Multi Round-Trip Requests (MRTR) (#1458)

src/ModelContextProtocol.Core/Server/MrtrContinuation.cs

@@ -0,0 +1,50 @@
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol.Server;
+
+/// <summary>
+/// Represents the lifecycle state for an MRTR handler invocation across retries.
+/// Created when the handler starts and stored in <c>_mrtrContinuations</c> when
+/// the handler suspends waiting for client input.
+/// </summary>
+internal sealed class MrtrContinuation
+{
+    private readonly CancellationTokenSource _handlerCts;
+
+    public MrtrContinuation(CancellationTokenSource handlerCts, Task<JsonNode?> handlerTask, MrtrContext mrtrContext)
+    {
+        _handlerCts = handlerCts;
+        HandlerTask = handlerTask;
+        MrtrContext = mrtrContext;
+    }
+
+    /// <summary>
+    /// Gets a token that cancels when the handler should be aborted.
+    /// Passed to the handler at creation and remains valid across retries.
+    /// </summary>
+    public CancellationToken HandlerToken => _handlerCts.Token;
+
+    /// <summary>
+    /// The handler task that is suspended awaiting input.
+    /// </summary>
+    public Task<JsonNode?> HandlerTask { get; }
+
+    /// <summary>
+    /// The MRTR context for the handler's async flow.
+    /// </summary>
+    public MrtrContext MrtrContext { get; }
+
+    /// <summary>
+    /// The exchange that is awaiting a response from the client.
+    /// Set each time the handler suspends on a new exchange.
+    /// </summary>
+    public MrtrExchange? PendingExchange { get; set; }
+
+    /// <summary>
+    /// Cancels the handler. Safe to call multiple times and concurrently -
+    /// <see cref="CancellationTokenSource.Cancel()"/> is thread-safe with itself.
+    /// The CTS is intentionally never disposed to avoid deadlock risks from
+    /// calling Cancel/Dispose inside synchronization primitives.
+    /// </summary>
+    public void CancelHandler() => _handlerCts.Cancel();
+}