Guard unprepared_statement blocks with the connection lock

In system tests, a single database connection is shared among all
the server threads. A call to unprepared_statement temporarily
modifies an instance variable on the connection object, which is
then visible to other concurrently running threads. This leads to
a situation where prepared statements may end up with the wrong
binds.

Addresses rails#36763

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -258,10 +258,12 @@ def seconds_idle # :nodoc:
       end
 
       def unprepared_statement
-        old_prepared_statements, @prepared_statements = @prepared_statements, false
-        yield
-      ensure
-        @prepared_statements = old_prepared_statements
+        @lock.synchronize do
+          old_prepared_statements, @prepared_statements = @prepared_statements, false
+          yield
+        ensure
+          @prepared_statements = old_prepared_statements
+        end
       end
 
       # Returns the human-readable name of the adapter. Use mixed case - one

activerecord/test/cases/statement_cache_test.rb

@@ -111,6 +111,15 @@ def test_unprepared_statements_dont_share_a_cache_with_prepared_statements
       assert_not_equal book, other_book
     end
 
+    def test_unprepared_statements_lock_connection
+      connection = Book.connection
+      lock = connection.lock
+
+      connection.unprepared_statement do
+        assert(lock.mon_locked? && lock.mon_owned?)
+      end
+    end
+
     def test_find_by_does_not_use_statement_cache_if_table_name_is_changed
       book = Book.create(name: "my book")