OpenAPI: Fix Circular reference in specific order gives empty schema (#63511)

* Add circular schema example

* Fix empty schema in certain orderings of schemas.

This is fixed by not adding/registering a schema when it contains a reference, as it is a incomplete/unresolved schema

* Revert launchsettings.json

* Add unit tests for circular references and remove it from the sample

* Move tests to OpenApiSchemaReferenceTransformerTests

* Fix NRT warnings

Author: desjoerd

src/OpenApi/src/Services/Schemas/OpenApiSchemaService.cs

@@ -337,7 +337,7 @@ internal static IOpenApiSchema ResolveReferenceForSchema(OpenApiDocument documen
             if (schema.Metadata.TryGetValue(OpenApiConstants.SchemaId, out var schemaId) &&
                 schemaId is string schemaIdString)
             {
-                return document.AddOpenApiSchemaByReference(schemaIdString, schema);
+                return new OpenApiSchemaReference(schemaIdString, document);
             }
             var relativeSchemaId = $"#/components/schemas/{rootSchemaId}{refIdString.Replace("#", string.Empty)}";
             return new OpenApiSchemaReference(relativeSchemaId, document);

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Transformers/Implementations/OpenApiSchemaReferenceTransformerTests.cs

@@ -1004,6 +1004,124 @@ await VerifyOpenApiDocument(builder, document =>
         });
     }
 
+    // Test for: https://github.com/dotnet/aspnetcore/issues/63503
+    [Fact]
+    public async Task HandlesCircularReferencesRegardlessOfPropertyOrder_SelfFirst()
+    {
+        var builder = CreateBuilder();
+        builder.MapPost("/", (DirectCircularModelSelfFirst dto) => { });
+
+        // Assert
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            Assert.NotNull(document.Components?.Schemas);
+            var schema = document.Components.Schemas["DirectCircularModelSelfFirst"];
+            Assert.Equal(JsonSchemaType.Object, schema.Type);
+            Assert.NotNull(schema.Properties);
+            Assert.Collection(schema.Properties,
+                property =>
+                {
+                    Assert.Equal("self", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                    Assert.Equal("#/components/schemas/DirectCircularModelSelfFirst", reference.Reference.ReferenceV3);
+                },
+                property =>
+                {
+                    Assert.Equal("referenced", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                });
+
+            // Verify that it does not result in an empty schema for a referenced schema
+            var referencedSchema = document.Components.Schemas["ReferencedModel"];
+            Assert.NotNull(referencedSchema.Properties);
+            Assert.NotEmpty(referencedSchema.Properties);
+            var idProperty = Assert.Single(referencedSchema.Properties);
+            Assert.Equal("id", idProperty.Key);
+            var idPropertySchema = Assert.IsType<OpenApiSchema>(idProperty.Value);
+            Assert.Equal(JsonSchemaType.Integer, idPropertySchema.Type);
+        });
+    }
+
+    // Test for: https://github.com/dotnet/aspnetcore/issues/63503
+    [Fact]
+    public async Task HandlesCircularReferencesRegardlessOfPropertyOrder_SelfLast()
+    {
+        var builder = CreateBuilder();
+        builder.MapPost("/", (DirectCircularModelSelfLast dto) => { });
+
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            Assert.NotNull(document.Components?.Schemas);
+            var schema = document.Components.Schemas["DirectCircularModelSelfLast"];
+            Assert.Equal(JsonSchemaType.Object, schema.Type);
+            Assert.NotNull(schema.Properties);
+            Assert.Collection(schema.Properties,
+                property =>
+                {
+                    Assert.Equal("referenced", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                },
+                property =>
+                {
+                    Assert.Equal("self", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                    Assert.Equal("#/components/schemas/DirectCircularModelSelfLast", reference.Reference.ReferenceV3);
+                });
+
+            // Verify that it does not result in an empty schema for a referenced schema
+            var referencedSchema = document.Components.Schemas["ReferencedModel"];
+            Assert.NotNull(referencedSchema.Properties);
+            Assert.NotEmpty(referencedSchema.Properties);
+            var idProperty = Assert.Single(referencedSchema.Properties);
+            Assert.Equal("id", idProperty.Key);
+            var idPropertySchema = Assert.IsType<OpenApiSchema>(idProperty.Value);
+            Assert.Equal(JsonSchemaType.Integer, idPropertySchema.Type);
+        });
+    }
+
+    // Test for: https://github.com/dotnet/aspnetcore/issues/63503
+    [Fact]
+    public async Task HandlesCircularReferencesRegardlessOfPropertyOrder_MultipleSelf()
+    {
+        var builder = CreateBuilder();
+        builder.MapPost("/", (DirectCircularModelMultiple dto) => { });
+
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            Assert.NotNull(document.Components?.Schemas);
+            var schema = document.Components.Schemas["DirectCircularModelMultiple"];
+            Assert.Equal(JsonSchemaType.Object, schema.Type);
+            Assert.NotNull(schema.Properties);
+            Assert.Collection(schema.Properties,
+                property =>
+                {
+                    Assert.Equal("selfFirst", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                    Assert.Equal("#/components/schemas/DirectCircularModelMultiple", reference.Reference.ReferenceV3);
+                },
+                property =>
+                {
+                    Assert.Equal("referenced", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                },
+                property =>
+                {
+                    Assert.Equal("selfLast", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                    Assert.Equal("#/components/schemas/DirectCircularModelMultiple", reference.Reference.ReferenceV3);
+                });
+
+            // Verify that it does not result in an empty schema for a referenced schema
+            var referencedSchema = document.Components.Schemas["ReferencedModel"];
+            Assert.NotNull(referencedSchema.Properties);
+            Assert.NotEmpty(referencedSchema.Properties);
+            var idProperty = Assert.Single(referencedSchema.Properties);
+            Assert.Equal("id", idProperty.Key);
+            var idPropertySchema = Assert.IsType<OpenApiSchema>(idProperty.Value);
+            Assert.Equal(JsonSchemaType.Integer, idPropertySchema.Type);
+        });
+    }
+
     // Test models for issue 61194
     private class Config
     {
@@ -1060,5 +1178,30 @@ public sealed class RefUser
         public string Name { get; set; } = "";
         public string Email { get; set; } = "";
     }
+
+    // Test models for issue 63503
+    private class DirectCircularModelSelfFirst
+    {
+        public DirectCircularModelSelfFirst Self { get; set; } = null!;
+        public ReferencedModel Referenced { get; set; } = null!;
+    }
+
+    private class DirectCircularModelSelfLast
+    {
+        public ReferencedModel Referenced { get; set; } = null!;
+        public DirectCircularModelSelfLast Self { get; set; } = null!;
+    }
+
+    private class DirectCircularModelMultiple
+    {
+        public DirectCircularModelMultiple SelfFirst { get; set; } = null!;
+        public ReferencedModel Referenced { get; set; } = null!;
+        public DirectCircularModelMultiple SelfLast { get; set; } = null!;
+    }
+
+    private class ReferencedModel
+    {
+        public int Id { get; set; }
+    }
 }
 #nullable restore