Description

When APICURIO_RULES_GLOBAL_COMPATIBILITY: FORWARD is enabled, registering new Protobuf schema versions via Confluent-compatible API (/apis/ccompat/v7) fails with InternalServerError 0 Syntax error in :1:1: unexpected label: CiV.... The error occurs because Confluent .NET Protobuf serializer registers schemas as base64-encoded FileDescriptorSet instead of .proto text, and the compatibility check parser expects .proto source text.

This appears related to #2440, but occurs specifically when global compatibility rules are enabled.

Environment

• Apicurio Registry version: 3.0.9 (using apicurio/apicurio-registry:3.0.9)
• API: Confluent-compatible /apis/ccompat/v7
• Client: Confluent.SchemaRegistry.Serdes.Protobuf (.NET) via KafkaFlow
• Configuration:

  APICURIO_RULES_GLOBAL_COMPATIBILITY: FORWARD
  KAFKA_BOOTSTRAP_SERVERS: kafka:29092

Steps to Reproduce

1. Configure Apicurio Registry with:

   environment:
     APICURIO_RULES_GLOBAL_COMPATIBILITY: FORWARD

2. Register first Protobuf schema version using Confluent .NET client:

   middlewares.AddSchemaRegistryProtobufSerializer(
       new ProtobufSerializerConfig { 
           SubjectNameStrategy = SubjectNameStrategy.TopicRecord 
       }
   );

3. First version registers successfully.

4. Modify the .proto file (e.g., add a field):

   message SomeMessage {
       // ... existing fields
       bool NewField = 15;  // new field
   }

5. Attempt to register the updated schema (produce a message with the new schema).

Expected vs Actual Behaviour

Expected

• Compatibility check should succeed (adding a new optional field in Proto3 is forward-compatible).
• New schema version should be registered successfully.

Actual

• Registration fails with:

  InternalServerError 0 Syntax error in :1:1: unexpected label: CiVLYWZrYS9Qcm90b2J1Zi9jaGFyZ2luZy1zZXNzaW9uLnByb3RvEhJBdG9tLkVtc3AuU2Vzc2lvbnM...
  

• The string starting with CiV... is a base64-encoded FileDescriptorSet.

Logs

System.Net.Http.HttpRequestException: [http://apicurio-registry:8080/apis/ccompat/v7/] InternalServerError 0 Syntax error in :1:1: unexpected label: CiVLYWZrYS9Qcm90b2J1Zi9jaGFyZ2luZy1zZXNzaW9uLnByb3RvE*
   at Confluent.SchemaRegistry.RestService.ExecuteOnOneInstanceAsync(Func`1 createRequest)
   at Confluent.SchemaRegistry.RestService.RequestAsync[T](String endPoint, HttpMethod method, Object[] jsonBody)
   at Confluent.SchemaRegistry.RestService.RegisterSchemaWithResponseAsync(String subject, Schema schema, Boolean normalize)
   at Confluent.Shared.CollectionUtils.ConcurrentDictionaryExtensions.GetOrAddAsync[TKey,TValue](ConcurrentDictionary`2 dictionary, TKey key, Func`2 valueFactory)
   at Confluent.SchemaRegistry.CachedSchemaRegistryClient.RegisterSchemaWithResponseAsync(String subject, Schema schema, Boolean normalize)
   at Confluent.SchemaRegistry.Serdes.ProtobufSerializer`1.SerializeAsync(T value, SerializationContext context)
   at KafkaFlow.ConfluentSerializerWrapper.InnerConfluentSerializerWrapper`1.SerializeAsync(Object message, Stream output, ISerializerContext context)
   at KafkaFlow.Middlewares.Serializer.SerializerProducerMiddleware.Invoke(IMessageContext context, MiddlewareDelegate next)
   at KafkaFlow.Producers.MessageProducer.ProduceAsync(String topic, Object messageKey, Object messageValue, IMessageHeaders headers, Nullable`1 partition)
   at KafkaFlow.Producers.MessageProducer.ProduceAsync(String topic, Object messageKey, Object messageValue, IMessageHeaders headers, Nullable`1 partition)
   at MediatR.NotificationPublishers.ForeachAwaitPublisher.Publish(IEnumerable`1 handlerExecutors, INotification notification, CancellationToken cancellationToken)