Check host libstdc++ for brew gcc dependency

For most formulae, the bottles need a minimum libstdc++ rather than a
minimum GCC version. This is particularly important when building on
Ubuntu where the default compiler version is older than libstdc++.

So, checking the host libstdc++ version is a more accurate way to
determine whether brew GCC is needed at runtime. This can be improved in
the future to check symbol versions (e.g. GLIBCXX, CXXABI, GLIBC) which
can allow some bottles to be installed even with older glibc/libstdc++.

Library/Homebrew/extend/os/linux/development_tools.rb

@@ -42,14 +42,14 @@ module OS
           @needs_libc_formula ||= OS::Linux::Glibc.below_ci_version?
         end
 
-        # Keep this method around for now to make it easier to add this functionality later.
-        # rubocop:disable Lint/UselessMethodDefinition
         sig { returns(Pathname) }
         def host_gcc_path
-          # TODO: override this if/when we to pick the GCC based on e.g. the Ubuntu version.
+          # Prioritise versioned path if installed
+          path = Pathname.new("/usr/bin/#{OS::LINUX_PREFERRED_GCC_COMPILER_FORMULA.tr("@", "-")}")
+          return path if path.exist?
+
           super
         end
-        # rubocop:enable Lint/UselessMethodDefinition
 
         sig { returns(T::Boolean) }
         def needs_compiler_formula?
@@ -60,12 +60,7 @@ module OS
           # Undocumented environment variable to make it easier to test compiler
           # formula automatic installation.
           @needs_compiler_formula = true if ENV["HOMEBREW_FORCE_COMPILER_FORMULA"]
-
+          @needs_compiler_formula ||= OS::Linux::Libstdcxx.below_ci_version?
-          @needs_compiler_formula ||= if host_gcc_path.exist?
-            ::DevelopmentTools.gcc_version(host_gcc_path.to_s) < OS::LINUX_GCC_CI_VERSION
-          else
-            true
-          end
         end
 
         sig { returns(T::Hash[String, T.nilable(String)]) }

Library/Homebrew/extend/os/linux/install.rb

@@ -2,6 +2,7 @@
 # frozen_string_literal: true
 
 require "os/linux/ld"
+require "os/linux/libstdcxx"
 require "utils/output"
 
 module OS
@@ -12,12 +13,12 @@ module OS
         # which are linked by the GCC formula. We only use the versioned shared libraries
         # as the other shared and static libraries are only used at build time where
         # GCC can find its own libraries.
-        GCC_RUNTIME_LIBS = %w[
+        GCC_RUNTIME_LIBS = T.let(%W[
           libatomic.so.1
           libgcc_s.so.1
           libgomp.so.1
-          libstdc++.so.6
+          #{OS::Linux::Libstdcxx::SONAME}
-        ].freeze
+        ].freeze, T::Array[String])
 
         sig { params(all_fatal: T::Boolean).void }
         def perform_preinstall_checks(all_fatal: false)

Library/Homebrew/extend/os/linux/linkage_checker.rb

@@ -2,12 +2,13 @@
 # frozen_string_literal: true
 
 require "compilers"
+require "os/linux/libstdcxx"
 
 module OS
   module Linux
     module LinkageChecker
       # Libraries provided by glibc and gcc.
-      SYSTEM_LIBRARY_ALLOWLIST = %w[
+      SYSTEM_LIBRARY_ALLOWLIST = %W[
         ld-linux-x86-64.so.2
         ld-linux-aarch64.so.1
         libanl.so.1
@@ -24,7 +25,7 @@ module OS
         libutil.so.1
         libgcc_s.so.1
         libgomp.so.1
-        libstdc++.so.6
+        #{OS::Linux::Libstdcxx::SONAME}
         libquadmath.so.0
       ].freeze
 

Library/Homebrew/extend/os/linux/system_config.rb

@@ -3,6 +3,7 @@
 
 require "compilers"
 require "os/linux/glibc"
+require "os/linux/libstdcxx"
 require "system_command"
 
 module OS
@@ -20,6 +21,13 @@ module OS
           version
         end
 
+        def host_libstdcxx_version
+          version = OS::Linux::Libstdcxx.system_version
+          return "N/A" if version.null?
+
+          version
+        end
+
         def host_gcc_version
           gcc = ::DevelopmentTools.host_gcc_path
           return "N/A" unless gcc.executable?
@@ -49,6 +57,7 @@ module OS
           out.puts "OS: #{OS::Linux.os_version}"
           out.puts "WSL: #{OS::Linux.wsl_version}" if OS::Linux.wsl?
           out.puts "Host glibc: #{host_glibc_version}"
+          out.puts "Host libstdc++: #{host_libstdcxx_version}"
           out.puts "#{::DevelopmentTools.host_gcc_path}: #{host_gcc_version}"
           out.puts "/usr/bin/ruby: #{host_ruby_version}" if RUBY_PATH != HOST_RUBY_PATH
           ["glibc", ::CompilerSelector.preferred_gcc, OS::LINUX_PREFERRED_GCC_RUNTIME_FORMULA, "xorg"].each do |f|

Library/Homebrew/os.rb

@@ -50,6 +50,7 @@ module OS
   LINUX_GLIBC_CI_VERSION = "2.35"
   LINUX_GLIBC_NEXT_CI_VERSION = "2.39"
   LINUX_GCC_CI_VERSION = "11.0"
+  LINUX_LIBSTDCXX_CI_VERSION = "6.0.30" # https://packages.ubuntu.com/jammy/libstdc++6
   LINUX_PREFERRED_GCC_COMPILER_FORMULA = "gcc@11" # https://packages.ubuntu.com/jammy/gcc
   LINUX_PREFERRED_GCC_RUNTIME_FORMULA = "gcc"
 

Library/Homebrew/os/linux/libstdcxx.rb

@@ -0,0 +1,47 @@
+# typed: strict
+# frozen_string_literal: true
+
+require "os/linux/ld"
+
+module OS
+  module Linux
+    # Helper functions for querying `libstdc++` information.
+    module Libstdcxx
+      SOVERSION = 6
+      SONAME = T.let("libstdc++.so.#{SOVERSION}".freeze, String)
+
+      sig { returns(T::Boolean) }
+      def self.below_ci_version?
+        system_version < LINUX_LIBSTDCXX_CI_VERSION
+      end
+
+      sig { returns(Version) }
+      def self.system_version
+        @system_version ||= T.let(nil, T.nilable(Version))
+        @system_version ||= if (path = system_path)
+          Version.new("#{SOVERSION}#{path.realpath.basename.to_s.delete_prefix!(SONAME)}")
+        else
+          Version::NULL
+        end
+      end
+
+      sig { returns(T.nilable(Pathname)) }
+      def self.system_path
+        @system_path ||= T.let(nil, T.nilable(Pathname))
+        @system_path ||= find_library(OS::Linux::Ld.library_paths(brewed: false))
+        @system_path ||= find_library(OS::Linux::Ld.system_dirs(brewed: false))
+      end
+
+      sig { params(paths: T::Array[String]).returns(T.nilable(Pathname)) }
+      private_class_method def self.find_library(paths)
+        paths.each do |path|
+          next if path.start_with?(HOMEBREW_PREFIX)
+
+          candidate = Pathname(path)/SONAME
+          return candidate if candidate.exist? && candidate.elf?
+        end
+        nil
+      end
+    end
+  end
+end

Library/Homebrew/test/os/linux/libstdcxx_spec.rb

@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+
+require "os/linux/libstdcxx"
+
+RSpec.describe OS::Linux::Libstdcxx do
+  describe "::below_ci_version?" do
+    it "returns false when system version matches CI version" do
+      allow(described_class).to receive(:system_version).and_return(Version.new(OS::LINUX_LIBSTDCXX_CI_VERSION))
+      expect(described_class.below_ci_version?).to be false
+    end
+
+    it "returns true when system version cannot be detected" do
+      allow(described_class).to receive(:system_version).and_return(Version::NULL)
+      expect(described_class.below_ci_version?).to be true
+    end
+  end
+
+  describe "::system_version" do
+    let(:tmpdir) { mktmpdir }
+    let(:libstdcxx) { tmpdir/described_class::SONAME }
+    let(:soversion) { Version.new(described_class::SOVERSION.to_s) }
+
+    before do
+      tmpdir.mkpath
+      described_class.instance_variable_set(:@system_version, nil)
+      allow(described_class).to receive(:system_path).and_return(libstdcxx)
+    end
+
+    after do
+      FileUtils.rm_rf(tmpdir)
+    end
+
+    it "returns NULL when unable to find system path" do
+      allow(described_class).to receive(:system_path).and_return(nil)
+      expect(described_class.system_version).to be Version::NULL
+    end
+
+    it "returns full version from filename" do
+      full_version = Version.new("#{soversion}.0.999")
+      libstdcxx_real = libstdcxx.sub_ext(".#{full_version}")
+      FileUtils.touch libstdcxx_real
+      FileUtils.ln_s libstdcxx_real, libstdcxx
+      expect(described_class.system_version).to eq full_version
+    end
+
+    it "returns major version when non-standard libstdc++ filename without full version" do
+      FileUtils.touch libstdcxx
+      expect(described_class.system_version).to eq soversion
+    end
+
+    it "returns major version when non-standard libstdc++ filename with unexpected realpath" do
+      libstdcxx_real = tmpdir/"libstdc++.so.real"
+      FileUtils.touch libstdcxx_real
+      FileUtils.ln_s libstdcxx_real, libstdcxx
+      expect(described_class.system_version).to eq soversion
+    end
+  end
+end