vague timeline for the v2 jvm backend

[cosmicexplorer]

Prototyped

#8760 has implementations of all of the below, which should be split out into separate PRs:

• (expose a v2 ruleset for BinaryToolBase #8859) Make BinaryToolBase consumable in v2.
• (JVM Bootstrap in the v2 engine #6856) Enabling the resolution of 3rdparty jars via bootstrapping coursier.
  • Make the coursier jar into a pants.binaries.binary_tool.Script.
  • Make a v2 ruleset which can invoke the coursier jar with (almost, if not) all of the arguments coursier is currently invoked with in v1.
• Make JvmToolMixin consumable in v2 via invoking coursier to resolve the classpath for the tool.
• Make a ruleset to resolve the thrift jar as a JvmToolMixin, and then to invoke thrift on a set of targets.
  • These might be two separate changes.
• Convert the export task to v2.
  • This isn't quite finished in ./pants --v2 export-v2 [HACKY PROTOTYPE -- NOT FOR MERGING] #8760, but I believe the hard part of transferring code from the v1 task has been completed. It is a lot of code at once to move, so this may be done over multiple PRs.

Not Yet Prototyped

These goals have not yet been prototyped:

• compile
• export-classpath

Related Work

• https://github.com/pantsbuild/pants/projects/13 -- the project covering the use of the native-image tool for zinc and rsc.

Needs Discussion

There is also an experimental but I believe correct and useful setup in #8760 which allows v2 rules to use v1 targets mostly verbatim (which would be a very different approach than we've previously been considering for #4535).

• Split out this change and describe the reasons why it might be useful (because we then don't have to change anything about the Target API).
• Merge this to unblock thrift gen and export above, which require this approach or something equivalent to work.

[stuhood]

Pants v2 now has alpha-quality support for the JVM: https://blog.pantsbuild.org/pants-2-9/ ! Further work can be tracked in https://github.com/pantsbuild/pants/projects/22?fullscreen=true