Extend SimulateSystem to MacOS.version

[apainintheneck]

Verification

• This issue's title and/or description do not reference a single formula e.g. brew install wget. If they do, open an issue at https://github.com/Homebrew/homebrew-core/issues/new/choose instead.

Provide a detailed description of the proposed feature

Currently, we use SimulateSystem in a few places to allow us to evaluate on_system blocks without having to change physical machines. This would extend that to allow us to simulate the system version. The difficult thing would be adding this in in such a way that it doesn't break current installs/builds when people are simulating things (using environment variables for instance).

What is the motivation for the feature?

This is needed to easily and accurately evaluate if a cask/formula runs on a specific OS. We could use this information to exclude unsupported OS versions from audits and the like.

Recent discussion in:

• calhash, deeper, maintenance, onyx: use array for allowed macOS releases homebrew-cask#154556 (comment)
• Handle nil cask urls caused by unsupported macOS version #15943 (comment)
• Improve cask audit #15977 (comment)

How will the feature be relevant to at least 90% of Homebrew users?

It would potentially reduce CI time somewhat since audits take less time and some command output would be more relevant since it could skip over unsupported casks/formulae. I haven't looked at CI though so I'm not sure if it will be relevant there.

What alternatives to the feature have been considered?

Option 1

We could mandate not defining depends_on macos: inside on_system blocks and only define them in the base level of the cask. This would require a new linter rule but then we wouldn't need to simulate anything to get the macOS version requirement for a cask.

For example, Homebrew/homebrew-cask#154556 changed some PRs to use the version array syntax which while being slightly more verbose ends up allowing us to tell the supported OS versions without simulating anything.

depends_on macos: [
  :catalina,
  :big_sur,
]

Option 2

We could rewrite the on_system DSL to automatically update a global MacOSRequirement for each on_system stanza. This would be a bit magical though and would require changing a large number of casks along with adding new linter rules but would just work without forcing people to think about where to put the depends_on macos: rules.

[apainintheneck]

Does anyone know if this is also relevant on the formula side?

[MikeMcQuaid]

Thanks for the write-up @apainintheneck! I agree this is needed.

My preference would be Option 2. I don't really like the DSL usage on Homebrew/homebrew-cask#154556

Does anyone know if this is also relevant on the formula side?

@apainintheneck Unsure but I wouldn't be surprised either way. Sorry I'm not more help 😁.

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