brew: re-exec under native arch if requested

[fxcoudert]

We are in the process of setting up a CI runner for macOS Big Sur on ARM. However, github's action-runner is not available for macOS ARM, so at least in the short term we will run the action-runner under Rosetta 2. In that use case, we need brew to still run natively, even when it is launched from a Rosetta process.

This PR allows this, and will let brew re-exec itself natively (under ARM architecture) if:

• requested by setting an environment variable, HOMEBREW_CHANGE_ARCH_TO_ARM=1
• we're on darwin, with ARM hardware (hw.optional.arm64)
• we were invoked under Rosetta (sysctl -n sysctl.proc_translated)

The idea and code is borrowed from @mistydemeo's pull request to do things the other away around #9287

---

With this patch, we have:

• native brew:

brew@28105 bin % brew config
HOMEBREW_VERSION: 2.6.0-82-gde1afcb-dirty
ORIGIN: https://github.com/Homebrew/brew
HEAD: de1afcbfc58fb3cd5779bd8fbb6b9995700dda4c
Last commit: 4 hours ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: 597d19eb9dd526fb1dd27e7105b2926b381a8c5e
Core tap last commit: 2 hours ago
Core tap branch: master
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_REPOSITORY: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_MAKE_JOBS: 8
Homebrew Ruby: 2.6.3 => /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby
CPU: octa-core 64-bit arm_vortex_tempest
Clang: 12.0 build 1200
Git: 2.24.3 => /Applications/Xcode.app/Contents/Developer/usr/bin/git
Curl: 7.64.1 => /usr/bin/curl
macOS: 11.0.1-arm64
CLT: 12.2.0.0.1.1604076827
Xcode: 12.2
Rosetta 2: false

• once we are under Rosetta 2:

brew@28105 bin % arch -x86_64 zsh
brew@28105 bin % brew config
HOMEBREW_VERSION: 2.6.0-82-gde1afcb-dirty
ORIGIN: https://github.com/Homebrew/brew
HEAD: de1afcbfc58fb3cd5779bd8fbb6b9995700dda4c
Last commit: 4 hours ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: 597d19eb9dd526fb1dd27e7105b2926b381a8c5e
Core tap last commit: 2 hours ago
Core tap branch: master
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_REPOSITORY: /opt/homebrew
HOMEBREW_CELLAR: /opt/homebrew/Cellar
HOMEBREW_CASK_OPTS: []
HOMEBREW_MAKE_JOBS: 4
Homebrew Ruby: 2.6.3 => /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby
CPU: quad-core 64-bit arrandale
Clang: 12.0 build 1200
Git: 2.24.3 => /Applications/Xcode.app/Contents/Developer/usr/bin/git
Curl: 7.64.1 => /usr/bin/curl
macOS: 11.0.1-x86_64
CLT: 12.2.0.0.1.1604076827
Xcode: 12.2
Rosetta 2: true
brew@28105 bin % HOMEBREW_CHANGE_ARCH_TO_ARM=1 brew config
HOMEBREW_VERSION: 2.6.0-82-gde1afcb-dirty
ORIGIN: https://github.com/Homebrew/brew
HEAD: de1afcbfc58fb3cd5779bd8fbb6b9995700dda4c
Last commit: 4 hours ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: 597d19eb9dd526fb1dd27e7105b2926b381a8c5e
Core tap last commit: 2 hours ago
Core tap branch: master
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_REPOSITORY: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_MAKE_JOBS: 8
Homebrew Ruby: 2.6.3 => /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby
CPU: octa-core 64-bit arm_vortex_tempest
Clang: 12.0 build 1200
Git: 2.24.3 => /Applications/Xcode.app/Contents/Developer/usr/bin/git
Curl: 7.64.1 => /usr/bin/curl
macOS: 11.0.1-arm64
CLT: 12.2.0.0.1.1604076827
Xcode: 12.2
Rosetta 2: false

[BrewTestBot]

Review period will end on 2020-12-07 at 17:06:19 UTC.

[fxcoudert]

@MikeMcQuaid if you approve, and if you think this is safely contained (and I believe it is, being protected behind HOMEBREW_CHANGE_ARCH_TO_ARM), I would like to merge before the 24-hour period, in order to be able to set up new CI runners over the week-end, while I have some time

[BrewTestBot]

Review period ended.

[fxcoudert]

I'm in a bit of a bind here… we recently decided to wait 1 week day to leave time for review, but I need this to merge in order to test it fully on ARM. And because I have a day job during the week, I really have time to work on this during the week-end.

So, I'm going to go on a limb here, and merge for now. @MikeMcQuaid approved the basic principle on a previous version: #9417

If there is the slightest hint of trouble, I'll revert immediately. And @MikeMcQuaid if on Monday or whenever you can review, you think it was not suitable, I'll revert and apologise.

[fxcoudert]

Completed testing by building a native ARM bottle through github actions here: Homebrew/homebrew-core#66288

[MikeMcQuaid]

I'm in a bit of a bind here… we recently decided to wait 1 week day to leave time for review, but I need this to merge in order to test it fully on ARM. And because I have a day job during the week, I really have time to work on this during the week-end.

So, I'm going to go on a limb here, and merge for now. @MikeMcQuaid approved the basic principle on a previous version: #9417

If there is the slightest hint of trouble, I'll revert immediately. And @MikeMcQuaid if on Monday or whenever you can review, you think it was not suitable, I'll revert and apologise.

@fxcoudert this was the right decision, thanks for unblocking yourself 🙇🏻