Dispose DI scope with async support in circuit host

Fixes part of #12918

This fixes the part of this issue that we're going to be able to do in
3.0 safely.

src/Components/Server/src/Circuits/CircuitHost.cs

@@ -170,7 +170,18 @@ namespace Microsoft.AspNetCore.Components.Server.Circuits
                 try
                 {
                     Renderer.Dispose();
-                    _scope.Dispose();
+
+                    // This cast is needed because it's possible the scope may not support async dispose.
+                    // Our DI container does, but other DI systems may not.
+                    if (_scope is IAsyncDisposable asyncDisposable)
+                    {
+                        await asyncDisposable.DisposeAsync();
+                    }
+                    else
+                    {
+                        _scope.Dispose();
+                    }
+
                     Log.DisposeSucceeded(_logger, CircuitId);
                 }
                 catch (Exception ex)

src/Components/Server/test/Circuits/CircuitHostTest.cs

@@ -39,6 +39,33 @@ namespace Microsoft.AspNetCore.Components.Server.Circuits
             Assert.Null(circuitHost.Handle.CircuitHost);
         }
 
+        [Fact]
+        public async Task DisposeAsync_DisposesScopeAsynchronouslyIfPossible()
+        {
+            // Arrange
+            var serviceScope = new Mock<IServiceScope>();
+            serviceScope
+                .As<IAsyncDisposable>()
+                .Setup(f => f.DisposeAsync())
+                .Returns(new ValueTask(Task.CompletedTask))
+                .Verifiable();
+
+            var remoteRenderer = GetRemoteRenderer();
+            var circuitHost = TestCircuitHost.Create(
+                Guid.NewGuid().ToString(),
+                serviceScope.Object,
+                remoteRenderer);
+
+            // Act
+            await circuitHost.DisposeAsync();
+
+            // Assert
+            serviceScope.Verify(s => s.Dispose(), Times.Never());
+            serviceScope.As<IAsyncDisposable>().Verify(s => s.DisposeAsync(), Times.Once());
+            Assert.True(remoteRenderer.Disposed);
+            Assert.Null(circuitHost.Handle.CircuitHost);
+        }
+
         [Fact]
         public async Task DisposeAsync_DisposesResourcesAndSilencesException()
         {