#11 it is

.rubocop.yml

@@ -2,6 +2,8 @@ AllCops:
   DisplayCopNames: true
   TargetRubyVersion: 2.3.3
 
+Layout/EmptyLineAfterGuardClause:
+  Enabled: false
 Style/ClassAndModuleChildren:
   Enabled: false
 Naming/FileName:

Rakefile

@@ -24,7 +24,7 @@ require 'rubygems'
 require 'rake'
 require 'rake/clean'
 
-CLEAN = FileList['coverage']
+CLEAN << 'coverage'
 
 def name
   @name ||= File.basename(Dir['*.gemspec'].first, '.*')

lib/random-port.rb

@@ -22,12 +22,5 @@
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 
-# The module for all classes.
-#
-# Author:: Yegor Bugayenko (yegor256@gmail.com)
-# Copyright:: Copyright (c) 2018 Yegor Bugayenko
-# License:: MIT
-module RandomPort
-end
-
-require 'random-port/pool'
+require_relative 'random-port/module'
+require_relative 'random-port/pool'

lib/random-port/module.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+# (The MIT License)
+#
+# Copyright (c) 2018 Yegor Bugayenko
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the 'Software'), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in all
+# copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+
+# The module for all classes.
+#
+# Author:: Yegor Bugayenko (yegor256@gmail.com)
+# Copyright:: Copyright (c) 2018 Yegor Bugayenko
+# License:: MIT
+module RandomPort
+end

lib/random-port/pool.rb

@@ -24,7 +24,7 @@
 
 require 'socket'
 require 'monitor'
-require_relative '../random-port'
+require_relative 'module'
 
 # Pool of TPC ports.
 #

random-port.gemspec

@@ -48,4 +48,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rdoc', '4.3.0'
   s.add_development_dependency 'rubocop', '0.58.1'
   s.add_development_dependency 'rubocop-rspec', '1.27.0'
+  s.add_development_dependency 'threads', '0.3.0'
 end

test/test_pool.rb

@@ -23,6 +23,7 @@
 # SOFTWARE.
 
 require 'minitest/autorun'
+require 'threads'
 require_relative '../lib/random-port/pool'
 
 # Pool test.
@@ -78,6 +79,18 @@ def test_acquires_and_releases_in_block
     assert_equal(123, result)
   end
 
+  def test_acquires_and_releases_in_threads
+    pool = RandomPort::Pool.new
+    Threads.new(100).assert do
+      pool.acquire(5) do |ports|
+        ports.each do |p|
+          server = TCPServer.new(p)
+          server.close
+        end
+      end
+    end
+  end
+
   def test_acquires_and_releases_safely
     pool = RandomPort::Pool.new
     assert_raises do