Hardened background work against disconnected circuits (#937)

app/MindWork AI Studio/Tools/Services/AIStudioCircuitHandler.cs

@@ -0,0 +1,61 @@
+using Microsoft.AspNetCore.Components.Server.Circuits;
+
+namespace AIStudio.Tools.Services;
+
+/// <summary>
+/// Follows the life of one circuit, so the rest of the app knows when its browser is unreachable.
+/// </summary>
+/// <remarks>
+/// The app keeps disconnected circuits for a long time on purpose, cf. the retention settings in
+/// Program.cs. That is what lets a user return to a working app after the machine woke up — but it also
+/// means that the components of reloaded or sleeping windows stay alive and keep receiving events. They
+/// may keep working: everything they do on the server is fine. Only JavaScript interop is impossible
+/// while the connection is gone. So this handler does two things, and deliberately nothing more:
+/// it publishes the connection state, and it cleans up once a circuit is truly over.
+/// </remarks>
+public sealed class AIStudioCircuitHandler(CircuitStateService circuitState, MessageBus messageBus, ILogger<AIStudioCircuitHandler> logger)
+    : CircuitHandler
+{
+    #region Overrides of CircuitHandler
+
+    public override Task OnCircuitOpenedAsync(Circuit circuit, CancellationToken cancellationToken)
+    {
+        circuitState.AssignCircuit(circuit.Id);
+        logger.LogInformation("The circuit '{CircuitId}' was opened.", circuit.Id);
+
+        return Task.CompletedTask;
+    }
+
+    public override Task OnConnectionUpAsync(Circuit circuit, CancellationToken cancellationToken)
+    {
+        circuitState.MarkAsConnected();
+        logger.LogInformation("The browser connection of the circuit '{CircuitId}' is up.", circuit.Id);
+
+        return Task.CompletedTask;
+    }
+
+    public override Task OnConnectionDownAsync(Circuit circuit, CancellationToken cancellationToken)
+    {
+        circuitState.MarkAsDisconnected();
+        logger.LogInformation("The browser connection of the circuit '{CircuitId}' is down. Its JavaScript interop is paused until it returns.", circuit.Id);
+
+        return Task.CompletedTask;
+    }
+
+    public override Task OnCircuitClosedAsync(Circuit circuit, CancellationToken cancellationToken)
+    {
+        circuitState.MarkAsDisconnected();
+
+        //
+        // The components of this circuit will not come back, so nobody would ever deregister them:
+        // Blazor disposes components of a retained circuit without giving them a chance to run their
+        // disposal in every case. Without this, the message bus would keep and serve them forever.
+        //
+        var numRemovedReceivers = messageBus.UnregisterCircuit(circuitState);
+        logger.LogInformation("The circuit '{CircuitId}' was closed. Removed {NumReceivers} message bus receiver(s) of that circuit.", circuit.Id, numRemovedReceivers);
+
+        return Task.CompletedTask;
+    }
+
+    #endregion
+}