Avoid allocation in TryGetAuthenticatedUser.

As we reported in dotnet/runtime#107861 it turns out that retrieving ClaimsPrincipal.Identity causes an unnecessary allocation. This works around that.

Solutions/Corvus.YarpPipelines/Corvus.YarpPipelines/ClaimsExtensions.cs

@@ -0,0 +1,44 @@
+// <copyright file="ClaimsExtensions.cs" company="Endjin Limited">
+// Copyright (c) Endjin Limited. All rights reserved.
+// </copyright>
+
+using System.Security.Claims;
+using System.Security.Principal;
+
+namespace Corvus.YarpPipelines;
+
+/// <summary>
+/// Low-allocation utilities for working with claims.
+/// </summary>
+internal static class ClaimsExtensions
+{
+    /// <summary>
+    /// A non-allocating version of <see cref="ClaimsPrincipal.Identity"/>.
+    /// </summary>
+    /// <param name="claimsPrincipal">
+    /// The principal from which to extract the primary identity.
+    /// </param>
+    /// <returns>The primary identity.</returns>
+    /// <remarks>
+    /// <see cref="ClaimsPrincipal.Identity"/> allocates an enumerator, because
+    /// it executes a foreach over the identities. In most cases, these identities
+    /// are in an IList, making zero-allocation enumeration possible, and since
+    /// the .NET runtime implementation doesn't do that (at least, not in .NET 8.0)
+    /// we have to do it ourselves. (See https://github.com/dotnet/runtime/issues/107861)
+    /// </remarks>
+    public static IIdentity? GetPrimaryIdentity(this ClaimsPrincipal claimsPrincipal)
+    {
+        if (claimsPrincipal.Identities is IList<ClaimsIdentity> identities)
+        {
+            for (int i = 0; i < identities.Count; i++)
+            {
+                if (identities[i] is not null)
+                {
+                    return identities[i];
+                }
+            }
+        }
+
+        return claimsPrincipal.Identity;
+    }
+}

Solutions/Corvus.YarpPipelines/Corvus.YarpPipelines/YarpRequestPipelineStateExtensions.cs

@@ -123,7 +123,8 @@ public static void SetUser(this YarpRequestPipelineState state, ClaimsPrincipal
     public static bool TryGetAuthenticatedUser(this YarpRequestPipelineState state, [NotNullWhen(true)] out ClaimsPrincipal? user)
     {
         user = state.RequestTransformContext.HttpContext.User;
-        return user is not null && user.Identity?.IsAuthenticated == true;
+
+        return user is not null && user.GetPrimaryIdentity()?.IsAuthenticated == true;
     }
 
     /// <summary>