Prevent deadlocks with load interlock and DB lock.

This fixes an issue where competing threads deadlock each other.

- Thread A holds the load interlock but is blocked on getting the DB lock
- Thread B holds the DB lock but is blocked on getting the load interlock (for example when there is a `Model.transaction` block that needs to autoload)

This solution allows for dependency loading in other threads while a thread is waiting to acquire the DB lock.

Fixes #31019

2 files changed, 72 insertions, 0 deletions

diff --git a/activesupport/lib/active_support/concurrency/load_interlock_aware_monitor.rb b/activesupport/lib/active_support/concurrency/load_interlock_aware_monitor.rb
new file mode 100644
index 0000000000..a8455c0048
--- /dev/null
+++ b/activesupport/lib/active_support/concurrency/load_interlock_aware_monitor.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require "monitor"
+
+module ActiveSupport
+  module Concurrency
+    # A monitor that will permit dependency loading while blocked waiting for
+    # the lock.
+    class LoadInterlockAwareMonitor < Monitor
+      # Enters an exclusive section, but allows dependency loading while blocked
+      def mon_enter
+        mon_try_enter ||
+          ActiveSupport::Dependencies.interlock.permit_concurrent_loads { super }
+      end
+    end
+  end
+end
diff --git a/activesupport/test/concurrency/load_interlock_aware_monitor_test.rb b/activesupport/test/concurrency/load_interlock_aware_monitor_test.rb
new file mode 100644
index 0000000000..2d0f45ec5f
--- /dev/null
+++ b/activesupport/test/concurrency/load_interlock_aware_monitor_test.rb
@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+require "abstract_unit"
+require "concurrent/atomic/count_down_latch"
+require "active_support/concurrency/load_interlock_aware_monitor"
+
+module ActiveSupport
+  module Concurrency
+    class LoadInterlockAwareMonitorTest < ActiveSupport::TestCase
+      def setup
+        @monitor = ActiveSupport::Concurrency::LoadInterlockAwareMonitor.new
+      end
+
+      def test_entering_with_no_blocking
+        assert @monitor.mon_enter
+      end
+
+      def test_entering_with_blocking
+        load_interlock_latch = Concurrent::CountDownLatch.new
+        monitor_latch = Concurrent::CountDownLatch.new
+
+        able_to_use_monitor = false
+        able_to_load = false
+
+        thread_with_load_interlock = Thread.new do
+          ActiveSupport::Dependencies.interlock.running do
+            load_interlock_latch.count_down
+            monitor_latch.wait
+
+            @monitor.synchronize do
+              able_to_use_monitor = true
+            end
+          end
+        end
+
+        thread_with_monitor_lock = Thread.new do
+          @monitor.synchronize do
+            monitor_latch.count_down
+            load_interlock_latch.wait
+
+            ActiveSupport::Dependencies.interlock.loading do
+              able_to_load = true
+            end
+          end
+        end
+
+        thread_with_load_interlock.join
+        thread_with_monitor_lock.join
+
+        assert able_to_use_monitor
+        assert able_to_load
+      end
+    end
+  end
+end