Perform the install triggered by auto_install in a subprocess

With `auto_install`, Bundler installs missing gems 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 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. 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

@@ -250,6 +250,36 @@
     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