Better cache invalidation for custom, local generators

Follows https://github.com/thesamet/sbt-protoc/pull/162

How can we accurately fingerprint protoc-bridge local (defined in the meta-build) generators behind PB.targets, to avoid false positive cache hits on local workspaces ~as ScalaPB/sbt-akka-grpc gets bumped for example~?

Looking up a potential serialVersionUID on Generator would allow generators to declare their version, but it leaves that responsibility to the generator author to bump this… ~I can’t think of any other way - maybe something related to JAR timestamps?~

[thesamet]

I recently wrote a guide that tries to consolidate the best practices and all magic involved in writing plugins: https://scalapb.github.io/docs/writing-plugins

It comes with an sbt plugin, as well as a g8 template to get projects set up correctly. This is used in scalapb-validate and zio-grpc.

Specifically, to run plugins to run from a local project, it provides codeGenClassPath and genModule. Currently it re-generates sources on any compile, but the improvement we identified above can go in here.

[thesamet]

I am not sure how far/generic you would like this to be, as I assume this plugin was only meant for e2e testing, not for defining generators in a monorepo?

Thanks for trying this! Right, when writing this plugin I had in mind projects like ScalaPB/zio-grpc/scalapb-validate which are meant to produce a protoc plugin and have one or two e2e projects to verify it. Will gladly accept PRs that improve performance and how classloader caching is done.