Update HandleLogoutRequestContext to allow attaching custom sign-out parameters

src/OpenIddict.Server/OpenIddictServerEvents.Session.cs

@@ -5,6 +5,7 @@
  */
 
 using System;
+using System.Collections.Generic;
 using OpenIddict.Abstractions;
 using SR = OpenIddict.Abstractions.OpenIddictResources;
 
@@ -115,10 +116,26 @@ namespace OpenIddict.Server
             /// </summary>
             public bool IsSignOutTriggered { get; private set; }
 
+            /// <summary>
+            /// Gets the additional parameters returned to the client application.
+            /// </summary>
+            public Dictionary<string, OpenIddictParameter> Parameters { get; private set; }
+                = new(StringComparer.Ordinal);
+
             /// <summary>
             /// Allows OpenIddict to return a sign-out response.
             /// </summary>
             public void SignOut() => IsSignOutTriggered = true;
+
+            /// <summary>
+            /// Allows OpenIddict to return a sign-out response.
+            /// </summary>
+            /// <param name="parameters">The additional parameters returned to the client application.</param>
+            public void SignOut(IDictionary<string, OpenIddictParameter> parameters)
+            {
+                IsSignOutTriggered = true;
+                Parameters = new(parameters, StringComparer.Ordinal);
+            }
         }
 
         /// <summary>

src/OpenIddict.Server/OpenIddictServerEvents.cs

@@ -733,6 +733,11 @@ namespace OpenIddict.Server
                 get => Transaction.Response!;
                 set => Transaction.Response = value;
             }
+
+            /// <summary>
+            /// Gets the additional parameters returned to the client application.
+            /// </summary>
+            public Dictionary<string, OpenIddictParameter> Parameters { get; } = new(StringComparer.Ordinal);
         }
     }
 }

src/OpenIddict.Server/OpenIddictServerHandlers.Session.cs

@@ -227,6 +227,14 @@ namespace OpenIddict.Server
                             Response = new OpenIddictResponse()
                         };
 
+                        if (notification.Parameters.Count > 0)
+                        {
+                            foreach (var parameter in notification.Parameters)
+                            {
+                                @event.Parameters.Add(parameter.Key, parameter.Value);
+                            }
+                        }
+
                         await _dispatcher.DispatchAsync(@event);
 
                         if (@event.IsRequestHandled)

src/OpenIddict.Server/OpenIddictServerHandlers.cs

@@ -82,12 +82,13 @@ namespace OpenIddict.Server
             GenerateUserCode.Descriptor,
             GenerateIdentityToken.Descriptor,
 
-            AttachTokenParameters.Descriptor,
+            AttachSignInParameters.Descriptor,
 
             /*
              * Sign-out processing:
              */
-            ValidateSignOutDemand.Descriptor)
+            ValidateSignOutDemand.Descriptor,
+            AttachSignOutParameters.Descriptor)
 
             .AddRange(Authentication.DefaultHandlers)
             .AddRange(Device.DefaultHandlers)
@@ -2832,16 +2833,16 @@ namespace OpenIddict.Server
         }
 
         /// <summary>
-        /// Contains the logic responsible of attaching the tokens and their metadata to the sign-in response.
+        /// Contains the logic responsible of attaching the appropriate parameters to the sign-in response.
         /// </summary>
-        public class AttachTokenParameters : IOpenIddictServerHandler<ProcessSignInContext>
+        public class AttachSignInParameters : IOpenIddictServerHandler<ProcessSignInContext>
         {
             /// <summary>
             /// Gets the default descriptor definition assigned to this handler.
             /// </summary>
             public static OpenIddictServerHandlerDescriptor Descriptor { get; }
                 = OpenIddictServerHandlerDescriptor.CreateBuilder<ProcessSignInContext>()
-                    .UseSingletonHandler<AttachTokenParameters>()
+                    .UseSingletonHandler<AttachSignInParameters>()
                     .SetOrder(GenerateIdentityToken.Descriptor.Order + 1_000)
                     .SetType(OpenIddictServerHandlerType.BuiltIn)
                     .Build();
@@ -3009,5 +3010,40 @@ namespace OpenIddict.Server
                 return default;
             }
         }
+
+        /// <summary>
+        /// Contains the logic responsible of attaching the appropriate parameters to the sign-out response.
+        /// </summary>
+        public class AttachSignOutParameters : IOpenIddictServerHandler<ProcessSignOutContext>
+        {
+            /// <summary>
+            /// Gets the default descriptor definition assigned to this handler.
+            /// </summary>
+            public static OpenIddictServerHandlerDescriptor Descriptor { get; }
+                = OpenIddictServerHandlerDescriptor.CreateBuilder<ProcessSignOutContext>()
+                    .UseSingletonHandler<AttachSignOutParameters>()
+                    .SetOrder(ValidateSignOutDemand.Descriptor.Order + 1_000)
+                    .SetType(OpenIddictServerHandlerType.BuiltIn)
+                    .Build();
+
+            /// <inheritdoc/>
+            public ValueTask HandleAsync(ProcessSignOutContext context)
+            {
+                if (context is null)
+                {
+                    throw new ArgumentNullException(nameof(context));
+                }
+
+                if (context.Parameters.Count > 0)
+                {
+                    foreach (var parameter in context.Parameters)
+                    {
+                        context.Response.SetParameter(parameter.Key, parameter.Value);
+                    }
+                }
+
+                return default;
+            }
+        }
     }
 }

test/OpenIddict.Server.IntegrationTests/OpenIddictServerIntegrationTests.Session.cs

@@ -478,6 +478,43 @@ namespace OpenIddict.Server.IntegrationTests
             Assert.Equal("Bob le Magnifique", (string?) response["name"]);
         }
 
+        [Fact]
+        public async Task HandleLogoutResponse_ResponseContainsCustomParameters()
+        {
+            // Arrange
+            await using var server = await CreateServerAsync(options =>
+            {
+                options.EnableDegradedMode();
+
+                options.AddEventHandler<HandleLogoutRequestContext>(builder =>
+                    builder.UseInlineHandler(context =>
+                    {
+                        context.SignOut();
+
+                        context.Parameters["custom_parameter"] = "custom_value";
+                        context.Parameters["parameter_with_multiple_values"] = new[]
+                        {
+                            "custom_value_1",
+                            "custom_value_2"
+                        };
+
+                        return default;
+                    }));
+            });
+
+            await using var client = await server.CreateClientAsync();
+
+            // Act
+            var response = await client.PostAsync("/connect/logout", new OpenIddictRequest
+            {
+                PostLogoutRedirectUri = "http://www.fabrikam.com/path"
+            });
+
+            // Assert
+            Assert.Equal("custom_value", (string?) response["custom_parameter"]);
+            Assert.Equal(new[] { "custom_value_1", "custom_value_2" }, (string[]?) response["parameter_with_multiple_values"]);
+        }
+
         [Fact]
         public async Task ApplyLogoutResponse_AllowsHandlingResponse()
         {