keg_relocate (linux): prepend gcc/lib/current to RPATH when needed

[carlocab]

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This should help keep bottles that require GCC working when
Homebrew/homebrew-core#106755 is merged.

This only works on freshly-poured bottles. Previously installed bottles
will still break on systems with a host GCC older than GCC 11.

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I've marked this as draft since this isn't ready to merge yet. For one thing, this line probably needs explanation in a comment. But I've opened this to suggest a potential way to minimise the pain of breaking all these Linux bottles for the GCC 12 migration.

As noted above, this fix only applies for bottles poured after brew includes this change. If desired, we could add a check to brew doctor that recommends a brew install foo if foo would benefit from having their RPATH fixed up by this change.

[BrewTestBot]

Review period will end on 2022-08-03 at 03:09:28 UTC.

[carlocab]

It's also worth noting that this will likely fix most, if not all, Linux-only GCC dependents in CI (as bottles are always freshly poured).

[carlocab]

It's worth noting that this replaces linkage to any Homebrew GCC, even if they're not the latest, with linkage to the newest Homebrew GCC.

This is safe to do only if GCC libraries are always backwards-compatible. My understanding is that they should be. At the very least, libstdc++ and libgcc_s should be.

[MikeMcQuaid]

Fine to 🚢 as-is but one question.

[fxcoudert]

This is safe to do only if GCC libraries are always backwards-compatible.

GCC libraries are properly versioned, so any non-backward compatible change will mean the library version will have changed. So this should not be a problem.

[BrewTestBot]

Review period skipped due to critical label.

[carlocab]

Ok, I think I'm happy to merge this.

Also, with this change in hand, we can then use a trick similar to what we did for the Linuxbrew migration to migrate installed Linux bottles to GCC 12:

brew/Library/Homebrew/cmd/update-report.rb

Lines 162 to 178 in 4baff3e

	LINUXBREW_CORE_MIGRATION_LIST.each do |name|
	begin
	formula = Formula[name]
	rescue FormulaUnavailableError
	next
	end
	next unless formula.any_version_installed?
	keg = formula.installed_kegs.last
	tab = Tab.for_keg(keg)
	# force a `brew upgrade` from the linuxbrew-core version to the homebrew-core version (even if lower)
	tab.source["versions"]["version_scheme"] = -1
	tab.write
	end
	Settings.write "linuxbrewmigrated", true
	end

I think this should result in an almost pain-free migration experience for most Linux users. (Aside from the bottles that have already been lost due to changes independent from bottle removal.)

Thoughts? We'll need to get the timing of tagged releases for brew, etc, right, of course.

[MikeMcQuaid]

Also, with this change in hand, we can then use a trick similar to what we did for the Linuxbrew migration to migrate installed Linux bottles to GCC 12:

Seems like a great idea 👍🏻

[carlocab]

Ok, I guess we'll want those Linux bottles back then. Homebrew/homebrew-core#107148