Merge pull request #36868 from eileencodes/make-prevent-writes-threadsafe

Fix thread safety of prevent_writes

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

@@ -1005,28 +1005,33 @@ def self.discard_unowned_pools(pid_map) # :nodoc:
         end
       end
 
-      attr_reader :prevent_writes
-
       def initialize
         # These caches are keyed by spec.name (ConnectionSpecification#name).
         @owner_to_pool = ConnectionHandler.create_owner_to_pool
-        @prevent_writes = false
 
         # Backup finalizer: if the forked child never needed a pool, the above
         # early discard has not occurred
         ObjectSpace.define_finalizer self, ConnectionHandler.unowned_pool_finalizer(@owner_to_pool)
       end
 
+      def prevent_writes # :nodoc:
+        Thread.current[:prevent_writes]
+      end
+
+      def prevent_writes=(prevent_writes) # :nodoc:
+        Thread.current[:prevent_writes] = prevent_writes
+      end
+
       # Prevent writing to the database regardless of role.
       #
       # In some cases you may want to prevent writes to the database
       # even if you are on a database that can write. `while_preventing_writes`
       # will prevent writes to the database for the duration of the block.
       def while_preventing_writes(enabled = true)
-        original, @prevent_writes = @prevent_writes, enabled
+        original, self.prevent_writes = self.prevent_writes, enabled
         yield
       ensure
-        @prevent_writes = original
+        self.prevent_writes = original
       end
 
       def connection_pool_list

activerecord/test/cases/database_selector_test.rb

@@ -143,12 +143,12 @@ def test_preventing_writes_turns_off_for_primary_write
       write = false
       resolver.read do
         assert ActiveRecord::Base.connected_to?(role: :writing)
-        assert ActiveRecord::Base.connection_handler.prevent_writes
+        assert_predicate ActiveRecord::Base.connection, :preventing_writes?
         read = true
 
         resolver.write do
           assert ActiveRecord::Base.connected_to?(role: :writing)
-          assert_not ActiveRecord::Base.connection_handler.prevent_writes
+          assert_not_predicate ActiveRecord::Base.connection, :preventing_writes?
           write = true
         end
       end
@@ -157,6 +157,43 @@ def test_preventing_writes_turns_off_for_primary_write
       assert read
     end
 
+    def test_preventing_writes_works_in_a_threaded_environment
+      resolver = ActiveRecord::Middleware::DatabaseSelector::Resolver.new(@session, delay: 5.seconds)
+      inside_preventing = Concurrent::Event.new
+      finished_checking = Concurrent::Event.new
+
+      @session.update_last_write_timestamp
+
+      t1 = Thread.new do
+        resolver.read do
+          inside_preventing.wait
+          assert ActiveRecord::Base.connected_to?(role: :writing)
+          assert_predicate ActiveRecord::Base.connection, :preventing_writes?
+          finished_checking.set
+        end
+      end
+
+      t2 = Thread.new do
+        resolver.write do
+          assert ActiveRecord::Base.connected_to?(role: :writing)
+          assert_not_predicate ActiveRecord::Base.connection, :preventing_writes?
+          inside_preventing.set
+          finished_checking.wait
+        end
+      end
+
+      t3 = Thread.new do
+        resolver.read do
+          assert ActiveRecord::Base.connected_to?(role: :writing)
+          assert_predicate ActiveRecord::Base.connection, :preventing_writes?
+        end
+      end
+
+      t1.join
+      t2.join
+      t3.join
+    end
+
     def test_read_from_replica_with_no_delay
       resolver = ActiveRecord::Middleware::DatabaseSelector::Resolver.new(@session, delay: 0.seconds)