Rollback does not work well with Go language transactional wrapper

I have recently started learning Go.

I found the following Github implementation of a wrapper for database transaction processing and decided to try it out.

(source) https://github.com/oreilly-japan/practical-go-programming/blob/master/ch09/transaction/wrapper/main.go

I am using PostgreSQL as the database.

Initially, it contains the following data.

testdb=> select * from products;
 product_id | price
------------+-------
 0001       |   200
 0002       |   100
 0003       |   150
 0004       |   300
(4 rows)

After Process A succeeds, Process B is intentionally made to fail, and a rollback of transaction A is expected. However, when we run it, the rollback does not occur and we end up with the following

In truth, since B failed, the process A should be rolled back and there should be no change in the database value.

I have inserted Logs in places to confirm this, but I am not sure. Why is the rollback not executed?

package main

import (
	"context"
	"database/sql"
	"fmt"
	"log"

	_ "github.com/jackc/pgx/v4/stdlib"
)

// transaction-wrapper-start
type txAdmin struct {
	*sql.DB
}

type Service struct {
	tx txAdmin
}

func (t *txAdmin) Transaction(ctx context.Context, f func(ctx context.Context) (err error)) error {
	log.Printf("transaction")
	tx, err := t.BeginTx(ctx, nil)
	if err != nil {
		return err
	}
	defer tx.Rollback()
	if err := f(ctx); err != nil {
		log.Printf("transaction err")
		return fmt.Errorf("transaction query failed: %w", err)
	}

	log.Printf("commit")
	return tx.Commit()
}

func (s *Service) UpdateProduct(ctx context.Context, productID string) error {
	updateFunc := func(ctx context.Context) error {
		log.Printf("first process")
        // Process A
		if _, err := s.tx.ExecContext(ctx, "UPDATE products SET price = 200 WHERE product_id = $1", productID); err != nil {
			log.Printf("first err")
			return err
		}
		log.Printf("second process")

        // Process B(They are intentionally failing.)
		if _, err := s.tx.ExecContext(ctx, "...", productID); err != nil {
			log.Printf("second err")
			return err
		}
		return nil
	}
	log.Printf("update")
	return s.tx.Transaction(ctx, updateFunc)
}

// transaction-wrapper-end
func main() {

	data, err := sql.Open("pgx", "host=localhost port=5432 user=testuser dbname=testdb password=password sslmode=disable")
	if nil != err {
		log.Fatal(err)
	}

	database := Service {tx: txAdmin{data}}

	ctx := context.Background()

	database.UpdateProduct(ctx, "0004")
}

output

2022/05/26 13:28:55 update     
2022/05/26 13:28:55 transaction
2022/05/26 13:28:55 first process
2022/05/26 13:28:55 second process
2022/05/26 13:28:55 second err
2022/05/26 13:28:55 transaction err

database changes(If the rollback works, the PRICE for id 0004 should remain 300.)

testdb=> select * from products;
 product_id | price
------------+-------
 0001       |   200
 0002       |   100
 0003       |   150
 0004       |   200
(4 rows)

Please tell me how I can use the wrapper to correctly process transactions.

=========
PS.
The following code without the wrapper worked properly.

package main

import (
	"context"
	"database/sql"
	"log"

	_ "github.com/jackc/pgx/v4/stdlib"
)

// transaction-defer-start
type Service struct {
	db *sql.DB
}

func (s *Service) UpdateProduct(ctx context.Context, productID string) (err error) {
	tx, err := s.db.Begin()
	if err != nil {
		return err
	}
	defer tx.Rollback()

	if _, err = tx.ExecContext(ctx, "UPDATE products SET price = 200 WHERE product_id = $1", productID); err != nil {
		log.Println("update err")
		return err
	}

	if _, err = tx.ExecContext(ctx, "...", productID); err != nil {
		log.Println("update err")
		return err
	}

	return tx.Commit()
}

// transaction-defer-end
func main() {
	var database Service
	dbConn, err := sql.Open("pgx", "host=localhost port=5432 user=testuser dbname=testdb password=passs sslmode=disable")
	if nil != err {
		log.Fatal(err)
	}
	database.db = dbConn

	ctx := context.Background()

	database.UpdateProduct(ctx, "0004")
	
}

As @Richard Huxton said, pass tx into a function f

here are the steps:

1. add one field on struct txAdmin to accommodate *sql.Tx, so txAdmin have DB and Tx fields
2. inside Transaction set tx to *txAdmin.Tx
3. inside UpdateProduct use *Service.tx.Tx for every query

so the final code looks like this:

package main

import (
	"context"
	"database/sql"
	"fmt"
	"log"

	_ "github.com/jackc/pgx/v4/stdlib"
)

// transaction-wrapper-start
type txAdmin struct {
	*sql.DB
	*sql.Tx
}

type Service struct {
	tx txAdmin
}

func (t *txAdmin) Transaction(ctx context.Context, f func(ctx context.Context) (err error)) error {
	log.Printf("transaction")
	tx, err := t.DB.BeginTx(ctx, nil)
	if err != nil {
		return err
	}

    // set tx to Tx
	t.Tx = tx

	defer tx.Rollback()
	if err := f(ctx); err != nil {
		log.Printf("transaction err")
		return fmt.Errorf("transaction query failed: %w", err)
	}

	log.Printf("commit")
	return tx.Commit()
}

func (s *Service) UpdateProduct(ctx context.Context, productID string) error {
	updateFunc := func(ctx context.Context) error {
		log.Printf("first process")
		// Process A
		if _, err := s.tx.Tx.ExecContext(ctx, "UPDATE products SET price = 200 WHERE product_id = $1", productID); err != nil {
			log.Printf("first err")
			return err
		}
		log.Printf("second process")

		// Process B(They are intentionally failing.)
		if _, err := s.tx.Tx.ExecContext(ctx, "...", productID); err != nil {
			log.Printf("second err")
			return err
		}
		return nil
	}
	log.Printf("update")
	return s.tx.Transaction(ctx, updateFunc)
}

// transaction-wrapper-end
func main() {
	data, err := sql.Open("pgx", "host=localhost port=5432 user=testuser dbname=testdb password=password sslmode=disable")
	if nil != err {
		log.Fatal(err)
	}

	database := Service{tx: txAdmin{DB: data}}

	ctx := context.Background()

	database.UpdateProduct(ctx, "0004")
}