Implement a make jobserver:

bundler/lib/bundler/installer/parallel_installer.rb

@@ -107,8 +107,27 @@ def failed_specs
     end
 
     def install_with_worker
-      enqueue_specs
-      process_specs until finished_installing?
+      with_jobserver do
+        enqueue_specs
+        process_specs until finished_installing?
+      end
+    end
+
+    def with_jobserver
+      r, w = IO.pipe
+      r.close_on_exec = false
+      w.close_on_exec = false
+      w.write("*" * @size)
+
+      old_makeflags = ENV["MAKEFLAGS"]
+      ENV["MAKEFLAGS"] = "#{old_makeflags} --jobserver-auth=#{r.fileno},#{w.fileno}"
+
+      yield
+    ensure
+      r.close
+      w.close
+
+      old_makeflags ? ENV["MAKEFLAGS"] = old_makeflags : ENV.delete("MAKEFLAGS")
     end
 
     def install_serially

bundler/lib/bundler/rubygems_gem_installer.rb

@@ -104,10 +104,18 @@ def generate_bin_script(filename, bindir)
     end
 
     def build_jobs
-      Bundler.settings[:jobs] || super
+      @jobserver_read_io&.read_nonblock(3, @jobserver_tokens)
+      available_jobs = @jobserver_tokens.empty? ? nil : @jobserver_tokens.size
+
+      available_jobs || Bundler.settings[:jobs] || super
+    rescue IO::WaitReadable
+      1
     end
 
     def build_extensions
+      @jobserver_tokens = +""
+      @jobserver_read_io, @jobserver_write_io = connect_to_jobserver
+
       extension_cache_path = options[:bundler_extension_cache_path]
       extension_dir = spec.extension_dir
       unless extension_cache_path && extension_dir
@@ -131,6 +139,11 @@ def build_extensions
           FileUtils.cp_r extension_dir, extension_cache_path
         end
       end
+    ensure
+      unless @jobserver_tokens.empty?
+        @jobserver_write_io.write(@jobserver_tokens)
+        @jobserver_write_io.flush
+      end
     end
 
     def spec
@@ -147,6 +160,15 @@ def gem_checksum
 
     private
 
+    def connect_to_jobserver
+      return unless ENV["MAKEFLAGS"]
+      read_fd, write_fd = ENV["MAKEFLAGS"].match(/--jobserver-auth=(\d+),(\d+)/)&.captures
+
+      return unless read_fd && write_fd
+
+      [IO.new(read_fd.to_i, autoclose: false), IO.new(write_fd.to_i, autoclose: false)]
+    end
+
     def prepare_extension_build(extension_dir)
       SharedHelpers.filesystem_access(extension_dir, :create) do
         FileUtils.mkdir_p extension_dir

bundler/spec/bundler/installer/parallel_installer_spec.rb

@@ -0,0 +1,139 @@
+# frozen_string_literal: true
+
+require "bundler/installer/parallel_installer"
+require "bundler/rubygems_gem_installer"
+require "rubygems/remote_fetcher"
+require "bundler"
+
+RSpec.describe Bundler::ParallelInstaller do
+  describe "connect to make jobserver" do
+    before do
+      unless Gem::Installer.private_method_defined?(:build_jobs)
+        skip "This example is runnable when RubyGems::Installer implements `build_jobs`"
+      end
+
+      require "support/artifice/compact_index"
+
+      @previous_client = Gem::Request::ConnectionPools.client
+      Gem::Request::ConnectionPools.client = Gem::Net::HTTP
+      Gem::RemoteFetcher.fetcher.close_all
+
+      build_repo2 do
+        build_gem "one", &:add_c_extension
+        build_gem "two", &:add_c_extension
+      end
+
+      gemfile <<~G
+        source "https://gem.repo2"
+
+        gem "one"
+        gem "two"
+      G
+      lockfile <<~L
+        GEM
+          remote: https://gem.repo2/
+          specs:
+            one (1.0)
+            two (1.0)
+
+        DEPENDENCIES
+          one
+          two
+      L
+
+      @old_ui = Bundler.ui
+      Bundler.ui = Bundler::UI::Silent.new
+    end
+
+    after do
+      Bundler.ui = @old_ui
+      Gem::Request::ConnectionPools.client = @previous_client
+      Artifice.deactivate
+    end
+
+    let(:definition) do
+      allow(Bundler).to receive(:root) { bundled_app }
+
+      definition = Bundler::Definition.build(bundled_app.join("Gemfile"), bundled_app.join("Gemfile.lock"), false)
+      definition.tap(&:setup_domain!)
+    end
+    let(:installer) { Bundler::Installer.new(bundled_app, definition) }
+    let(:gem_one) { definition.specs.find {|spec| spec.name == "one" } }
+    let(:gem_two) { definition.specs.find {|spec| spec.name == "two" } }
+
+    it "takes all available slots" do
+      redefine_build_jobs do
+        Bundler::ParallelInstaller.call(installer, definition.specs, 5, false, true)
+      end
+
+      # Take 3 slots out of the 5 available.
+      expect(File.read(File.join(gem_one.extension_dir, "gem_make.out"))).to include("make -j3")
+      # Take the remaining 2 slots.
+      expect(File.read(File.join(gem_two.extension_dir, "gem_make.out"))).to include("make -j2")
+    end
+
+    it "fallback to non parallel when no slots are available" do
+      redefine_build_jobs do
+        Bundler::ParallelInstaller.call(installer, definition.specs, 3, false, true)
+      end
+
+      # Take 3 slots out of the 3 available.
+      expect(File.read(File.join(gem_one.extension_dir, "gem_make.out"))).to include("make -j3")
+      # Fallback to one slot (non parallel).
+      expect(File.read(File.join(gem_two.extension_dir, "gem_make.out"))).to_not include("make -j")
+    end
+
+    it "uses one jobs when installing serially" do
+      Bundler.settings.temporary(jobs: 1) do
+        Bundler::ParallelInstaller.call(installer, definition.specs, 1, false, true)
+      end
+
+      expect(File.read(File.join(gem_one.extension_dir, "gem_make.out"))).to_not include("make -j")
+      expect(File.read(File.join(gem_two.extension_dir, "gem_make.out"))).to_not include("make -j")
+    end
+
+    it "release the job slots" do
+      build_repo2 do
+        build_gem "one", &:add_c_extension
+        build_gem "two" do |spec|
+          spec.add_c_extension
+          spec.add_dependency(:one) # ParallelInstaller will wait for `one` to be fully installed.
+        end
+      end
+
+      Bundler::ParallelInstaller.call(installer, definition.specs, 3, false, true)
+
+      # Take 3 slots out of the 3 available.
+      expect(File.read(File.join(gem_one.extension_dir, "gem_make.out"))).to include("make -j3")
+      # Take 3 slots that were released.
+      expect(File.read(File.join(gem_two.extension_dir, "gem_make.out"))).to include("make -j3")
+    end
+
+    def redefine_build_jobs
+      old_method = Bundler::RubyGemsGemInstaller.instance_method(:build_jobs)
+      Bundler::RubyGemsGemInstaller.remove_method(:build_jobs)
+
+      gem_one_waiting = true
+      gem_two_waiting = true
+
+      Bundler::RubyGemsGemInstaller.define_method(:build_jobs) do
+        if spec.name == "one"
+          value = old_method.bind(self).call
+          gem_one_waiting = false
+          sleep(0.1) while gem_two_waiting
+        elsif spec.name == "two"
+          sleep(0.1) while gem_one_waiting
+          value = old_method.bind(self).call
+          gem_two_waiting = false
+        end
+
+        value
+      end
+
+      yield
+    ensure
+      Bundler::RubyGemsGemInstaller.remove_method(:build_jobs)
+      Bundler::RubyGemsGemInstaller.define_method(:build_jobs, old_method)
+    end
+  end
+end

bundler/spec/commands/install_spec.rb

@@ -1352,7 +1352,7 @@ def run
       expect(gem_make_out).not_to include("make -j8")
     end
 
-    it "pass down the BUNDLE_JOBS to RubyGems when running the compilation of an extension" do
+    it "uses 3 slots from the available pool when running the compilation of an extension" do
       ENV.delete("MAKEFLAGS")
 
       install_gemfile(<<~G, env: { "BUNDLE_JOBS" => "8" })
@@ -1362,10 +1362,10 @@ def run
 
       gem_make_out = File.read(File.join(@gemspec.extension_dir, "gem_make.out"))
 
-      expect(gem_make_out).to include("make -j8")
+      expect(gem_make_out).to include("make -j3")
     end
 
-    it "uses nprocessors by default" do
+    it "consumes 3 slots from the pool when BUNDLE_JOBS isn't set" do
       ENV.delete("MAKEFLAGS")
 
       install_gemfile(<<~G)
@@ -1375,7 +1375,7 @@ def run
 
       gem_make_out = File.read(File.join(@gemspec.extension_dir, "gem_make.out"))
 
-      expect(gem_make_out).to include("make -j#{Etc.nprocessors + 1}")
+      expect(gem_make_out).to include("make -j3")
     end
   end
 

bundler/spec/support/windows_tag_group.rb

@@ -90,6 +90,7 @@ module WindowsTagGroup
         "spec/update/gems/fund_spec.rb",
         "spec/bundler/stub_specification_spec.rb",
         "spec/bundler/retry_spec.rb",
+        "spec/bundler/installer/parallel_installer_spec.rb",
         "spec/bundler/installer/spec_installation_spec.rb",
         "spec/bundler/spec_set_spec.rb",
         "spec/quality_es_spec.rb",

lib/rubygems/ext/builder.rb

@@ -41,8 +41,9 @@ def self.make(dest_path, results, make_dir = Dir.pwd, sitedir = nil, targets = [
     # nmake doesn't support parallel build
     unless is_nmake
       have_make_arguments = make_program.size > 1
+      n_jobs ||= 0
 
-      if !have_make_arguments && !ENV["MAKEFLAGS"] && n_jobs
+      if !have_make_arguments && n_jobs > 1 && !ENV["MAKEFLAGS"]&.match(/-j\d?(\s|\Z)/)
         make_program << "-j#{n_jobs}"
       end
     end