Hand off the interlock to the new thread in AC::Live

Most importantly, the original request thread must yield its share lock
while waiting for the live thread to commit -- otherwise a request's
base and live threads can deadlock against each other.

1 files changed, 26 insertions, 0 deletions

diff --git a/activesupport/test/share_lock_test.rb b/activesupport/test/share_lock_test.rb
index 12953d99a6..68fa5bb69e 100644
--- a/activesupport/test/share_lock_test.rb
+++ b/activesupport/test/share_lock_test.rb
@@ -287,6 +287,32 @@ class ShareLockTest < ActiveSupport::TestCase
     assert_threads_not_stuck threads
   end
 
+  def test_manual_yield
+    ready = Concurrent::CyclicBarrier.new(2)
+    done = Concurrent::CyclicBarrier.new(2)
+
+    threads = [
+      Thread.new do
+        @lock.sharing do
+          ready.wait
+          @lock.exclusive(purpose: :x) {}
+          done.wait
+        end
+      end,
+
+      Thread.new do
+        @lock.sharing do
+          ready.wait
+          @lock.yield_shares(compatible: [:x]) do
+            done.wait
+          end
+        end
+      end,
+    ]
+
+    assert_threads_not_stuck threads
+  end
+
   def test_in_shared_section_incompatible_non_upgrading_threads_cannot_preempt_upgrading_threads
     scratch_pad       = []
     scratch_pad_mutex = Mutex.new