swagger-codegen-maven-plugin: generateApis not always honored

Description

I have a multi-module project that uses swagger-codegen-maven-plugin to generate Java stubs. In one of the modules (module A), we don't have generateApis set (which should mean that it is treated as true); for the other (module B), we have it set to false.

Usually, this works, but occasionally we don't get the API classes generated (just the model classes) in module A or else we do get the API classes generated in module B.

If it matters, we are using multithreaded builds.

Swagger-codegen version

2.2.3, 2.3.1.

Swagger declaration file content or url

Proprietary code, I haven't generated an isolated test case yet.

Here's a gist with the swagger configuration in the two poms:

https://gist.github.com/davidcarltonsumo/a941f184abaa017928fa3f298645cb9f

Command line used for generation

Steps to reproduce

I don't have a test case that reproduces it at all reliably.

Related issues/PRs

Issue #6260 might be relevant.

Suggest a fix/enhancement

Reading through the code, my only current hypothesis is that the use of system properties ("api" in this case) to pass configuration information might be relevant - if both modules are executing within the same JVM, then perhaps the value of the property is bleeding over from one execution to the other? I don't understand the swagger-codegen code well enough to understand why system properties are being used here instead of having CodeGenMojo pass the configuration to DefaultGenerator via some other method (e.g. a constructor argument).

Start by reading CodeGenMojo and DefaultGenerator, focusing on how the "api" system property carries generateApis between plugin executions. Compare this with issue #6260 and reproduce the behavior in a multithreaded multi-module Maven build. Done means module A consistently generates APIs by default while module B consistently honors generateApis=false.