Allow the ResourceReadyEvent to block waiters

playground/mongo/Mongo.AppHost/Program.cs

@@ -1,15 +1,40 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using MongoDB.Bson.Serialization.Attributes;
+using MongoDB.Bson;
+using MongoDB.Driver;
+
 var builder = DistributedApplication.CreateBuilder(args);
 
 var db = builder.AddMongoDB("mongo")
     .WithMongoExpress(c => c.WithHostPort(3022))
     .AddDatabase("db");
 
+builder.Eventing.Subscribe<ResourceReadyEvent>(db.Resource, async (@event, ct) =>
+{
+    // Artificial delay to demonstrate the waiting
+    await Task.Delay(TimeSpan.FromSeconds(10), ct);
+
+    // Seed the database with some data
+    var cs = await db.Resource.ConnectionStringExpression.GetValueAsync(ct);
+    using var client = new MongoClient(cs);
+
+    const string collectionName = "entries";
+
+    var myDb = client.GetDatabase("db");
+    await myDb.CreateCollectionAsync(collectionName, cancellationToken: ct);
+
+    for (int i = 0; i < 10; i++)
+    {
+        await myDb.GetCollection<Entry>(collectionName).InsertOneAsync(new Entry(), cancellationToken: ct);
+    }
+});
+
 builder.AddProject<Projects.Mongo_ApiService>("api")
        .WithExternalHttpEndpoints()
-       .WithReference(db).WaitFor(db);
+       .WithReference(db)
+       .WaitFor(db);
 
 #if !SKIP_DASHBOARD_REFERENCE
 // This project is only added in playground projects to support development/debugging
@@ -22,3 +47,10 @@
 #endif
 
 builder.Build().Run();
+
+public sealed class Entry
+{
+    [BsonId]
+    [BsonRepresentation(BsonType.ObjectId)]
+    public string? Id { get; set; }
+}

src/Aspire.Hosting/ApplicationModel/CustomResourceSnapshot.cs

@@ -61,6 +61,11 @@ public ResourceStateSnapshot? State
     /// </summary>
     public int? ExitCode { get; init; }
 
+    /// <summary>
+    /// A flag indicating whether the resource ready event has completed.
+    /// </summary>
+    internal Task? ResourceReadyEventTask { get; init; }
+
     /// <summary>
     /// Gets the health status of the resource.
     /// </summary>

src/Aspire.Hosting/ApplicationModel/ResourceNotificationService.cs

@@ -161,6 +161,13 @@ private async Task WaitUntilHealthyAsync(IResource resource, IResource dependenc
             await WaitForResourceHealthyAsync(dependency.Name, cancellationToken).ConfigureAwait(false);
         }
 
+        // Now wait for the resource ready event to be executed.
+        resourceLogger.LogInformation("Waiting for resource ready to execute for '{Name}'.", dependency.Name);
+        resourceEvent = await WaitForResourceAsync(dependency.Name, re => re.Snapshot.ResourceReadyEventTask is { } t && t.IsCompleted, cancellationToken: cancellationToken).ConfigureAwait(false);
+
+        // Observe the result of the resource ready event task
+        await resourceEvent.Snapshot.ResourceReadyEventTask!.WaitAsync(cancellationToken).ConfigureAwait(false);
+
         resourceLogger.LogInformation("Finished waiting for resource '{Name}'.", dependency.Name);
 
         static bool IsContinuableState(CustomResourceSnapshot snapshot) =>

src/Aspire.Hosting/Health/ResourceHealthCheckService.cs

@@ -45,18 +45,34 @@ private async Task MonitorResourceHealthAsync(ResourceEvent initialEvent, Cancel
         var resource = initialEvent.Resource;
         var resourceReadyEventFired = false;
 
+        void FireResourceReadyEvent()
+        {
+            // We don't want to block the monitoring loop while we fire the event.
+            _ = Task.Run(async () =>
+            {
+                var resourceReadyEvent = new ResourceReadyEvent(resource, services);
+
+                // Execute the publish and store the task so that waiters can await it and observe the result.
+                var task = eventing.PublishAsync(resourceReadyEvent, cancellationToken);
+
+                await resourceNotificationService.PublishUpdateAsync(resource, s => s with
+                {
+                    ResourceReadyEventTask = task
+                })
+                    .ConfigureAwait(false);
+            },
+            cancellationToken);
+        }
+
         if (!resource.TryGetAnnotationsIncludingAncestorsOfType<HealthCheckAnnotation>(out var annotations))
         {
             // NOTE: If there are no health check annotations then there
             //       is currently nothing to monitor. At this point in time we don't
             //       dynamically add health checks at runtime. If this changes then we
             //       would need to revisit this and scan for transitive health checks
             //       on a periodic basis (you wouldn't want to do it on every pass.
-            var resourceReadyEvent = new ResourceReadyEvent(resource, services);
-            await eventing.PublishAsync(
-                resourceReadyEvent,
-                EventDispatchBehavior.NonBlockingSequential,
-                cancellationToken).ConfigureAwait(false);
+            FireResourceReadyEvent();
+
             return;
         }
 
@@ -76,11 +92,8 @@ await eventing.PublishAsync(
                 if (!resourceReadyEventFired && report.Status == HealthStatus.Healthy)
                 {
                     resourceReadyEventFired = true;
-                    var resourceReadyEvent = new ResourceReadyEvent(resource, services);
-                    await eventing.PublishAsync(
-                        resourceReadyEvent,
-                        EventDispatchBehavior.NonBlockingSequential,
-                        cancellationToken).ConfigureAwait(false);
+
+                    FireResourceReadyEvent();
                 }
 
                 var latestEvent = _latestEvents.GetValueOrDefault(resource.Name);

tests/Aspire.Hosting.Tests/WaitForTests.cs

@@ -5,6 +5,7 @@
 using Aspire.Hosting.Utils;
 using Microsoft.AspNetCore.InternalTesting;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
 using Xunit;
 using Xunit.Abstractions;
 
@@ -216,6 +217,69 @@ await rns.PublishUpdateAsync(dependency.Resource, s => s with
         await app.StopAsync();
     }
 
+    [Fact]
+    [RequiresDocker]
+    public async Task WaitForObservedResultOfResourceReadyEvent()
+    {
+        using var builder = TestDistributedApplicationBuilder.Create().WithTestAndResourceLogging(testOutputHelper);
+
+        builder.Services.AddLogging(b =>
+        {
+            b.AddFakeLogging();
+        });
+
+        var resourceReadyTcs = new TaskCompletionSource();
+        var dependency = builder.AddResource(new CustomResource("test"));
+        var nginx = builder.AddContainer("nginx", "mcr.microsoft.com/cbl-mariner/base/nginx", "1.22")
+                           .WithReference(dependency)
+                           .WaitFor(dependency);
+
+        builder.Eventing.Subscribe<ResourceReadyEvent>(dependency.Resource, (e, ct) => resourceReadyTcs.Task);
+
+        using var app = builder.Build();
+
+        // StartAsync will currently block until the dependency resource moves
+        // into a Finished state, so rather than awaiting it we'll hold onto the
+        // task so we can inspect the state of the Nginx resource which should
+        // be in a waiting state if everything is working correctly.
+        var startupCts = AsyncTestHelpers.CreateDefaultTimeoutTokenSource(TestConstants.LongTimeoutDuration);
+        var startTask = app.StartAsync(startupCts.Token);
+
+        // We don't want to wait forever for Nginx to move into a waiting state,
+        // it should be super quick, but we'll allow 60 seconds just in case the
+        // CI machine is chugging (also useful when collecting code coverage).
+        var waitingStateCts = AsyncTestHelpers.CreateDefaultTimeoutTokenSource(TestConstants.LongTimeoutDuration);
+
+        var rns = app.Services.GetRequiredService<ResourceNotificationService>();
+        await rns.WaitForResourceAsync(nginx.Resource.Name, "Waiting", waitingStateCts.Token);
+
+        // Now that we know we successfully entered the Waiting state, we can swap
+        // the dependency into a running state which will unblock startup and
+        // we can continue executing.
+        await rns.PublishUpdateAsync(dependency.Resource, s => s with
+        {
+            State = KnownResourceStates.Running
+        });
+
+        resourceReadyTcs.SetException(new InvalidOperationException("The resource ready event failed!"));
+
+        // This time we want to wait for Nginx to move into a Running state to verify that
+        // it successfully started after we moved the dependency resource into the Finished, but
+        // we need to give it more time since we have to download the image in CI.
+        var runningStateCts = AsyncTestHelpers.CreateDefaultTimeoutTokenSource(TestConstants.LongTimeoutDuration);
+        await rns.WaitForResourceAsync(nginx.Resource.Name, KnownResourceStates.FailedToStart, runningStateCts.Token);
+
+        await startTask;
+
+        var collector = app.Services.GetFakeLogCollector();
+        var logs = collector.GetSnapshot();
+
+        // Just looking for a common message in Docker build output.
+        Assert.Contains(logs, log => log.Message.Contains("The resource ready event failed!"));
+
+        await app.StopAsync();
+    }
+
     [Fact]
     [RequiresDocker]
     public async Task EnsureDependencyResourceThatReturnsNonMatchingExitCodeResultsInDependentResourceFailingToStart()