OpenAPI Extensions no longer work

[kjc14f]

Describe the bug
It seems that the ability to add Extesntions to the OpenAPI object no longer works.
Previously you could provide .extensions() a Map<String, Object> which would reflect in the OpenAPI JSON.
Our application relied heavily on these custom extensions which is how I noticed the issue initially.

After trying several different versions, this functionality appears to have broken starting version 1.6.2 and still appears to be presnt in the latest version 2.0.2.

To Reproduce
Steps to reproduce the behavior:

Spring Boot : 2.7.9 (has to be 2.x.x so you can switch between Springdoc 1.6.1 and 1.6.2 easily)
JDK: 17.0.4
OS: Tested on Window 11 + RHEL8
Gradle 7.6.1

Example project to reproduce the bug: https://github.com/kjc14f/springdoc-test

Create a very simple Config class that sets an extremely simple extension(s):

@Configuration
public class SwaggerConfig {

  @Bean
  public OpenAPI customOpenAPI() {
    return new OpenAPI()
        .extensions(Map.of("TEST", "HELLO"));
  }

}

In build.gradle add implementation 'org.springdoc:springdoc-openapi-ui:1.6.1' as a dependency.

Run the app and view http://localhost:8080/v3/api-docs, you will notice "TEST": "HELLO" which is correct.

Version 1.6.1 working:

{
    "openapi": "3.0.1",
    "info": {
        "title": "OpenAPI definition",
        "version": "v0"
    },
    "servers": [
        {
            "url": "http://localhost:8080",
            "description": "Generated server url"
        }
    ],
    "paths": {},
    "components": {},
    "TEST": "HELLO"
}

Stop the app, switch the import to org.springdoc:springdoc-openapi-ui:1.6.2 and restart the app.
You will now see the extension "TEST": "HELLO" disappear.

Version >= 1.6.2 broken:

{
    "openapi": "3.0.1",
    "info": {
        "title": "OpenAPI definition",
        "version": "v0"
    },
    "servers": [
        {
            "url": "http://localhost:8080",
            "description": "Generated server url"
        }
    ],
    "paths": {},
    "components": {}
}

Expected behavior

OpenAPI JSON should show the extensions set on the OpenAPI object on all versions of Springdoc.
In the above example "TEST": "HELLO"

[kjc14f]

I did some more investigating and it looks like the breaking change was introduced here: https://github.com/springdoc/springdoc-openapi/blob/master/springdoc-openapi-common/src/main/java/org/springdoc/core/OpenAPIService.java#L251
calculatedOpenAPI = objectMapper.readValue(objectMapper.writeValueAsString(openAPI), OpenAPI.class);

Previously it was simply doing calculatedOpenAPI = openAPI which worked fine.
I guess this was changed to the above as a deep copy of the openAPI object was required.
Serializing to/from JSON does not work properly for this class though as the extensions are of type Map<String, Object> so they don't desrialize to a field as the deserializer is looking for the literal word extensions in the JSON at the moment.

It looks like bit of an awkward one as the OpenAPI class comes from another library and it is not Serializable which limits options!

[ezoerner]

Unfortunately extensions are broken in deeper components inside OpenAPI as well. Extensions added to info are also lost. This fix only fixed extensions at the top level on OpenAPI.

[ezoerner]

Using v1.6.15, but I don't see any issue open for this that would have been addressed for a more recent version of SpringDoc. Please re-open this issue or create a new one to address the fact that extensions also don't work for the Info that is in the OpenAPI object (and possible other subobjects of OpenAPI).

UPDATE: I opened a new issue for this #2449