From eccc533e7490cb858ff6995eee1e8ad96b796b32 Mon Sep 17 00:00:00 2001
From: Brent Wheeldon <brent.wheeldon@gmail.com>
Date: Thu, 2 Nov 2017 14:53:43 -0400
Subject: [PATCH] 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
---
 .../connection_adapters/abstract_adapter.rb   |  3 +-
 .../load_interlock_aware_monitor.rb           | 17 ++++++
 .../load_interlock_aware_monitor_test.rb      | 55 +++++++++++++++++++
 3 files changed, 74 insertions(+), 1 deletion(-)
 create mode 100644 activesupport/lib/active_support/concurrency/load_interlock_aware_monitor.rb
 create mode 100644 activesupport/test/concurrency/load_interlock_aware_monitor_test.rb

diff --git a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
index 84fd812392f5a..e523a97aa5d51 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
@@ -4,6 +4,7 @@
 require "active_record/connection_adapters/sql_type_metadata"
 require "active_record/connection_adapters/abstract/schema_dumper"
 require "active_record/connection_adapters/abstract/schema_creation"
+require "active_support/concurrency/load_interlock_aware_monitor"
 require "arel/collectors/bind"
 require "arel/collectors/sql_string"
 
@@ -105,7 +106,7 @@ def initialize(connection, logger = nil, config = {}) # :nodoc:
         @schema_cache        = SchemaCache.new self
         @quoted_column_names, @quoted_table_names = {}, {}
         @visitor = arel_visitor
-        @lock = Monitor.new
+        @lock = ActiveSupport::Concurrency::LoadInterlockAwareMonitor.new
 
         if self.class.type_cast_config_to_boolean(config.fetch(:prepared_statements) { true })
           @prepared_statements = true
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 0000000000000..a8455c00483f7
--- /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 0000000000000..2d0f45ec5f380
--- /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