Guard for the ftp protocol feature of OpenURI

lib/open-uri.rb

@@ -750,7 +750,12 @@ module URI
         OpenURI.open_http(buf, self, proxy, options)
         return
       end
+
+      begin
         require 'net/ftp'
+      rescue LoadError
+        abort "net/ftp is not found. You may need to `gem install net-ftp` to install net/ftp."
+      end
 
       path = self.path
       path = path.sub(%r{\A/}, '%2F') # re-encode the beginning slash because uri library decodes it.

test/open-uri/test_open-uri.rb

@@ -703,6 +703,9 @@ class TestOpenURI < Test::Unit::TestCase
 
   # 192.0.2.0/24 is TEST-NET.  [RFC3330]
 
+  begin
+    require 'net/ftp'
+
     def test_ftp_invalid_request
       assert_raise(ArgumentError) { URI("ftp://127.0.0.1/").read }
       assert_raise(ArgumentError) { URI("ftp://127.0.0.1/a%0Db").read }
@@ -841,6 +844,9 @@ class TestOpenURI < Test::Unit::TestCase
         end
       }
     end
+  rescue LoadError
+    # net-ftp is the bundled gems at Ruby 3.1
+  end
 
   def test_ftp_over_http_proxy
     TCPServer.open("127.0.0.1", 0) {|proxy_serv|