Perform the install triggered by auto_install in a subprocess

When `auto_install` is enabled and gems are missing, Bundler installs them
in the same process that then runs `Bundler.setup`. Installing requires
`openssl` for HTTPS remotes, and no Gemfile requirement is in effect yet,
so RubyGems activates the newest installed version, which is the default
gem while the locked one is still missing. Activation can't be undone, so
the `Bundler.setup` that follows raises a `Gem::LoadError` when the
lockfile pins `openssl` to another version.

Do the install in a forked child, like `bundler/inline` already does for
the same reason, so the process that boots the application never activates
gems that were only needed to install. Platforms without `fork` keep
installing in process.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: meganemura

lib/bundler.rb

@@ -187,7 +187,7 @@ def auto_install
       rescue GemNotFound, GitError
         ui.info "Automatically installing missing gems."
         reset!
-        CLI::Install.new({}).run
+        auto_install_missing_gems
         reset!
       end
     end
@@ -605,6 +605,44 @@ def self_manager
 
     private
 
+    # When possible we do the install in a subprocess because to install gems
+    # we need to require some default gems like `openssl` (for HTTPS remotes)
+    # which may later conflict with the Gemfile requirements. `bundler/inline`
+    # re-resolves when that happens. We can't: the `Bundler.setup` that follows
+    # must activate what the lockfile says.
+    def auto_install_missing_gems
+      do_install = -> { CLI::Install.new({}).run }
+
+      if Process.respond_to?(:fork)
+        [$stdout, $stderr].each(&:flush) # don't let the fork inherit buffered output
+
+        _, status = Process.waitpid2(Process.fork do
+          exit_status = 1
+
+          begin
+            # Errors here never reach the parent's handler. Report them in the
+            # child, and let the parent exit with the status. Required inside the
+            # fork so the CLI's vendored Thor stays out of the parent.
+            require_relative "bundler/friendly_errors"
+
+            with_friendly_errors(&do_install)
+            exit_status = 0
+          rescue SystemExit => e
+            exit_status = e.status
+          ensure
+            # Skip `at_exit` handlers, they belong to the booting program.
+            # `exit!` doesn't flush, so flush by hand.
+            [$stdout, $stderr].each(&:flush)
+            exit!(exit_status)
+          end
+        end)
+
+        exit(status.exitstatus || status.to_i) unless status.success?
+      else
+        do_install.call
+      end
+    end
+
     def unbundle_env(env)
       if env.key?("BUNDLER_ORIG_MANPATH")
         env["MANPATH"] = env["BUNDLER_ORIG_MANPATH"]

spec/bundler/bundler_spec.rb

@@ -207,6 +207,36 @@
     end
   end
 
+  describe "#auto_install" do
+    let(:install) { double("install") }
+
+    before do
+      skip "requires Process.fork" unless Process.respond_to?(:fork)
+
+      definition = double("definition")
+      allow(definition).to receive(:specs).and_raise(Bundler::GemNotFound)
+      allow(Bundler).to receive(:definition).and_return(definition)
+      allow(Bundler::CLI::Install).to receive(:new).with({}).and_return(install)
+    end
+
+    it "installs in a subprocess, so that gems activated to install don't conflict with the Gemfile" do
+      installer_pid = tmp("auto_install_pid")
+      allow(install).to receive(:run) { File.write(installer_pid, Process.pid) }
+
+      Bundler.settings.temporary(auto_install: true) { Bundler.auto_install }
+
+      expect(installer_pid.read.to_i).not_to eq(Process.pid)
+    end
+
+    it "exits with the status code of a failed install" do
+      allow(install).to receive(:run).and_raise(Bundler::InstallError)
+
+      expect do
+        Bundler.settings.temporary(auto_install: true) { Bundler.auto_install }
+      end.to raise_error(SystemExit) {|error| expect(error.status).to eq(5) }
+    end
+  end
+
   describe "#mkdir_p" do
     it "creates a folder at the given path" do
       install_gemfile <<-G

spec/runtime/setup_gems_spec.rb

@@ -792,6 +792,29 @@ def require(path)
     expect(out).to include("Installing myrack 1.0.0")
   end
 
+  it "performs an automatic bundle install of a default gem locked to another version" do
+    build_repo4 do
+      build_gem "psych", "999"
+      build_gem "myrack", "1.0.0"
+    end
+
+    gemfile <<-G
+      source "https://gem.repo4"
+      gem "psych"
+      gem "myrack"
+    G
+
+    bundle_config "auto_install 1"
+
+    ruby <<-RUBY, artifice: "compact_index"
+      require 'bundler/setup'
+      puts Gem.loaded_specs["psych"].version
+    RUBY
+    expect(err).to be_empty
+    expect(out).to include("Installing psych 999")
+    expect(out).to include("999")
+  end
+
   context "in a read-only filesystem" do
     before do
       gemfile <<-G