GroupManager.cs

@@ -1,9 +1,12 @@
 // Copyright (c) Microsoft. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+using System;
+using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.SignalR;
+using Microsoft.Azure.SignalR.Protocol;
 
 namespace Microsoft.Azure.SignalR.Management
 {
@@ -14,5 +17,7 @@ public abstract class GroupManager : IGroupManager
         public abstract Task RemoveFromGroupAsync(string connectionId, string groupName, CancellationToken cancellationToken = default);
 
         public abstract Task RemoveFromAllGroupsAsync(string connectionId, CancellationToken cancellationToken = default);
+
+        public virtual IAsyncEnumerable<GroupMember> ListConnectionsInGroup(string groupName, int? max, CancellationToken token) => throw new NotImplementedException();
     }
 }