Avoid space-containing absolute path in RUBYOPT

lib/bundler/shared_helpers.rb

@@ -347,7 +347,12 @@ def set_path
 
     def set_rubyopt
       rubyopt = [ENV["RUBYOPT"]].compact
-      setup_require = "-r#{File.expand_path("setup", __dir__)}"
+      setup_path = File.expand_path("setup", __dir__)
+      # RUBYOPT is split on whitespace with no quoting mechanism, so an
+      # absolute path containing spaces would be torn apart. Fall back to
+      # requiring by feature name; set_rubylib puts our lib directory first
+      # on the child's load path.
+      setup_require = /\s/.match?(setup_path) ? "-rbundler/setup" : "-r#{setup_path}"
       return if !rubyopt.empty? && rubyopt.first.include?(setup_require)
       rubyopt.unshift setup_require
       Bundler::SharedHelpers.set_env "RUBYOPT", rubyopt.join(" ")

spec/bundler/shared_helpers_spec.rb

@@ -399,6 +399,34 @@
       it_behaves_like "ENV['RUBYOPT'] gets set correctly"
     end
 
+    context "when bundler install path contains whitespace" do
+      let(:install_path) { "/opt/ruby with space/lib" }
+
+      before do
+        allow(File).to receive(:expand_path).and_return("#{install_path}/bundler/setup")
+        allow(Gem).to receive(:bin_path).and_return("#{install_path}/bundler/setup")
+      end
+
+      # RUBYOPT is split on whitespace with no quoting mechanism, so an
+      # absolute -r path containing spaces would make every child ruby fail
+      # to boot with "invalid switch in RUBYOPT".
+      it "requires bundler/setup by feature name instead of absolute path" do
+        subject.set_bundle_environment
+        expect(ENV["RUBYOPT"].split(" ")).to start_with("-rbundler/setup")
+      end
+
+      it "does not inject the space-containing path into RUBYOPT" do
+        subject.set_bundle_environment
+        expect(ENV["RUBYOPT"]).not_to include(install_path)
+      end
+
+      it "is idempotent" do
+        subject.set_bundle_environment
+        subject.set_bundle_environment
+        expect(ENV["RUBYOPT"].split(" ").grep(%r{\A-r.*bundler/setup\z}).length).to eq(1)
+      end
+    end
+
     context "ENV['RUBYLIB'] does not exist" do
       before { ENV.delete("RUBYLIB") }