第四节 db.View()实现分析

View()主要用来执行只读事务。事务的开启、提交、回滚都交由tx控制。

// View executes a function within the context of a managed read-only transaction.
// Any error that is returned from the function is returned from the View() method.
//
// Attempting to manually rollback within the function will cause a panic.
func (db *DB) View(fn func(*Tx) error) error {
    t, err := db.Begin(false)
    if err != nil {
        return err
    }

    // Make sure the transaction rolls back in the event of a panic.
    defer func() {
        if t.db != nil {
            t.rollback()
        }
    }()

    // Mark as a managed tx so that the inner function cannot manually rollback.
    t.managed = true

    // If an error is returned from the function then pass it through.
    err = fn(t)
    t.managed = false
    if err != nil {
        _ = t.Rollback()
        return err
    }

    if err := t.Rollback(); err != nil {
        return err
    }

    return nil
}