Make sure nested at_exit blocks don't break exit status

Per teamcapybara/capybara#178, it's possible
to have at_exit handlers installed inside Runner's run method that
execute after the 'exit' call in autorun. These at_exit handlers
can actually change the exit status (in this case, manual testing
reveals that capybara's call to @browser.quit in driver.rb seems to
be the problem).

Avoid this issue entirely by isolating the at_exit handler that calls
exit, ensuring it can't nest at_exit handlers.

features/command_line/exit_status.feature

@@ -42,11 +42,25 @@ Feature: exit status
     When I run `rspec nested_ko_spec.rb`
     Then the exit status should be 1
     And the output should contain "1 example, 1 failure"
-      
+
   Scenario: exit with 0 when no examples are run
     Given a file named "a_no_examples_spec.rb" with:
       """
       """
     When I run `rspec a_no_examples_spec.rb`
     Then the exit status should be 0
     And the output should contain "0 examples"
+
+  Scenario: exit with 1 when one example fails even if a nested exit handler calls exit(0)
+    Given a file named "ko_with_exit_handler_spec.rb" with:
+      """
+      describe "KO" do
+        it "fails" do
+          at_exit { exit(0) }
+          raise "KO"
+        end
+      end
+      """
+    When I run `rspec ko_with_exit_handler_spec.rb`
+    Then the exit status should be 1
+    And the output should contain "1 example, 1 failure"

lib/rspec/core/runner.rb

@@ -8,7 +8,12 @@ class Runner
       def self.autorun
         return if autorun_disabled? || installed_at_exit? || running_in_drb?
         @installed_at_exit = true
-        at_exit { exit(run(ARGV, $stderr, $stdout).to_i) }
+        at_exit do
+          exit_status = 0
+          # make sure this one really runs last to ensure proper exit status
+          at_exit { exit(exit_status) }
+          exit_status = run(ARGV, $stderr, $stdout).to_i
+        end
       end
       AT_EXIT_HOOK_BACKTRACE_LINE = "#{__FILE__}:#{__LINE__ - 2}:in `autorun'"