fix: throw on circular reference

src/Microsoft.OpenApi/Services/OpenApiWorkspace.cs

@@ -81,7 +81,7 @@
         /// Registers a document's components into the workspace
         /// </summary>
         /// <param name="document"></param>
         public void RegisterComponents(OpenApiDocument document)
         {
             if (document?.Components == null) return;
 
@@ -268,7 +268,7 @@
         /// <param name="value"></param>
         public void AddDocumentId(string? key, Uri? value)
         {
             if (!string.IsNullOrEmpty(key) && key is not null && value is not null && !_documentsIdRegistry.ContainsKey(key))
             {
                 _documentsIdRegistry[key] = value;
             }
@@ -371,12 +371,15 @@
             return default;          
         }
 
         internal static IOpenApiSchema? ResolveSubSchema(IOpenApiSchema schema, string[] pathSegments, Stack<IOpenApiSchema> visitedSchemas)
         {
             // Prevent infinite recursion in case of circular references
             if (visitedSchemas.Contains(schema))
             {
-                return null;
+                if (schema is OpenApiSchemaReference openApiSchemaReference)
+                    throw new InvalidOperationException($"Circular reference detected while resolving schema: {openApiSchemaReference.Reference.ReferenceV3}");
+                else
+                    throw new InvalidOperationException($"Circular reference detected while resolving schema");
             }
             visitedSchemas.Push(schema);
             // Traverse schema object to resolve subschemas

test/Microsoft.OpenApi.Readers.Tests/V31Tests/RelativeReferenceTests.cs

@@ -502,9 +502,9 @@ public void ExitsEarlyOnCyclicalReferences()
             document.RegisterComponents();
 
             var tagsSchemaRef = Assert.IsType<OpenApiSchemaReference>(categorySchema.Properties["tags"]);
-            Assert.Null(tagsSchemaRef.Items);
+            Assert.Throws<InvalidOperationException>(() => tagsSchemaRef.Items);
             Assert.Equal("#/components/schemas/Category/properties/parent/properties/tags", tagsSchemaRef.Reference.ReferenceV3);
-            Assert.Null(tagsSchemaRef.Target);
+            Assert.Throws<InvalidOperationException>(() => tagsSchemaRef.Target);
 
             var parentSchemaRef = Assert.IsType<OpenApiSchemaReference>(categorySchema.Properties["parent"]);
             Assert.Equal("#/components/schemas/Category", parentSchemaRef.Reference.ReferenceV3);