linux: use preferred_gcc instead of gcc

[iMichka]

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[BrewTestBot]

Review period will end on 2021-02-09 at 19:09:44 UTC.

[iMichka]

I need to run some sanity checks locally before shipping this. Proposed by @sjackman

[sjackman]

In extend/os/linux/linkage_checker.rb

-@undeclared_deps -= ["gcc", "glibc"]
+@undeclared_deps -= [preferred_compiler, "glibc"]

[sjackman]

In extend/os/linux/system_config.rb

-["glibc", "gcc", "xorg"].each do |f|
+["glibc", preferred_compiler, "xorg"].each do |f|

[MikeMcQuaid]

👍🏻 once this is 💚. I really like how minimal and clean this change is ❤️

[BrewTestBot]

Review period ended.

[iMichka]

I had to stub gcc@5 in the tests to make it pass. That's the only test change I had to make.

[sjackman]

I did some quick tests in the homebrew/brew Docker image.
✅ brew install -sv hello uses /usr/bin/gcc-9 provided by the host
✅ brew install gcc@5 && brew install -sv hello uses /home/linuxbrew/.linuxbrew/bin/gcc-5

Currently gcc and gcc@5 conflict, because they both provide GCC 5 until https://github.com/Homebrew/linuxbrew-core/pull/21380 is merged.

I need to think about migration issues for existing users that have gcc installed, because their host system is old (GCC < 5). We want it to smoothly install gcc@5 for those users (and upgrade gcc to 10, but I'm less concerned about that). I don't believe gcc@5 will be installed automatically, but I need to test that.

[sjackman]

We want it to smoothly install gcc@5 for those users (and upgrade gcc to 10, but I'm less concerned about that). I don't believe gcc@5 will be installed automatically, but I need to test that.

This sort of works! Once gcc is upgraded to GCC 10 and no longer provides ~/.linuxbrew/opt/gcc/bin/gcc, when the user has no other executable named gcc version ≥ 5 in their PATH, then Homebrew will install preferred_gcc, aka gcc@5.

[iMichka]

Regarding the transition phase from gcc to gcc-5 in linuxbrew-core: stuff might get broken for complex cases, especially for people which do not have gcc-5 installed. But I think we can guide them to run some commands manually to clean the mess up if anything goes wrong. But your tests tend to demonstrate that there will be no issue.

[sjackman]

Thank you, Michka!