Respect XDG directory settings on macOS

[aaronsky]

Description of the feature request:

Per #16937, while the XDG Base Directory spec is more commonly adopted on Linux, there's little I'm aware of in the way of supporting it on macOS for those users who set the environment variables. Is this something that can be added as a secondary option for that platform, or is there something special about /private/var/tmp as an output root?

Which category does this issue belong to?

Core

What underlying problem are you trying to solve with this feature?

A similar issue as interrogated with #16937, but for macOS instead of just Linux.

Which operating system are you running Bazel on?

macOS 15.3

What is the output of bazel info release?

release 8.0.1

[meteorcloudy]

/cc @tetromino any idea on this one?

[aaronsky]

Following up on one of the questions up top about whether there's anything special about /private/var/tmp for Bazel on macOS, I noticed that updating the OS causes this location to be blown away. I would understand if this location was chosen with this or a related fact in mind. If this is a coincidence or unimportant, I'd love to have this option on macOS (particularly because environment variables are not interpolated in .bazelrc as far as I'm aware, so I have to hardcode the path in the absence of this).

[meteorcloudy]

See https://chatgpt.com/share/67a33ebf-0508-8013-ac91-478bb2ca0563

Have you tried --output_base and --output_user_root to set Bazel's output paths? See also https://bazel.build/remote/output-directories#layout

[aaronsky]

Yes, but as I said, I can't set startup --output_user_root=${XDG_CACHE_DIR}/bazel in my $HOME/.bazelrc because environment variables are not interpolated in bazelrc. I have to hardcode it to a known fixed value, in this case startup --output_user_root=~/.cache/bazel. It would be nice for macOS to get the same environment-respecting feature that Linux has, if the environment variable is set – even if XDG adoption isn't as widespread on macOS as it is Linux, it's both valid to do so and not uncommon.

[meteorcloudy]

OK, I cannot think of any reason to forbidden this, are you willing to contribute a PR to Bazel to add this support?

[aaronsky]

Yeah, I'm willing to try! I was actually just in the middle of standing up a dev environment for #25165, which is the higher priority at the $dayjob.