feat(runtime): track node lifecycle and QUIC link state

backend/fleet/src/Fleet.Link/FleetLink.cs

@@ -58,6 +58,9 @@ public sealed class FleetLinkOptions
     /// <summary>附加到 <c>fleet.heartbeat</c> 的进程内计数（不能包含主机信息）。</summary>
     public Func<JsonElement?>? HeartbeatExtra { get; init; }
 
+    /// <summary>Optional reason for an orderly client shutdown, so the server can distinguish updates from service stops.</summary>
+    public Func<string>? ShutdownReason { get; init; }
+
     public TimeSpan HeartbeatInterval { get; init; } = TimeSpan.FromSeconds(10);
 
     public int MaxFrameBytes { get; init; } = 16 << 20;
@@ -181,7 +184,8 @@ public sealed class FleetLink
         _secure = transport.Secure;
 
         var session = new RpcSession<LinkContext>(transport, _router, new LinkContext(this), _time);
-        using CancellationTokenSource sessionCts = CancellationTokenSource.CreateLinkedTokenSource(ct);
+        // Keep the active WebSocket receive alive until the planned shutdown marker and Close frame have been sent.
+        using var sessionCts = new CancellationTokenSource();
         Task run = session.RunAsync(sessionCts.Token);
         try
         {
@@ -192,7 +196,7 @@ public sealed class FleetLink
                 NowUtcMs = _time.GetUtcNow().ToUnixTimeMilliseconds(),
                 Meta = _options.HelloMeta?.Invoke(),
             };
-            Result<HelloResult> helloResult = await session.RequestAsync(FleetMethods.Hello, hello, FleetRpcContext.Default.HelloParams, FleetRpcContext.Default.HelloResult, sessionCts.Token);
+            Result<HelloResult> helloResult = await session.RequestAsync(FleetMethods.Hello, hello, FleetRpcContext.Default.HelloParams, FleetRpcContext.Default.HelloResult, ct);
             if (!helloResult.IsSuccess)
             {
                 return "hello failed: " + helloResult.Error.Code;
@@ -208,7 +212,23 @@ public sealed class FleetLink
                 await OnConnected(session, sessionCts.Token);
             }
 
-            Task finished = await Task.WhenAny(run, pump, heartbeat);
+            Task cancellation = Task.Delay(Timeout.Infinite, ct);
+            Task finished = await Task.WhenAny(run, pump, heartbeat, cancellation);
+            if (ct.IsCancellationRequested)
+            {
+                try
+                {
+                    using var closeTimeout = new CancellationTokenSource(TimeSpan.FromSeconds(1), _time);
+                    string reason = _options.ShutdownReason?.Invoke() ?? "stopping";
+                    await session.NotifyAsync(FleetMethods.Stopping, new StoppingParams { Reason = reason }, FleetRpcContext.Default.StoppingParams, closeTimeout.Token);
+                    await session.CloseAsync(reason, closeTimeout.Token);
+                }
+                catch (Exception ex) when (ex is OperationCanceledException or WebSocketException or IOException or ObjectDisposedException or InvalidOperationException)
+                {
+                    // The socket may already be gone; shutdown must still complete.
+                }
+            }
+
             await sessionCts.CancelAsync();
             await Task.WhenAll(Quiet(run), Quiet(pump), Quiet(heartbeat));
             return finished == run ? "closed by server" : "link task failed";

backend/fleet/src/Fleet.Rpc/FleetMethods.cs

@@ -10,6 +10,7 @@ public static class FleetMethods
     public const string Auth = "fleet.auth";
     public const string Hello = "fleet.hello";
     public const string Heartbeat = "fleet.heartbeat";
+    public const string Stopping = "fleet.stopping";
     public const string StateSync = "fleet.state.sync";
     public const string OutboxAck = "fleet.outbox.ack";
     public const string Revoked = "fleet.revoked";
@@ -28,6 +29,7 @@ public static class FleetMethodTable
         MethodSpec.Request(FleetMethods.Auth, RpcSide.Node).AsInline(),
         MethodSpec.Request(FleetMethods.Hello, RpcSide.Node).AsInline(),
         MethodSpec.Notification(FleetMethods.Heartbeat, RpcSide.Node).AsInline(),
+        MethodSpec.Notification(FleetMethods.Stopping, RpcSide.Node).AsInline(),
         MethodSpec.Request(FleetMethods.StateSync, RpcSide.Server),
         MethodSpec.Notification(FleetMethods.OutboxAck, RpcSide.Server),
         MethodSpec.Notification(FleetMethods.Revoked, RpcSide.Server),
@@ -156,6 +158,12 @@ public sealed class RevokedParams
     public string Reason { get; set; } = "";
 }
 
+public sealed class StoppingParams
+{
+    [JsonPropertyName("reason")]
+    public string Reason { get; set; } = "stopping";
+}
+
 [JsonSourceGenerationOptions(DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull)]
 [JsonSerializable(typeof(RpcMessage))]
 [JsonSerializable(typeof(ChallengeParams))]
@@ -164,6 +172,7 @@ public sealed class RevokedParams
 [JsonSerializable(typeof(HelloParams))]
 [JsonSerializable(typeof(HelloResult))]
 [JsonSerializable(typeof(HeartbeatParams))]
+[JsonSerializable(typeof(StoppingParams))]
 [JsonSerializable(typeof(StateSyncParams))]
 [JsonSerializable(typeof(StateSyncResult))]
 [JsonSerializable(typeof(OutboxAckParams))]

backend/fleet/src/Fleet.Server.Abstractions/ServerContracts.cs

@@ -38,7 +38,7 @@ public enum NodeEventKind
     Heartbeat,
 }
 
-public sealed record NodeEvent(NodeEventKind Kind, NodeRecord Node, long AtMs, HelloParams? Hello = null, JsonElement? Extra = null);
+ public sealed record NodeEvent(NodeEventKind Kind, NodeRecord Node, long AtMs, HelloParams? Hello = null, JsonElement? Extra = null, string? Reason = null);
 
 public interface INodeEventSink
 {

backend/fleet/src/Fleet.Server.Link/NodeConnection.Receive.cs

@@ -64,6 +64,17 @@ internal sealed partial class NodeConnection
             case FleetMethods.Heartbeat:
                 await HandleHeartbeatAsync(message, ct);
                 return;
+            case FleetMethods.Stopping:
+                try
+                {
+                    string? reason = message.Params?.Deserialize(FleetRpcContext.Default.StoppingParams)?.Reason;
+                    _closeReason = reason is "updating" ? "updating" : "stopping";
+                }
+                catch (JsonException)
+                {
+                    _closeReason = "stopping";
+                }
+                return;
         }
 
         if (_node!.Status != NodeStatus.Active)

backend/fleet/src/Fleet.Server.Link/NodeConnection.State.cs

@@ -102,7 +102,7 @@ internal sealed partial class NodeConnection
 
     private async Task FireAsync(NodeEventKind kind, CancellationToken ct, HelloParams? hello = null, JsonElement? extra = null)
     {
-        var nodeEvent = new NodeEvent(kind, _node!, _host.Time.GetUtcNow().ToUnixTimeMilliseconds(), hello, extra);
+        var nodeEvent = new NodeEvent(kind, _node!, _host.Time.GetUtcNow().ToUnixTimeMilliseconds(), hello, extra, kind == NodeEventKind.Offline ? _closeReason : null);
         foreach (INodeEventSink sink in _host.Sinks)
         {
             try

backend/fleet/src/Fleet.Server.Link/NodeConnection.Wire.cs

@@ -17,6 +17,7 @@ internal sealed partial class NodeConnection
             ValueWebSocketReceiveResult result = await _socket.ReceiveAsync(buffer, ct);
             if (result.MessageType == WebSocketMessageType.Close)
             {
+                _closeReason ??= _socket.CloseStatusDescription;
                 return null;
             }