AbstractAdapter: only synchronize when necessary

Ref: rails#28083

Right now all database operations are synchronized with a monitor even though
it's useless in the vast majority of cases.

The synchronization is only useful when transactional fixtures are enabled.

In production, connections are never shared across threads, as such this extra
synchronization is wasteful.

Author: byroot

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -173,6 +173,10 @@ def lock_thread=(lock_thread)
         else
           @lock_thread = nil
         end
+
+        if (active_connection = @thread_cached_conns[connection_cache_key(current_thread)])
+          active_connection.synchronized = lock_thread
+        end
       end
 
       # Retrieve the connection associated with the current thread, or call
@@ -352,7 +356,9 @@ def clear_reloadable_connections!
       # Raises:
       # - ActiveRecord::ConnectionTimeoutError no connection can be obtained from the pool.
       def checkout(checkout_timeout = @checkout_timeout)
-        checkout_and_verify(acquire_connection(checkout_timeout))
+        connection = checkout_and_verify(acquire_connection(checkout_timeout))
+        connection.synchronized = @lock_thread
+        connection
       end
 
       # Check-in a database connection back into the pool, indicating that you

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/null_lock"
 require "active_support/concurrency/load_interlock_aware_monitor"
 require "arel/collectors/bind"
 require "arel/collectors/composite"
@@ -155,7 +156,7 @@ def initialize(config_or_deprecated_connection, deprecated_logger = nil, depreca
         @idle_since = Process.clock_gettime(Process::CLOCK_MONOTONIC)
         @visitor = arel_visitor
         @statements = build_statement_pool
-        @lock = ActiveSupport::Concurrency::LoadInterlockAwareMonitor.new
+        self.synchronized = false
 
         @prepared_statements = self.class.type_cast_config_to_boolean(
           @config.fetch(:prepared_statements) { default_prepared_statements }
@@ -171,6 +172,14 @@ def initialize(config_or_deprecated_connection, deprecated_logger = nil, depreca
         @verified = false
       end
 
+      def synchronized=(synchronized) # :nodoc:
+        @lock = if synchronized
+          ActiveSupport::Concurrency::LoadInterlockAwareMonitor.new
+        else
+          ActiveSupport::Concurrency::NullLock
+        end
+      end
+
       EXCEPTION_NEVER = { Exception => :never }.freeze # :nodoc:
       EXCEPTION_IMMEDIATE = { Exception => :immediate }.freeze # :nodoc:
       private_constant :EXCEPTION_NEVER, :EXCEPTION_IMMEDIATE

activesupport/lib/active_support/concurrency/null_lock.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module ActiveSupport
+  module Concurrency
+    module NullLock # :nodoc:
+      extend self
+
+      def synchronize
+        yield
+      end
+    end
+  end
+end