.NET: Add BackgroundAgentsProvider.ReleaseSessionAsync to cancel and release per-session background tasks

dotnet/src/Microsoft.Agents.AI/Harness/BackgroundAgents/BackgroundAgentRuntimeState.cs

@@ -2,6 +2,7 @@
 
 using System.Collections.Generic;
 using System.Text.Json.Serialization;
+using System.Threading;
 using System.Threading.Tasks;
 
 namespace Microsoft.Agents.AI;
@@ -17,6 +18,18 @@ namespace Microsoft.Agents.AI;
 /// </remarks>
 internal sealed class BackgroundAgentRuntimeState
 {
+    /// <summary>
+    /// Gets an object used to synchronize access to the runtime references held by this instance.
+    /// </summary>
+    /// <remarks>
+    /// Background task registration happens on the agent's tool-invocation path, while
+    /// <see cref="BackgroundAgentsProvider.ReleaseSessionAsync"/> may be called concurrently by a host.
+    /// All mutations of the dictionaries below, and of <see cref="IsReleased"/>, must be performed under this lock
+    /// so that a task can never be registered into an already-released runtime.
+    /// </remarks>
+    [JsonIgnore]
+    public object SyncRoot { get; } = new();
+
     /// <summary>
     /// Gets the mapping of task IDs to their in-flight <see cref="Task{AgentResponse}"/> instances.
     /// </summary>
@@ -29,4 +42,25 @@ internal sealed class BackgroundAgentRuntimeState
     /// </summary>
     [JsonIgnore]
     public Dictionary<int, AgentSession> BackgroundTaskSessions { get; } = [];
+
+    /// <summary>
+    /// Gets the mapping of task IDs to the <see cref="CancellationTokenSource"/> controlling their run.
+    /// </summary>
+    /// <remarks>
+    /// A source is created when a task is started or continued, and is disposed and removed when the task is
+    /// finalized, cleared, or when the session is released via <see cref="BackgroundAgentsProvider.ReleaseSessionAsync"/>.
+    /// </remarks>
+    [JsonIgnore]
+    public Dictionary<int, CancellationTokenSource> TaskCancellations { get; } = [];
+
+    /// <summary>
+    /// Gets or sets a value indicating whether this runtime has been released via
+    /// <see cref="BackgroundAgentsProvider.ReleaseSessionAsync"/>.
+    /// </summary>
+    /// <remarks>
+    /// Once released, all in-flight tasks have been cancelled and awaited, and the runtime references have been
+    /// dropped. Tools that would start new background work refuse to run against a released runtime.
+    /// </remarks>
+    [JsonIgnore]
+    public bool IsReleased { get; set; }
 }