Safer tests by not mutating current process

If you don't know if code will mutate a process information, such as environment variables then run it in a fork to ensure it does not affect the parent process.

lib/hatchet/app.rb

@@ -218,6 +218,32 @@ def in_directory(directory = self.directory)
       end
     end
 
+    # A safer alternative to in_directory
+    # this method is used to run code that may mutate the
+    # current process anything run in this block is executed
+    # in a different fork
+    def in_directory_fork(&block)
+      Tempfile.create("stdout") do |tmp_file|
+        pid = fork do
+          $stdout.reopen(tmp_file, "w")
+          $stderr.reopen(tmp_file, "w")
+          $stdout.sync = true
+          $stderr.sync = true
+          in_directory do |dir|
+            yield dir
+          end
+          Kernel.exit!(0) # needed for https://github.com/seattlerb/minitest/pull/683
+        end
+        Process.waitpid(pid)
+
+        if $?.success?
+          puts File.read(tmp_file)
+        else
+          raise File.read(tmp_file)
+        end
+      end
+    end
+
     # creates a new app on heroku, "pushes" via anvil or git
     # then yields to self so you can call self.run or
     # self.deployed?

test/hatchet/local_repo_test.rb

@@ -1,6 +1,18 @@
 require 'test_helper'
 
-class AppTest < Minitest::Test
+class LocalRepoTest < Minitest::Test
+  def test_in_directory_fork
+    env_name = SecureRandom.hex
+    ENV[env_name] = env_name
+
+    Hatchet::App.new("default_ruby").in_directory_fork do
+      ENV.delete(env_name)
+      assert_nil ENV[env_name]
+    end
+
+    assert_equal env_name, ENV[env_name]
+  end
+
   def test_repos_checked_into_git
     app = Hatchet::App.new("test/different-folder-for-checked-in-repos/default_ruby")
     app.in_directory do