[feature] Skip toolchain installation when system already has the matching version

[moltar]

Is your feature request related to a problem? Please describe.

When running moon docker setup inside a Docker container that already has the exact Node.js version Moon is configured to use (e.g. using an official node:20 base image), Moon still downloads and installs Node.js through proto. This is redundant — it slows down Docker builds and adds ~200 MB of bloat to the image since Node.js ends up installed twice.

The current workaround is MOON_TOOLCHAIN_FORCE_GLOBALS=node, but this is all-or-nothing — it unconditionally skips installation with no fallback. If the system binary is missing or the wrong version, commands will fail at runtime.

Describe the solution you'd like

Before downloading/installing a toolchain, Moon should check if a matching version already exists on the system PATH. If the system binary satisfies the configured version requirement, skip the proto installation and use the system binary directly.

This could work as a new option, for example:

# .moon/toolchain.yml
node:
  version: "20.0.0"
  preferSystemBinary: true  # Use system node if it matches the version

Or it could be an additional mode for MOON_TOOLCHAIN_FORCE_GLOBALS, e.g. MOON_TOOLCHAIN_FORCE_GLOBALS=node:prefer meaning "use system if available and matching, otherwise install."

Describe alternatives you've considered

• MOON_TOOLCHAIN_FORCE_GLOBALS=node — works but has no fallback if the system binary is missing or wrong version. It also doesn't verify the version matches.
• Pre-installing the version in proto's cache before running moon docker setup — adds unnecessary complexity to the Dockerfile.

Additional context

This is especially relevant for Docker workflows where base images already bundle the correct runtime. Avoiding a redundant download would improve build times and reduce image layer size.

[milesj]

The problem with this is that every tool has a different way of printing the version, which means we have to either:

a) brute force it by continually spawning child processes till we figure out which arguments are correct
b) keep a map internally of every tool and the arguments required to get the version

Both of which are not ideal. This doesn't even take into account the actual stdout that is printed, which can contain more than the version, so we would have to parse it correctly. Parsing the git version has already turned into a massive pain.

[moltar]

I agree with the problem described. Maybe it can be specified in the config which versions are already installed globally. But I understand that it's quite a corner case. If you don't think this is useful feel free to close. Thank you for looking into this.