[Configuration cache] Using plugin in a Java project with configuration cache enabled gives unexpected warning

[jjohannes]

This is a follow up issue to #408

If you apply the latest version of the protobuf plugin to a pure Java project like this (latest Gradle 6.6 nightly):

plugins {
  id 'java-library'
  id 'com.google.protobuf' version '0.8.13-SNAPSHOT'
}

And use the configurations cache

./gradlew assemble --configuration-cache

You'll get the following configuration cache warning:

value provider(?) failed to unpack provider
  fieldvariantName of com.google.protobuf.gradle.GenerateProtoTask
    task:Server-Protocol-Buffer:generateProto of type com.google.protobuf.gradle.GenerateProtoTask
      java.lang.IllegalStateException: variant should not be used in a Java project

This is because Gradle tries to read the variantName field which is backed by the variant property. The getter of that property then throws the "variant should not be used in a Java project" error.

I think this can be fixed by making variantName transient similar to how it was done for the backing variant field in #408. //cc @gavra0

[voidzcy]

Are you interested in creating a PR for the fix?