[BUG][CLI] crash when evaluating definitions without parameters

[codymikol]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When you provide openapi-generator an openapi definition that includes a url with path variables, but no defined parameters, it will throw a null pointer exception.

Exception in thread "main" java.lang.NullPointerException
        at java.base/java.util.HashSet.<init>(HashSet.java:120)
        at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:464)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:507)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:423)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:61)

openapi-generator version

5.0.0-SNAPSHOT

OpenAPI declaration file content or url

https://github.com/codymikol/openapi-generator-kotlin-bug-demo/blob/master/generated-openapi-spec.json

Generation Details

This seems to happen regardless of generator-name, and the only other CLI options I am providing are the input-spec and output.

Steps to reproduce

I have created a minimal repro repository over here
this includes a shell script with all of the exact steps that I took. The generated openapi spec is hardcoded in the repo, but I have also included a minimal Spring project that generates the spec via springfox 3 if you'd like to generate that yourself.

Note that the script uses the latest published openapi-generator and will need some tweaking to point at your own build. I have tested with both and they have the same result

Related issues/PRs

I did a search, but found nothing similar.

Suggest a fix

I plan on looking into this myself, this is mostly to help document the issue in case I end up not having enough time. Any suggestions would be great.

[codymikol]

I'll leave this for my findings while digging into this

It looks like when this is evaluated, the result.messages property is null and this is what causes the NPE.

SwaggerParseResult result = new OpenAPIParser().readLocation(inputSpec, authorizationValues, options);

[codymikol]

The root cause of this issue seems to come from swagger-parser. It will return null for messages and throw a NPE. I think what we can do is add some decent error handling for when this kind of error does occur.

[codymikol]

I have added a PR that will not fix this, but will make it much more obvious what has gone wrong.