Pants may not implement REv2.1 and REv2.2, meaning it is no longer compatible with Buildbarn after 2026-04-01

[EdSchouten]

Hi there,

Not long after REv2.0 was released, some minor changes were made to the protocol:

• Command.output_{files,directories} was replaced with Command.output_paths.
• Platform properties were promoted from the Command message to the Action. This allows the scheduler to route actions without loading a potentially big Command message from storage.

Given that these changes were made ages ago (2019 and 2020, respectively), I am planning on merging a change on the Buildbarn side on 2026-04-01 to remove support for these features entirely. After this change lands, Buildbarn will announce the following version numbers:

deprecated_api_version: v2.2.0
low_api_version: v2.3.0
high_api_version: v2.11.0

Looking at the Pants source code, it seems that it still uses the old schemes for specifying output paths and platform properties. In order to guarantee compatibility with Buildbarn after 2026-04-01, I would recommend implementing these newer features.

[spider-yamet]

Hi @EdSchouten Hope you are doing great.
May I pick up this issue? I'd love to work on this issue if it is open for contributors.
Looking forward to your response.
cc: @eric @nitay @gkossakowski

Best Regards

[benjyw]

See #23026