minitest/minitest#467 - store pid that registered at_exit hook

gistfile1.txt

diff --git a/lib/minitest.rb b/lib/minitest.rb
index 2d4344e..5c50120 100644
--- a/lib/minitest.rb
+++ b/lib/minitest.rb
@@ -43,12 +43,14 @@ module Minitest
   # Registers Minitest to run at process exit
 
   def self.autorun
+    registered_pid = $$
     at_exit {
       next if $! and not ($!.kind_of? SystemExit and $!.success?)
 
       exit_code = nil
 
       at_exit {
+        next unless registered_pid == $$
         @@after_run.reverse_each(&:call)
         exit exit_code || false
       }
diff --git a/test/minitest/test_minitest_unit.rb b/test/minitest/test_minitest_unit.rb
index e8b5974..a60df9d 100644
--- a/test/minitest/test_minitest_unit.rb
+++ b/test/minitest/test_minitest_unit.rb
@@ -1883,4 +1883,10 @@ def test_method
       end
     end
   end
+
+  def test_forking_process
+    pid = fork{ true }
+    Process.waitpid(pid)
+    assert $?.success?, "forked process did not exit successfully"
+  end
 end