Preserve BUNDLE_GEMFILE and add a test for it

.gitignore

@@ -12,6 +12,8 @@ tmp
 t/
 .rbx/
 Gemfile.lock
+Gemfile.duplicate
+Gemfile.duplicate.lock
 .idea/
 /test/test_puma.state
 /test/test_server.sock

lib/puma/launcher.rb

@@ -277,8 +277,10 @@ def prune_bundler
 
       log '* Pruning Bundler environment'
       home = ENV['GEM_HOME']
+      gemfile = ENV['BUNDLE_GEMFILE']
       Bundler.with_clean_env do
         ENV['GEM_HOME'] = home
+        ENV['BUNDLE_GEMFILE'] = gemfile
         ENV['PUMA_BUNDLER_PRUNED'] = '1'
         wild = File.expand_path(File.join(puma_lib_dir, "../bin/puma-wild"))
         args = [Gem.ruby, wild, '-I', dirs.join(':'), deps.join(',')] + @original_argv

test/rackup/hello-bundler-env.ru

@@ -0,0 +1 @@
+run lambda { |env| [200, {"Content-Type" => "text/plain"}, ["Hello BUNDLE_GEMFILE #{ENV["BUNDLE_GEMFILE"]}"]] }

test/test_integration.rb

@@ -51,8 +51,8 @@ def server_cmd(argv)
     "#{base} -b tcp://127.0.0.1:#{@tcp_port} #{argv}"
   end
 
-  def server(argv)
-    @server = IO.popen(server_cmd(argv), "r")
+  def server(argv, env = {})
+    @server = IO.popen(env, server_cmd(argv), "r")
 
     wait_for_server_to_boot(@server)
 
@@ -75,8 +75,8 @@ def stop_forked_server(pid)
     Process.wait2(pid)
   end
 
-  def restart_server_and_listen(argv)
-    server(argv)
+  def restart_server_and_listen(argv, env = {})
+    server(argv, env)
     connection = connect
     initial_reply = read_body(connection)
     restart_server(@server, connection)
@@ -239,6 +239,18 @@ def test_restart_closes_keepalive_sockets_workers
     assert_equal "Hello World", new_reply
   end
 
+  def test_restart_with_prune_bundler_keeps_bundle_gemfile_env
+    gemfile_path = File.expand_path("../Gemfile", __dir__)
+    duplicate_gemfile_path = gemfile_path + ".duplicate"
+    FileUtils.cp(gemfile_path, gemfile_path + ".duplicate")
+    initial_reply, new_reply = restart_server_and_listen("-q -w 2 --prune-bundler test/rackup/hello-bundler-env.ru", "BUNDLE_GEMFILE" => "Gemfile.duplicate")
+    assert_match(/Hello BUNDLE_GEMFILE.*Gemfile\.duplicate/, initial_reply)
+    assert_match(/Hello BUNDLE_GEMFILE.*Gemfile\.duplicate/, new_reply)
+  ensure
+    FileUtils.rm_f(duplicate_gemfile_path)
+    FileUtils.rm_f(duplicate_gemfile_path + '.lock')
+  end
+
   def test_sigterm_closes_listeners_on_forked_servers
     skip NO_FORK_MSG unless HAS_FORK
     pid = start_forked_server("-w 2 -q test/rackup/1second.ru")