Make RequestTransformContext.DestinationPrefix read-write

src/ReverseProxy/Transforms/RequestTransformContext.cs

@@ -53,8 +53,9 @@ public QueryTransformContext Query
     /// <summary>
     /// The URI prefix for the proxy request. This includes the scheme and host and can optionally include a
     /// port and path base. The 'Path' and 'Query' properties will be appended to this after the transforms have run.
+    /// Changing this value can have side effects on load balancing and health checks.
     /// </summary>
-    public string DestinationPrefix { get; init; } = default!;
+    public string DestinationPrefix { get; set; } = default!;
 
     /// <summary>
     /// A <see cref="CancellationToken"/> indicating that the request is being aborted.

test/ReverseProxy.Tests/Forwarder/HttpForwarderTests.cs

@@ -153,10 +153,12 @@
         httpContext.Response.Body = proxyResponseStream;
 
         var destinationPrefix = "https://localhost:123/a/b/";
+        Uri? originalRequestUri = null;
         var transforms = new DelegateHttpTransforms()
         {
             OnRequest = (context, request, destination) =>
             {
+                originalRequestUri = request.RequestUri;
                 request.RequestUri = new Uri(destination + "prefix"
                     + context.Request.Path + context.Request.QueryString);
                 request.Headers.Remove("transformHeader");
@@ -214,6 +216,7 @@
 
         var proxyError = await sut.SendAsync(httpContext, destinationPrefix, client, ForwarderRequestConfig.Empty, transforms);
 
+        Assert.Null(originalRequestUri); // Should only be set by the transformer
         Assert.Equal(ForwarderError.None, proxyError);
         Assert.Equal(234, httpContext.Response.StatusCode);
         var reasonPhrase = httpContext.Features.Get<IHttpResponseFeature>().ReasonPhrase;

test/ReverseProxy.Tests/Forwarder/HttpTransformerTests.cs

@@ -13,6 +13,7 @@
 using Microsoft.Net.Http.Headers;
 using Xunit;
 using Yarp.ReverseProxy.Transforms;
+using Yarp.ReverseProxy.Transforms.Builder;
 using Yarp.ReverseProxy.Transforms.Builder.Tests;
 using Yarp.Tests.Common;
 
@@ -116,6 +117,28 @@ public async Task TransformRequestAsync_ContentLengthAndTransferEncoding_Content
         Assert.False(proxyRequest.Headers.TryGetValues(HeaderNames.TransferEncoding, out var _));
     }
 
+    [Fact]
+    public async Task TransformRequestAsync_SetDestinationPrefix()
+    {
+        const string updatedDestinationPrefix = "https://contoso.com";
+        var transformer = TransformBuilderTests.CreateTransformBuilder().CreateInternal(context =>
+        {
+            context.AddRequestTransform(transformContext =>
+            {
+                transformContext.DestinationPrefix = updatedDestinationPrefix;
+                return ValueTask.CompletedTask;
+            });
+        });
+        var httpContext = new DefaultHttpContext();
+        var proxyRequest = new HttpRequestMessage(HttpMethod.Get, requestUri: (string)null)
+        {
+            Content = new ByteArrayContent(Array.Empty<byte>()),
+        };
+
+        await transformer.TransformRequestAsync(httpContext, proxyRequest, "https://localhost", CancellationToken.None);
+        Assert.Equal(new Uri(updatedDestinationPrefix), proxyRequest.RequestUri);
+    }
+
     [Theory]
     [InlineData(HttpStatusCode.Continue)]
     [InlineData(HttpStatusCode.SwitchingProtocols)]

test/ReverseProxy.Tests/Transforms/DestinationPrefixTransformTests.cs

@@ -0,0 +1,31 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System.Threading.Tasks;
+using Xunit;
+
+namespace Yarp.ReverseProxy.Transforms.Tests;
+
+public class DestinationPrefixTransformTests
+{
+    [Fact]
+    public async Task UpdateDestinationPrefix()
+    {
+        const string newDestinationPrefix = "http://localhost:8080";
+        var context = new RequestTransformContext()
+        {
+            DestinationPrefix = "http://contoso.com:5000"
+        };
+        var transform = new DestinationPrefixTransform(newDestinationPrefix);
+        await transform.ApplyAsync(context);
+    }
+
+    private class DestinationPrefixTransform(string newDestinationPrefix) : RequestTransform
+    {
+        public override ValueTask ApplyAsync(RequestTransformContext context)
+        {
+            context.DestinationPrefix = newDestinationPrefix;
+            return ValueTask.CompletedTask;
+        }
+    }
+}