fix: SpannerAdapter requires prepared statements to be enabled (#323)

* fix: `SpannerAdapter` requires prepared statements to be enabled

The adapter relies on prepared statements to extract Spanner native
request options from the binds in the query engine call stack.

This is at odds with the query tags feature in Rails 7.1, as it disables
prepared statements for all adapters when it is enabled. This in turn

The spanner AR adapter relies on prepared statements to extract native
database functionality on the binds. The query tags feature, [when enabled](https://github.com/rails/rails/blob/7-1-stable/activerecord/lib/active_record/railtie.rb#L419),
[disables prepared statements](https://github.com/rails/rails/blob/main/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb#L160-L162),
breaking the Spanner adapters queries, like inserts and updates.

* test: verify that prepared statement is used

* fix: only disable prepared statements if supported

---------

Co-authored-by: Knut Olav Løite <[email protected]>

lib/active_record/connection_adapters/spanner_adapter.rb

@@ -74,6 +74,9 @@ def initialize connection, logger, connection_options, config
         @connection_options = connection_options
         super connection, logger, config
         @raw_connection ||= connection
+
+        # Spanner does not support unprepared statements
+        @prepared_statements = true
       end
 
       def max_identifier_length

test/activerecord_spanner_mock_server/spanner_active_record_with_mock_server_test.rb

@@ -78,6 +78,39 @@ def test_insert!
       assert_equal "id", mutation.insert.columns[2]
     end
 
+    def test_insert_with_disabled_prepared_statements
+      if ActiveRecord.respond_to?(:disable_prepared_statements)
+        ActiveRecord.disable_prepared_statements = true
+        ActiveRecord::Base.establish_connection(
+          adapter: "spanner",
+          emulator_host: "localhost:#{@port}",
+          project: "test-project",
+          instance: "test-instance",
+          database: "testdb",
+        )
+        assert ActiveRecord::Base.connection.prepared_statements?
+      end
+
+      insert_sql = "INSERT INTO `singers` (`first_name`, `last_name`, `id`) VALUES (@p1, @p2, @p3)"
+      @mock.put_statement_result insert_sql, StatementResult.new(1)
+      ActiveRecord::Base.transaction do
+        singer = { first_name: "Alice", last_name: "Ecila" }
+        Singer.create singer
+      end
+
+      requests = @mock.requests
+      request = requests.select { |req| req.is_a?(ExecuteSqlRequest) && req.sql == insert_sql }.first
+      assert_equal "Alice", request.params["p1"]
+      assert_equal "Ecila", request.params["p2"]
+      assert_equal :STRING, request.param_types["p1"].code
+      assert_equal :STRING, request.param_types["p2"].code
+      assert_equal :INT64, request.param_types["p3"].code
+    ensure
+      if ActiveRecord.respond_to?(:disable_prepared_statements)
+        ActiveRecord.disable_prepared_statements = false
+      end
+    end
+
     def test_upsert
       singer = { first_name: "Alice", last_name: "Ecila" }
       singer = Singer.upsert singer