Make prepared statement status thread and instance-specific

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -10,6 +10,7 @@
 require "arel/collectors/composite"
 require "arel/collectors/sql_string"
 require "arel/collectors/substitute_binds"
+require "concurrent/atomic/thread_local_var"
 
 module ActiveRecord
   module ConnectionAdapters # :nodoc:
@@ -77,7 +78,7 @@ class AbstractAdapter
       SIMPLE_INT = /\A\d+\z/
 
       attr_accessor :pool
-      attr_reader :visitor, :owner, :logger, :lock, :prepared_statements
+      attr_reader :visitor, :owner, :logger, :lock
       alias :in_use? :owner
 
       set_callback :checkin, :after, :enable_lazy_transactions!
@@ -128,10 +129,10 @@ def initialize(connection, logger = nil, config = {}) # :nodoc:
         @lock = ActiveSupport::Concurrency::LoadInterlockAwareMonitor.new
 
         if self.class.type_cast_config_to_boolean(config.fetch(:prepared_statements) { true })
-          @prepared_statements = true
+          @prepared_statement_status = Concurrent::ThreadLocalVar.new(true)
           @visitor.extend(DetermineIfPreparableVisitor)
         else
-          @prepared_statements = false
+          @prepared_statement_status = Concurrent::ThreadLocalVar.new(false)
         end
 
         @advisory_locks_enabled = self.class.type_cast_config_to_boolean(
@@ -174,6 +175,10 @@ def schema_migration # :nodoc:
                               end
       end
 
+      def prepared_statements
+        @prepared_statement_status.value
+      end
+
       class Version
         include Comparable
 
@@ -258,10 +263,7 @@ def seconds_idle # :nodoc:
       end
 
       def unprepared_statement
-        old_prepared_statements, @prepared_statements = @prepared_statements, false
-        yield
-      ensure
-        @prepared_statements = old_prepared_statements
+        @prepared_statement_status.bind(false) { yield }
       end
 
       # Returns the human-readable name of the adapter. Use mixed case - one

activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb

@@ -39,8 +39,8 @@ class Mysql2Adapter < AbstractMysqlAdapter
       include MySQL::DatabaseStatements
 
       def initialize(connection, logger, connection_options, config)
-        super
-        @prepared_statements = false unless config.key?(:prepared_statements)
+        superclass_config = config.reverse_merge(prepared_statements: false)
+        super(connection, logger, connection_options, superclass_config)
         configure_connection
       end
 

activerecord/test/cases/prepared_statement_status_test.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "models/course"
+require "models/entrant"
+
+module ActiveRecord
+  class PreparedStatementStatusTest < ActiveRecord::TestCase
+    def test_prepared_statement_status_is_thread_and_instance_specific
+      course_conn = Course.connection
+      entrant_conn = Entrant.connection
+
+      inside = Concurrent::Event.new
+      preventing = Concurrent::Event.new
+      finished = Concurrent::Event.new
+
+      assert_not_same course_conn, entrant_conn
+
+      if current_adapter?(:Mysql2Adapter)
+        # The mysql adapter does not use prepared
+        # statements by default.
+        assert_not course_conn.prepared_statements
+        assert_not entrant_conn.prepared_statements
+      else
+        t1 = Thread.new do
+          course_conn.unprepared_statement do
+            inside.set
+            preventing.wait
+            assert_not course_conn.prepared_statements
+            assert entrant_conn.prepared_statements
+            finished.set
+          end
+        end
+
+        t2 = Thread.new do
+          entrant_conn.unprepared_statement do
+            inside.wait
+            assert course_conn.prepared_statements
+            assert_not entrant_conn.prepared_statements
+            preventing.set
+            finished.wait
+          end
+        end
+
+        t1.join
+        t2.join
+      end
+    end
+  end
+end