Add thread safety check to rakefile

Rakefile

@@ -18,3 +18,33 @@ Rake::RDocTask.new do |rdoc|
   rdoc.rdoc_files.include('lib/**/*.rb')
   rdoc.rdoc_files.include(Bundler::GemHelper.gemspec.extra_rdoc_files)
 end
+
+task :thread_safety_check, [:thread_count] do |_, args|
+  thread_count = Integer(args[:thread_count] || "1000", 10)
+  $LOAD_PATH.unshift(File.join(__dir__, 'lib'))
+
+  require 'http/cookie' or raise "already required"
+
+  # see https://github.com/sparklemotion/http-cookie/issues/27
+  puts "checking for thread safety bug with #{thread_count} threads on #{RUBY_DESCRIPTION}"
+
+  results = thread_count.times.map do
+    Thread.new do
+      begin
+        HTTP::CookieJar::HashStore.new
+        nil
+      rescue => e
+        e
+      end
+    end
+  end.map(&:value)
+
+  exceptions = results.reject(&:nil?)
+  if exceptions.any?
+    classes = exceptions.map { |e| e.class.name }.uniq
+    warn "#{exceptions.size} of #{thread_count} threads saw exceptions: #{classes.inspect}"
+    raise exceptions.first
+  else
+    puts "no issues detected"
+  end
+end