Fix self-referential schema handling to close #58006

Author: captainsafia

src/OpenApi/src/Services/Schemas/OpenApiSchemaStore.cs

@@ -159,6 +159,14 @@ private void AddOrUpdateAnyOfSubSchemaByReference(OpenApiSchema schema)
     private void AddOrUpdateSchemaByReference(OpenApiSchema schema, string? baseTypeSchemaId = null, bool captureSchemaByRef = false)
     {
         var targetReferenceId = baseTypeSchemaId is not null ? $"{baseTypeSchemaId}{GetSchemaReferenceId(schema)}" : GetSchemaReferenceId(schema);
+        // Schemas that already have a reference provided by JsonSchemaExporter are skipped here
+        // and handled by the OpenApiSchemaReferenceTransformer instead. This case typically kicks
+        // in for self-referencing schemas where JsonSchemaExporter inlines references to avoid
+        // infinite recursion.
+        if (schema.Reference is not null)
+        {
+            return;
+        }
         if (SchemasByReference.TryGetValue(schema, out var referenceId) || captureSchemaByRef)
         {
             // If we've already used this reference ID else where in the document, increment a counter value to the reference

src/OpenApi/src/Transformers/Implementations/OpenApiSchemaReferenceTransformer.cs

@@ -101,6 +101,16 @@ public Task TransformAsync(OpenApiDocument document, OpenApiDocumentTransformerC
             return new OpenApiSchema { Reference = new OpenApiReference { Type = ReferenceType.Schema, Id = referenceId } };
         }
 
+        // Handle schemas where the references have been inline by the JsonSchemaExporter. In this case,
+        // the `#` ID is generated by the exporter since it has no base document to baseline against. In this
+        // case we we want to replace the reference ID with the schema ID that was generated by the
+        // `CreateSchemaReferenceId` method in the OpenApiSchemaService.
+        if (!isTopLevel && schema.Reference is { Id: "#" }
+            && schema.Annotations.TryGetValue(OpenApiConstants.SchemaId, out var schemaId))
+        {
+            return new OpenApiSchema { Reference = new OpenApiReference { Type = ReferenceType.Schema, Id = schemaId?.ToString() } };
+        }
+
         if (schema.AllOf is not null)
         {
             for (var i = 0; i < schema.AllOf.Count; i++)

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiSchemaService/OpenApiSchemaService.RequestBodySchemas.cs

@@ -659,4 +659,43 @@ private class ExampleWithSkippedUnmappedMembers
     {
         public int Number { get; init; }
     }
+
+    [Fact]
+    public async Task SupportsTypesWithSelfReferencedProperties()
+    {
+        // Arrange
+        var builder = CreateBuilder();
+
+        // Act
+        builder.MapPost("/api", (Parent parent) => { });
+
+        // Assert
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            var operation = document.Paths["/api"].Operations[OperationType.Post];
+            var requestBody = operation.RequestBody;
+            var content = Assert.Single(requestBody.Content);
+            var schema = content.Value.Schema.GetEffective(document);
+            Assert.Collection(schema.Properties,
+                property =>
+                {
+                    Assert.Equal("selfReferenceList", property.Key);
+                    Assert.Equal("array", property.Value.Type);
+                    Assert.Equal("Parent", property.Value.Items.Reference.Id);
+                },
+                property =>
+                {
+                    Assert.Equal("selfReferenceDictionary", property.Key);
+                    Assert.Equal("object", property.Value.Type);
+                    Assert.Equal("Parent", property.Value.AdditionalProperties.Reference.Id);
+                });
+        });
+    }
+
+    public class Parent
+    {
+        public IEnumerable<Parent> SelfReferenceList { get; set; } = [ ];
+        public IDictionary<string, Parent> SelfReferenceDictionary { get; set; } = new Dictionary<string, Parent>();
+    }
+
 }