am-downloader/credits.go

package main

import (
	"errors"
	"fmt"
	"strconv"
	"time"

	"gorm.io/gorm"
)

var (
	errInsufficientCredits = errors.New("insufficient credits")
	errReservationState    = errors.New("invalid credit reservation state")
)

// reserveCredit places one credit on hold for a task. The hold is released on
// failure or converted into a debit after a successful new download.
func (a *App) reserveCredit(userID, taskID uint, amount int64) (*CreditReservation, error) {
	if amount <= 0 {
		return nil, fmt.Errorf("invalid credit amount")
	}
	var reservation CreditReservation
	err := a.db.Transaction(func(tx *gorm.DB) error {
		var account CreditAccount
		if err := tx.Where("user_id = ?", userID).First(&account).Error; err != nil {
			if !errors.Is(err, gorm.ErrRecordNotFound) {
				return err
			}
			account = CreditAccount{UserID: userID}
			if err := tx.Create(&account).Error; err != nil {
				return err
			}
		}
		var existing CreditReservation
		if err := tx.Where("task_id = ? AND status = ?", taskID, "reserved").Order("id desc").First(&existing).Error; err == nil {
			reservation = existing
			return nil
		} else if !errors.Is(err, gorm.ErrRecordNotFound) {
			return err
		}
		if account.Balance-account.Reserved < amount {
			return errInsufficientCredits
		}
		account.Reserved += amount
		if err := tx.Save(&account).Error; err != nil {
			return err
		}
		reservation = CreditReservation{UserID: userID, TaskID: taskID, Amount: amount, Status: "reserved"}
		if err := tx.Create(&reservation).Error; err != nil {
			return err
		}
		return tx.Create(&CreditTransaction{UserID: userID, Type: "reserve", Amount: 0, BalanceAfter: account.Balance, ReferenceType: "task", ReferenceID: strconv.FormatUint(uint64(taskID), 10), Reason: "download reservation"}).Error
	})
	return &reservation, err
}

func (a *App) commitCredit(task *Task) error {
	if task.CreditReservationID == 0 {
		return nil
	}
	return a.db.Transaction(func(tx *gorm.DB) error {
		var r CreditReservation
		if err := tx.First(&r, task.CreditReservationID).Error; err != nil {
			return err
		}
		if r.Status == "committed" {
			return nil
		}
		if r.Status != "reserved" {
			return errReservationState
		}
		var account CreditAccount
		if err := tx.Where("user_id = ?", r.UserID).First(&account).Error; err != nil {
			return err
		}
		account.Reserved -= r.Amount
		account.Balance -= r.Amount
		if account.Reserved < 0 || account.Balance < 0 {
			return errReservationState
		}
		if err := tx.Save(&account).Error; err != nil {
			return err
		}
		now := time.Now()
		r.Status, r.CompletedAt = "committed", &now
		if err := tx.Save(&r).Error; err != nil {
			return err
		}
		return tx.Create(&CreditTransaction{UserID: r.UserID, Type: "debit", Amount: -r.Amount, BalanceAfter: account.Balance, ReferenceType: "task", ReferenceID: strconv.FormatUint(uint64(task.ID), 10), Reason: "successful download"}).Error
	})
}

// refundCredit releases a reservation without debiting the user's balance.
// It is idempotent so download failure paths can safely call fail repeatedly.
func (a *App) refundCredit(task *Task) error {
	if task.CreditReservationID == 0 {
		return nil
	}
	return a.db.Transaction(func(tx *gorm.DB) error {
		var r CreditReservation
		if err := tx.First(&r, task.CreditReservationID).Error; err != nil {
			return err
		}
		if r.Status == "refunded" {
			return nil
		}
		if r.Status == "committed" {
			// A committed reservation belongs to a completed download. A later
			// storage/retry error must never debit or refund it a second time.
			return nil
		}
		if r.Status != "reserved" {
			return errReservationState
		}
		var account CreditAccount
		if err := tx.Where("user_id = ?", r.UserID).First(&account).Error; err != nil {
			return err
		}
		account.Reserved -= r.Amount
		if account.Reserved < 0 {
			return errReservationState
		}
		if err := tx.Save(&account).Error; err != nil {
			return err
		}
		now := time.Now()
		r.Status, r.CompletedAt = "refunded", &now
		if err := tx.Save(&r).Error; err != nil {
			return err
		}
		return tx.Create(&CreditTransaction{UserID: r.UserID, Type: "refund", Amount: 0, BalanceAfter: account.Balance, ReferenceType: "task", ReferenceID: strconv.FormatUint(uint64(task.ID), 10), Reason: "download failed"}).Error
	})
}

func (a *App) creditBalance(userID uint) (CreditAccount, error) {
	var account CreditAccount
	err := a.db.Where("user_id = ?", userID).First(&account).Error
	return account, err
}