os/{mac,mac/sdk}: refactor out sdk_version

[carlocab]

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The changes here rely on #10129. Merge that PR before this one. (Tests will pass when that is done.)

This PR:

1. Removes sdk_version, as it is no longer in use (cf. os/mac/pkgconfig: merge pc files for 11.1 and 11.0 #10129 (comment))
2. Combines the logic in

   brew/Library/Homebrew/os/mac/sdk.rb

   Line 64 in da0d7ef

   return sdk if OS::Mac.version >= :big_sur && sdk.version.major == OS::Mac.version.major

   and

   brew/Library/Homebrew/os/mac/sdk.rb

   Lines 96 to 101 in da0d7ef

   	sdk_path = File.join(sdk_prefix, "MacOSX.sdk")
   	if OS::Mac.version >= :big_sur && File.directory?(sdk_path)
   	sdk_settings = File.join(sdk_path, "SDKSettings.json")
   	version = JSON.parse(File.read(sdk_settings))["Version"] if File.exist?(sdk_settings)
   	paths[OS::Mac::Version.new(version)] = sdk_path if version.present?
   	end

   using the behaviour of OS::Mac.version in Big Sur.

Since OS::Mac.version returns only the major version in Big Sur, we use that to populate the sdk_paths hash with the key-value pair OS::Mac.version.major => /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk.

This alleviates the concern expressed here: #10112 (comment)

This also allows you to request for the versioned SDK path if required:

❯ brew ruby -e 'puts MacOS.sdk_path_if_needed'
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
❯ brew ruby -e 'puts MacOS.sdk_path_if_needed(OS::Mac::Version.new("11.1"))'
/Library/Developer/CommandLineTools/SDKs/MacOSX11.1.sdk
❯ brew ruby -e 'puts MacOS.sdk_path_if_needed(OS::Mac::Version.new("10.15"))'
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk

Finer-grained control over the exact SDK path could prove useful when macOS 12 hits.

[BrewTestBot]

Review period will end on 2020-12-29 at 00:00:00 UTC.

[carlocab]

One limitation of this is that latest_sdk

https://github.com/Homebrew/brew/blob/3a7de2275ef2477e1ee141c0d0b8f37925d145df/Library/Homebrew/os/mac/sdk.rb#L34-L39

still returns a path to MacOSX11.x.sdk. latest_sdk is used in

https://github.com/Homebrew/brew/blob/3a7de2275ef2477e1ee141c0d0b8f37925d145df/Library/Homebrew/os/mac/sdk.rb#L45-L54

I can fix that if that's not desirable, but I currently only have ugly hacks in mind.

[MikeMcQuaid]

Nice work here so far @carlocab!

[carlocab]

I don't think I actually need the if sdk_settings.exist? block (I can inline the if statement to the JSON.parseline), but I think writing it that way also makes it easier to read.

[MikeMcQuaid]

still returns a path to MacOSX11.x.sdk.

Might be worth making it private as a result?

[carlocab]

Well, BaseSDKLocator is already private (at least, I think that's what private_constant does), so it actually takes a minute to figure out how to get to latest_sdk and then make it give you the attribute you want.

It doesn't hurt, however. Updated commits will be up when brew tests finishes running.

[MikeMcQuaid]

Looks good, thanks again @carlocab!

[carlocab]

Cheers @MikeMcQuaid. Thanks for bearing with my glaring lack of familiarity with Ruby.

[BrewTestBot]

Review period ended.

[carlocab]

Weird, there look to be two "CI / test everything (macOS)" runs, but the first failed because it never really started. [1] Going to rebase and force push to try to restart the tests.

[1] https://github.com/Homebrew/brew/runs/1618013201