Adding special scale out header to 429 responses

Author: mathewc

src/WebJobs.Script.WebHost/WebScriptHostRequestManager.cs

@@ -56,7 +56,10 @@ protected override HttpResponseMessage RejectRequest(HttpRequestMessage request)
             var function = request.GetPropertyOrDefault<FunctionDescriptor>(ScriptConstants.AzureFunctionsHttpFunctionKey);
             _metricsLogger.LogEvent(MetricEventNames.FunctionInvokeThrottled, function.Name);
 
-            return base.RejectRequest(request);
+            var response = base.RejectRequest(request);
+            response.Headers.Add(ScriptConstants.AntaresScaleOutHeaderName, "1");
+
+            return response;
         }
     }
 }

src/WebJobs.Script/ScriptConstants.cs

@@ -55,6 +55,7 @@ public static class ScriptConstants
         public const string DefaultFunctionKeyName = "default";
 
         public const string AntaresLogIdHeaderName = "X-ARR-LOG-ID";
+        public const string AntaresScaleOutHeaderName = "X-FUNCTION-SCALEOUT";
         public const string DynamicSku = "Dynamic";
         public const string DefaultProductionSlotName = "production";
 

test/WebJobs.Script.Tests/WebScriptHostRequestManagerTests.cs

@@ -82,6 +82,8 @@ public async Task ProcessRequestAsync_PerformanceThrottle_ReturnsExpectedResult(
             for (int i = 0; i < 3; i++)
             {
                 response = await _requestManager.ProcessRequestAsync(request, ProcessRequest, CancellationToken.None);
+                var scaleOutHeader = response.Headers.GetValues(ScriptConstants.AntaresScaleOutHeaderName).Single();
+                Assert.Equal("1", scaleOutHeader);
                 Assert.Equal((HttpStatusCode)429, response.StatusCode);
                 await Task.Delay(100);
             }