[release/13.0] EndpointReference evaluation should wait on missing AllocatedEndpoint (#13074)

src/Aspire.Hosting/ApplicationModel/EndpointReference.cs

@@ -293,10 +293,16 @@ public class EndpointReferenceExpression(EndpointReference endpointReference, En
         {
             // We are going to take the first snapshot that matches the context network ID. In general there might be multiple endpoints for a single service,
             // and in future we might need some sort of policy to choose between them, but for now we just take the first one.
-            var nes = Endpoint.EndpointAnnotation.AllAllocatedEndpoints.Where(nes => nes.NetworkID == networkContext).FirstOrDefault();
+            var endpointSnapshots = Endpoint.EndpointAnnotation.AllAllocatedEndpoints;
+            var nes = endpointSnapshots.Where(nes => nes.NetworkID == networkContext).FirstOrDefault();
             if (nes is null)
             {
-                return null;
+                nes = new NetworkEndpointSnapshot(new ValueSnapshot<AllocatedEndpoint>(), networkContext);
+                if (!endpointSnapshots.TryAdd(networkContext, nes.Snapshot))
+                {
+                    // Someone else added it first, use theirs.
+                    nes = endpointSnapshots.Where(nes => nes.NetworkID == networkContext).First();
+                }
             }
 
             var allocatedEndpoint = await nes.Snapshot.GetValueAsync(cancellationToken).ConfigureAwait(false);

tests/Aspire.Hosting.Tests/ExpressionResolverTests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using Aspire.Hosting.Dcp;
+using Aspire.Hosting.Testing;
 using Aspire.Hosting.Tests.Utils;
 using Microsoft.AspNetCore.InternalTesting;
 using Microsoft.Extensions.Options;
@@ -232,6 +233,28 @@ public async Task ContainerToContainerEndpointShouldResolve()
 
         Assert.Equal("http://myContainer:8080", config["ConnectionStrings__myContainer"]);
     }
+
+    [Fact]
+    public async Task ExpressionResolutionShouldWaitOnMissingAllocatedEndpoint()
+    {
+        var builder = DistributedApplicationTestingBuilder.Create();
+
+        var dependency = builder
+           .AddResource(new TestHostResource("hostResource"))
+           .WithHttpEndpoint(name: "hostEndpoint");
+
+        var consumer = builder.AddResource(new MyContainerResource("containerResource"))
+           .WithImage("redis")
+           .WithReference(dependency.GetEndpoint("hostEndpoint"));
+
+        var endpointAnnotation = dependency.Resource.Annotations.OfType<EndpointAnnotation>().Single();
+        endpointAnnotation.AllocatedEndpoint = new AllocatedEndpoint(endpointAnnotation, "localhost", 1234);
+
+        await Assert.ThrowsAsync<TimeoutException>(async () =>
+        {
+            _ = await EnvironmentVariableEvaluator.GetEnvironmentVariablesAsync(consumer.Resource, DistributedApplicationOperation.Run, TestServiceProvider.Instance).AsTask().TimeoutAfter(TimeSpan.FromSeconds(2));
+        });
+    }
 }
 
 sealed class MyContainerResource : ContainerResource, IResourceWithConnectionString
@@ -286,3 +309,8 @@ public TestExpressionResolverResource(string exprName) : base("testresource")
 
     public ReferenceExpression ConnectionStringExpression => Expressions[_exprName];
 }
+
+public sealed class TestHostResource : Resource, IResourceWithEndpoints
+{
+    public TestHostResource(string name) : base(name) { }
+}