Throw if a request transform overrides the request HttpContent

src/ReverseProxy/Forwarder/HttpForwarder.cs

@@ -430,6 +430,11 @@ public async ValueTask<ForwarderError> SendAsync(
         // :: Step 3: Copy request headers Client --► Proxy --► Destination
         await transformer.TransformRequestAsync(context, destinationRequest, destinationPrefix, activityToken.Token);
 
+        if (!ReferenceEquals(requestContent, destinationRequest.Content) && destinationRequest.Content is not EmptyHttpContent)
+        {
+            throw new InvalidOperationException("Replacing the YARP outgoing request HttpContent is not supported. You should configure the HttpContext.Request instead.");
+        }
+
         // The transformer generated a response, do not forward.
         if (RequestUtilities.IsResponseSet(context.Response))
         {
@@ -450,7 +455,6 @@ public async ValueTask<ForwarderError> SendAsync(
             context.Features.Get<IHttpResponseBodyFeature>()?.DisableBuffering();
         }
 
-        // TODO: What if they replace the HttpContent object? That would mess with our tracking and error handling.
         return (destinationRequest, requestContent, tryDowngradingH2WsOnFailure);
     }
 

test/ReverseProxy.Tests/Forwarder/HttpForwarderTests.cs

@@ -498,6 +498,43 @@ public async Task TransformRequestAsync_WritesToResponse_ShortCircuits()
         events.AssertContainProxyStages(Array.Empty<ForwarderStage>());
     }
 
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public async Task TransformRequestAsync_ModifiesRequestContent_Throws(bool originalHasBody)
+    {
+        var events = TestEventListener.Collect();
+        TestLogger.Collect();
+
+        var httpContext = new DefaultHttpContext();
+        httpContext.Request.Method = "POST";
+        httpContext.Features.Set<IHttpRequestBodyDetectionFeature>(new TestBodyDetector { CanHaveBody = originalHasBody });
+
+        var destinationPrefix = "https://localhost/foo";
+
+        var transforms = new DelegateHttpTransforms()
+        {
+            CopyRequestHeaders = true,
+            OnRequest = (context, request, destination) =>
+            {
+                request.Content = new StringContent("modified content");
+                return Task.CompletedTask;
+            }
+        };
+
+        var sut = CreateProxy();
+        var client = MockHttpHandler.CreateClient(
+            (HttpRequestMessage request, CancellationToken cancellationToken) =>
+            {
+                throw new NotImplementedException();
+            });
+
+        var proxyError = await sut.SendAsync(httpContext, destinationPrefix, client, ForwarderRequestConfig.Empty, transforms);
+
+        AssertErrorInfo<InvalidOperationException>(ForwarderError.RequestCreation, StatusCodes.Status502BadGateway, proxyError, httpContext, destinationPrefix);
+        Assert.Empty(events.GetProxyStages());
+    }
+
     // Tests proxying an upgradeable request.
     [Theory]
     [InlineData("WebSocket")]