From ca7c48c520f2baae032e0429f19552abed77e009 Mon Sep 17 00:00:00 2001
From: Martin Costello <martin@martincostello.com>
Date: Mon, 7 Jan 2019 00:42:36 +0000
Subject: [PATCH] Fix ArgumentException from duplicate key (#6416)

---
 .../src/Matching/HttpMethodMatcherPolicy.cs   | 11 ++++---
 .../HttpMethodMatcherPolicyIntegrationTest.cs | 32 ++++++++++++++++++-
 2 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/src/Http/Routing/src/Matching/HttpMethodMatcherPolicy.cs b/src/Http/Routing/src/Matching/HttpMethodMatcherPolicy.cs
index f1ec26d67d..af3981521a 100644
--- a/src/Http/Routing/src/Matching/HttpMethodMatcherPolicy.cs
+++ b/src/Http/Routing/src/Matching/HttpMethodMatcherPolicy.cs
@@ -1,13 +1,10 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
 using System.Collections.Generic;
-using System.IO;
-using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http;
-using Microsoft.AspNetCore.Routing.Patterns;
 using Microsoft.Extensions.Internal;
 using Microsoft.Extensions.Primitives;
 
@@ -274,7 +271,11 @@ namespace Microsoft.AspNetCore.Routing.Matching
                 (context) =>
                 {
                     context.Response.StatusCode = 405;
-                    context.Response.Headers.Add("Allow", allow);
+
+                    // Prevent ArgumentException from duplicate key if header already added, such as when the
+                    // request is re-executed by an error handler (see https://github.com/aspnet/AspNetCore/issues/6415)
+                    context.Response.Headers["Allow"] = allow;
+
                     return Task.CompletedTask;
                 },
                 EndpointMetadataCollection.Empty,
diff --git a/src/Http/Routing/test/UnitTests/Matching/HttpMethodMatcherPolicyIntegrationTest.cs b/src/Http/Routing/test/UnitTests/Matching/HttpMethodMatcherPolicyIntegrationTest.cs
index 1aca0d5a64..fd223d9b7e 100644
--- a/src/Http/Routing/test/UnitTests/Matching/HttpMethodMatcherPolicyIntegrationTest.cs
+++ b/src/Http/Routing/test/UnitTests/Matching/HttpMethodMatcherPolicyIntegrationTest.cs
@@ -1,4 +1,4 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
@@ -288,6 +288,36 @@ namespace Microsoft.AspNetCore.Routing.Matching
             MatcherAssert.AssertMatch(context, httpContext, endpoint2, ignoreValues: true);
         }
 
+        [Fact] // See https://github.com/aspnet/AspNetCore/issues/6415
+        public async Task NotMatch_HttpMethod_Returns405Endpoint_ReExecute()
+        {
+            // Arrange
+            var endpoint1 = CreateEndpoint("/hello", httpMethods: new string[] { "GET", "PUT" });
+            var endpoint2 = CreateEndpoint("/hello", httpMethods: new string[] { "DELETE" });
+
+            var matcher = CreateMatcher(endpoint1, endpoint2);
+            var (httpContext, context) = CreateContext("/hello", "POST");
+
+            // Act
+            await matcher.MatchAsync(httpContext, context);
+
+            // Assert
+            Assert.NotSame(endpoint1, context.Endpoint);
+            Assert.NotSame(endpoint2, context.Endpoint);
+
+            Assert.Same(HttpMethodMatcherPolicy.Http405EndpointDisplayName, context.Endpoint.DisplayName);
+
+            // Invoke the endpoint
+            await context.Endpoint.RequestDelegate(httpContext);
+            Assert.Equal(405, httpContext.Response.StatusCode);
+            Assert.Equal("DELETE, GET, PUT", httpContext.Response.Headers["Allow"]);
+
+            // Invoke the endpoint again to verify headers not duplicated
+            await context.Endpoint.RequestDelegate(httpContext);
+            Assert.Equal(405, httpContext.Response.StatusCode);
+            Assert.Equal("DELETE, GET, PUT", httpContext.Response.Headers["Allow"]);
+        }
+
         private static Matcher CreateMatcher(params RouteEndpoint[] endpoints)
         {
             var services = new ServiceCollection()