Merge pull request rails#46519 from Shopify/skip-adapter-synchronization

AbstractAdapter: only synchronize when necessary

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