Define OS::Mac on Linux

[sjackman]

Define OS::Mac on Linux for formula API compatibility.

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Define MacOS.version, MacOS.full_version, and MacOS::Xcode.version to Version::NULL on Linux so that brew readall succeeds and Homebrew/brew can tap Homebrew/core on Linux.

[maxim-belkin]

Hmmm... wouldn't it be better to define two new methods instead, i.e. olderthan? and newerthan? so that, as an example

MacOS.version <= :snow_leopard

becomes

MacOS.version.older_than? :snow_leopard

And make both of the methods return false on Linux... Otherwise, the above comparison will be returning true on Linux

[maxim-belkin]

$ uname -s && brew irb <<< "Version::NULL < :snow_leopard"
Linux
==> Interactive Homebrew Shell
Example commands available with: brew irb --examples
Switch to inspect mode.
Version::NULL < :snow_leopard
true

[sjackman]

The primary intention of this patch is to make it possible to tap Homebrew/core using Homebrew/brew running on Linux. We can discuss in a separate issue possible changes to the Homebrew formula API.

[maxim-belkin]

Four methods would be required, actually 🤔 :

<    :  older_than?
<=   :  not_newer_than?
>=   :  not_older_than?
>    :  newer_than?

[maxim-belkin]

Sure, but this is a hack. MacOS.version is meaningless on Linux and should not be returning NULL. If anything, it should give something like "not defined". We can not assume that all Linux distributions correspond to old macOS.

[sjackman]

Version::NULL does mean not defined. You can use Version.null? to test whether a version is null.

[maxim-belkin]

Version::NULL < something evaluates to true for any number and any string, whereas
Version::NULL > something evaluates to false even for negative numbers.
So, whenever there is MacOS.version < something comparison in a formula, it will be considered as true on Linux. So, unless it's augmented/prefixed by OS.mac? && this might lead to incorrect/undesired behavior or unnecessary patches being applied

$ uname -s
Linux

$ cd $(brew --repo homebrew/core)/Formula

$ grep -w "MacOS.version <" * | wc -l
312

$ grep -w "MacOS.version <" * | grep 'OS.mac?' | wc -l
14

[MikeMcQuaid]

Version::NULL does mean not defined. You can use Version.null? to test whether a version is null.

I agree, let's keep with this approach.

@sjackman Does this patch enable an otherwise unpatched Homebrew/brew to brew readall on Homebrew/homebrew-core?

[ilovezfs]

So, whenever there is MacOS.version < something comparison in a formula, it will be considered as true on Linux. So, unless it's augmented/prefixed by OS.mac? && this might lead to incorrect/undesired behavior or unnecessary patches being applied

This is caused by spaghetti formulae that attempt to interweave macOS and Linux support and/or reuse macOS formulae from Homebrew/homebrew-core.

[MikeMcQuaid]

Thanks again @sjackman!

[sjackman]

Does this patch enable an otherwise unpatched Homebrew/brew to brew readall on Homebrew/homebrew-core?

Yes.

Thanks for merging, Mike!