Fix base URI during prerendering (#8910)

src/Components/Server/src/Circuits/CircuitPrerenderer.cs

@@ -65,7 +65,16 @@ namespace Microsoft.AspNetCore.Components.Server.Circuits
 
         private string GetFullBaseUri(HttpRequest request)
         {
-            return UriHelper.BuildAbsolute(request.Scheme, request.Host, request.PathBase);
+            var result = UriHelper.BuildAbsolute(request.Scheme, request.Host, request.PathBase);
+
+            // PathBase may be "/" or "/some/thing", but to be a well-formed base URI
+            // it has to end with a trailing slash
+            if (!result.EndsWith('/'))
+            {
+                result += '/';
+            }
+
+            return result;
         }
     }
 }

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

@@ -0,0 +1,132 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Components.Server.Circuits;
+using Microsoft.AspNetCore.Components.Services;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.DependencyInjection;
+using Moq;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Components.Server.Tests.Circuits
+{
+    public class CircuitPrerendererTest
+    {
+        // Because CircuitPrerenderer is a point of integration with HttpContext,
+        // it's not a good candidate for unit testing. The majority of prerendering
+        // unit tests should be elsewhere in HtmlRendererTests inside the
+        // Microsoft.AspNetCore.Components.Tests projects.
+        //
+        // The only unit tests added here should specifically be about how we're
+        // interacting with the HttpContext for configuring the prerenderer.
+
+        [Fact]
+        public async Task ExtractsUriFromHttpContext_EmptyPathBase()
+        {
+            // Arrange
+            var circuitFactory = new TestCircuitFactory();
+            var circuitPrerenderer = new CircuitPrerenderer(circuitFactory);
+            var httpContext = new Mock<HttpContext>();
+            var httpRequest = new Mock<HttpRequest>().SetupAllProperties();
+            httpContext.Setup(h => h.Request).Returns(httpRequest.Object);
+            httpRequest.Object.Scheme = "https";
+            httpRequest.Object.Host = new HostString("example.com", 1234);
+            httpRequest.Object.Path = "/some/path";
+
+            var prerenderingContext = new ComponentPrerenderingContext
+            {
+                ComponentType = typeof(UriDisplayComponent),
+                Parameters = ParameterCollection.Empty,
+                Context = httpContext.Object
+            };
+
+            // Act
+            var result = await circuitPrerenderer.PrerenderComponentAsync(prerenderingContext);
+
+            // Assert
+            Assert.Equal(new[]
+            {
+                "The current URI is ",
+                "https://example.com:1234/some/path",
+                " within base URI ",
+                "https://example.com:1234/"
+            }, result);
+        }
+
+        [Fact]
+        public async Task ExtractsUriFromHttpContext_NonemptyPathBase()
+        {
+            // Arrange
+            var circuitFactory = new TestCircuitFactory();
+            var circuitPrerenderer = new CircuitPrerenderer(circuitFactory);
+            var httpContext = new Mock<HttpContext>();
+            var httpRequest = new Mock<HttpRequest>().SetupAllProperties();
+            httpContext.Setup(h => h.Request).Returns(httpRequest.Object);
+            httpRequest.Object.Scheme = "https";
+            httpRequest.Object.Host = new HostString("example.com", 1234);
+            httpRequest.Object.PathBase = "/my/dir";
+            httpRequest.Object.Path = "/some/path";
+
+            var prerenderingContext = new ComponentPrerenderingContext
+            {
+                ComponentType = typeof(UriDisplayComponent),
+                Parameters = ParameterCollection.Empty,
+                Context = httpContext.Object
+            };
+
+            // Act
+            var result = await circuitPrerenderer.PrerenderComponentAsync(prerenderingContext);
+
+            // Assert
+            Assert.Equal(new[]
+            {
+                "The current URI is ",
+                "https://example.com:1234/my/dir/some/path",
+                " within base URI ",
+                "https://example.com:1234/my/dir/"
+            }, result);
+        }
+
+        class TestCircuitFactory : CircuitFactory
+        {
+            public override CircuitHost CreateCircuitHost(HttpContext httpContext, CircuitClientProxy client, string uriAbsolute, string baseUriAbsolute)
+            {
+                var serviceCollection = new ServiceCollection();
+                serviceCollection.AddScoped<IUriHelper>(_ =>
+                {
+                    var uriHelper = new RemoteUriHelper();
+                    uriHelper.Initialize(uriAbsolute, baseUriAbsolute);
+                    return uriHelper;
+                });
+                var serviceScope = serviceCollection.BuildServiceProvider().CreateScope();
+                return TestCircuitHost.Create(serviceScope);
+            }
+        }
+
+        class UriDisplayComponent : IComponent
+        {
+            private RenderHandle _renderHandle;
+
+            [Inject] IUriHelper UriHelper { get; set; }
+
+            public void Configure(RenderHandle renderHandle)
+            {
+                _renderHandle = renderHandle;
+            }
+
+            public Task SetParametersAsync(ParameterCollection parameters)
+            {
+                _renderHandle.Render(builder =>
+                {
+                    builder.AddContent(0, "The current URI is ");
+                    builder.AddContent(1, UriHelper.GetAbsoluteUri());
+                    builder.AddContent(2, " within base URI ");
+                    builder.AddContent(3, UriHelper.GetBaseUri());
+                });
+
+                return Task.CompletedTask;
+            }
+        }
+    }
+}

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

@@ -43,7 +43,7 @@ namespace Microsoft.AspNetCore.Components.Server.Circuits
             if (remoteRenderer == null)
             {
                 remoteRenderer = new RemoteRenderer(
-                    Mock.Of<IServiceProvider>(),
+                    serviceScope.ServiceProvider ?? Mock.Of<IServiceProvider>(),
                     new RendererRegistry(),
                     jsRuntime,
                     clientProxy,