dep: make net-ftp an optional dependency

and let users know what's wrong if they try to use it with a version
of Ruby that doesn't provide it by default.

CHANGELOG.md

@@ -1,5 +1,10 @@
 ## mini_portile changelog
 
+### 2.5.3 / unreleased
+
+Make `net-ftp` an optional dependency, since requiring it as a hard dependency in v2.5.2 caused warnings to be emitted by Ruby 2.7 and earlier. A warning message is emitted if FTP functionality is called and `net-ftp` isn't available; this should only happen in Ruby 3.1 and later.
+
+
 ### 2.5.2 / 2021-05-28
 
 #### Dependencies

Gemfile

@@ -1,4 +1,6 @@
 source 'https://rubygems.org'
 
+gem "net-ftp" if Gem::Requirement.new("> 3.1.0.dev").satisfied_by?(Gem::Version.new(RUBY_VERSION))
+
 # Specify your gem's dependencies in mini_portile2.gemspec
 gemspec

lib/mini_portile2/mini_portile.rb

@@ -1,7 +1,6 @@
 require 'rbconfig'
 require 'net/http'
 require 'net/https'
-require 'net/ftp'
 require 'fileutils'
 require 'tempfile'
 require 'digest'
@@ -528,6 +527,7 @@ def download_file_file(uri, full_path)
   end
 
   def download_file_ftp(uri, full_path)
+    require "net/ftp"
     filename = File.basename(uri.path)
     with_tempfile(filename, full_path) do |temp_file|
       total = 0
@@ -551,6 +551,8 @@ def download_file_ftp(uri, full_path)
       end
       output
     end
+  rescue LoadError
+    raise LoadError, "Ruby #{RUBY_VERSION} does not provide the net-ftp gem, please add it as a dependency if you need to use FTP"
   rescue Net::FTPError
     return false
   end

mini_portile2.gemspec

@@ -33,8 +33,6 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = ">= 2.3.0"
 
-  spec.add_dependency "net-ftp", "~> 0.1"
-
   spec.add_development_dependency "bundler", "~> 2.1"
   spec.add_development_dependency "minitar", "~> 0.7"
   spec.add_development_dependency "minitest", "~> 5.11"

test/test_download.rb

@@ -18,10 +18,12 @@ def server_must_receive_connection(connections = 3, &block)
       end
     end
 
-    block.call
-
-    thread.kill
-    server.close
+    begin
+      block.call
+    ensure
+      thread.kill
+      server.close
+    end
 
     request_count.must_be :>, 0
   end