Order of execution is only guaranteed if upgrading

If the thread isn't yet holding any form of lock, it has no claim over
what may / may not run while it's blocked.

activesupport/lib/active_support/concurrency/share_lock.rb

@@ -52,7 +52,7 @@ def start_exclusive(purpose: nil, compatible: [], no_wait: false)
               return false if no_wait
 
               loose_shares = @sharing.delete(Thread.current)
-              @waiting[Thread.current] = compatible
+              @waiting[Thread.current] = compatible if loose_shares
 
               @cv.wait_while { busy?(purpose) }
 

activesupport/test/share_lock_test.rb

@@ -147,7 +147,9 @@ def test_exclusive_ordering
 
           assert_threads_not_stuck threads
           scratch_pad_mutex.synchronize do
-            assert_equal [:load, :load, :unload, :unload], scratch_pad
+            if use_upgrading
+              assert_equal [:load, :load, :unload, :unload], scratch_pad
+            end
             scratch_pad.clear
           end
         end