Preserve client order of activity baggage items (#26302)

Author: AndreyTretyak

src/Hosting/Hosting/src/Internal/HostingApplicationDiagnostics.cs

@@ -267,14 +267,14 @@ private Activity StartActivity(HttpContext httpContext, out bool hasDiagnosticLi
                 // We expect baggage to be empty by default
                 // Only very advanced users will be using it in near future, we encourage them to keep baggage small (few items)
                 string[] baggage = headers.GetCommaSeparatedValues(HeaderNames.CorrelationContext);
-                if (baggage.Length > 0)
+
+                // AddBaggage adds items at the beginning  of the list, so we need to add them in reverse to keep the same order as the client
+                // An order could be important if baggage has two items with the same key (that is allowed by the contract)
+                for (var i = baggage.Length - 1; i >= 0; i--)
                 {
-                    foreach (var item in baggage)
+                    if (NameValueHeaderValue.TryParse(baggage[i], out var baggageItem))
                     {
-                        if (NameValueHeaderValue.TryParse(item, out var baggageItem))
-                        {
-                            activity.AddBaggage(baggageItem.Name.ToString(), HttpUtility.UrlDecode(baggageItem.Value.ToString()));
-                        }
+                        activity.AddBaggage(baggageItem.Name.ToString(), HttpUtility.UrlDecode(baggageItem.Value.ToString()));
                     }
                 }
             }

src/Hosting/Hosting/test/HostingApplicationDiagnosticsTests.cs

@@ -301,6 +301,43 @@ public void ActivityParentIdAndBaggeReadFromHeaders()
             Assert.Contains(Activity.Current.Baggage, pair => pair.Key == "Key2" && pair.Value == "value2");
         }
 
+        [Fact]
+        public void ActivityBaggagePreservesItemsOrder()
+        {
+            var diagnosticListener = new DiagnosticListener("DummySource");
+            var hostingApplication = CreateApplication(out var features, diagnosticListener: diagnosticListener);
+
+            diagnosticListener.Subscribe(new CallbackDiagnosticListener(pair => { }),
+                s =>
+                {
+                    if (s.StartsWith("Microsoft.AspNetCore.Hosting.HttpRequestIn"))
+                    {
+                        return true;
+                    }
+                    return false;
+                });
+
+            features.Set<IHttpRequestFeature>(new HttpRequestFeature()
+            {
+                Headers = new HeaderDictionary()
+                {
+                    {"Request-Id", "ParentId1"},
+                    {"Correlation-Context", "Key1=value1, Key2=value2, Key1=value3"} // duplicated keys allowed by the contract
+                }
+            });
+            hostingApplication.CreateContext(features);
+            Assert.Equal("Microsoft.AspNetCore.Hosting.HttpRequestIn", Activity.Current.OperationName);
+
+            var expectedBaggage = new []
+            {
+                KeyValuePair.Create("Key1","value1"),
+                KeyValuePair.Create("Key2","value2"),
+                KeyValuePair.Create("Key1","value3")
+            };
+
+            Assert.Equal(expectedBaggage, Activity.Current.Baggage);
+        }
+
         [Fact]
         public void ActivityBaggageValuesAreUrlDecodedFromHeaders()
         {