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

1 files changed, 17 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