From ff02295bddb8c972e4f89be0c77f638920d58b7d Mon Sep 17 00:00:00 2001
From: martincostello <martin@martincostello.com>
Date: Sun, 16 Aug 2026 15:31:19 +0100
Subject: [PATCH] Make LegacyPropagator list sort linear

Replace O(n^2) behaviour with O(n) to reverse the list of keys parsed from the baggage header by `LegacyPropagator`, matching the implementation in `W3CPropagator`.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .../src/System/Diagnostics/LegacyPropagator.cs         |  6 ++++--
 .../tests/PropagatorTests.cs                           | 10 ++++++++--
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/LegacyPropagator.cs b/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/LegacyPropagator.cs
index 04a478fde94c40..c2d04607e43ab2 100644
--- a/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/LegacyPropagator.cs
+++ b/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/LegacyPropagator.cs
@@ -167,8 +167,7 @@ internal static bool TryExtractBaggage(string baggageString, out IEnumerable<Key
                 {
                     baggageList ??= new List<KeyValuePair<string, string?>>();
 
-                    // Insert in reverse order for asp.net compatibility.
-                    baggageList.Insert(0, new KeyValuePair<string, string?>(
+                    baggageList.Add(new KeyValuePair<string, string?>(
                                                 WebUtility.UrlDecode(baggageString.Substring(keyStart, keyEnd - keyStart)).Trim(s_trimmingSpaceCharacters),
                                                 WebUtility.UrlDecode(baggageString.Substring(valueStart, currentIndex - valueStart)).Trim(s_trimmingSpaceCharacters)));
                 }
@@ -182,6 +181,9 @@ internal static bool TryExtractBaggage(string baggageString, out IEnumerable<Key
                 currentIndex++; // Move to next key-value entry
             } while (currentIndex < baggageString.Length);
 
+            // Reverse order for asp.net compatibility.
+            baggageList?.Reverse();
+
             baggage = baggageList;
             return baggageList != null;
         }
diff --git a/src/libraries/System.Diagnostics.DiagnosticSource/tests/PropagatorTests.cs b/src/libraries/System.Diagnostics.DiagnosticSource/tests/PropagatorTests.cs
index 9f5319c36f5a19..96f4760b41f258 100644
--- a/src/libraries/System.Diagnostics.DiagnosticSource/tests/PropagatorTests.cs
+++ b/src/libraries/System.Diagnostics.DiagnosticSource/tests/PropagatorTests.cs
@@ -41,12 +41,18 @@ public void TestAllPropagators()
                 TestLegacyPropagatorUsingW3CActivity(
                                 DistributedContextPropagator.Current,
                                 "Legacy1=true",
-                                new List<KeyValuePair<string, string>>() { new KeyValuePair<string, string>("     LegacyKey1     ", "    LegacyValue1    ") });
+                                new List<KeyValuePair<string, string>>() {
+                                    new KeyValuePair<string, string>("     LegacyKey1     ", "    LegacyValue1    "),
+                                    new KeyValuePair<string, string>("LegacyKey1b", "LegacyValue1b"),
+                                    new KeyValuePair<string, string>("LegacyKey1c", "LegacyValue1c") });
 
                 TestLegacyPropagatorUsingHierarchicalActivity(
                                 DistributedContextPropagator.Current,
                                 "Legacy2=true",
-                                new List<KeyValuePair<string, string>>() { new KeyValuePair<string, string>("LegacyKey2", "LegacyValue2") });
+                                new List<KeyValuePair<string, string>>() {
+                                    new KeyValuePair<string, string>("LegacyKey2", "LegacyValue2"),
+                                    new KeyValuePair<string, string>("LegacyKey2b", "LegacyValue2b"),
+                                    new KeyValuePair<string, string>("LegacyKey2c", "LegacyValue2c") });
 
                 TestFields(DistributedContextPropagator.Current);