fix(billing): 防止余额计费持续透支

backend/internal/repository/usage_billing_repo.go

@@ -113,11 +113,12 @@ func (r *usageBillingRepository) applyUsageBillingEffects(ctx context.Context, t
 	}
 
 	if cmd.BalanceCost > 0 {
-		newBalance, err := deductUsageBillingBalance(ctx, tx, cmd.UserID, cmd.BalanceCost)
+		newBalance, sufficient, err := deductUsageBillingBalance(ctx, tx, cmd.UserID, cmd.BalanceCost)
 		if err != nil {
 			return err
 		}
 		result.NewBalance = &newBalance
+		result.BalanceOverdrafted = !sufficient
 	}
 
 	if cmd.APIKeyQuotaCost > 0 {
@@ -173,9 +174,23 @@ func incrementUsageBillingSubscription(ctx context.Context, tx *sql.Tx, subscrip
 	return service.ErrSubscriptionNotFound
 }
 
-func deductUsageBillingBalance(ctx context.Context, tx *sql.Tx, userID int64, amount float64) (float64, error) {
+func deductUsageBillingBalance(ctx context.Context, tx *sql.Tx, userID int64, amount float64) (float64, bool, error) {
 	var newBalance float64
 	err := tx.QueryRowContext(ctx, `
+		UPDATE users
+		SET balance = balance - $1,
+			updated_at = NOW()
+		WHERE id = $2 AND deleted_at IS NULL AND balance >= $1
+		RETURNING balance
+	`, amount, userID).Scan(&newBalance)
+	if err == nil {
+		return newBalance, true, nil
+	}
+	if !errors.Is(err, sql.ErrNoRows) {
+		return 0, false, err
+	}
+
+	err = tx.QueryRowContext(ctx, `
 		UPDATE users
 		SET balance = balance - $1,
 			updated_at = NOW()
@@ -183,12 +198,12 @@ func deductUsageBillingBalance(ctx context.Context, tx *sql.Tx, userID int64, am
 		RETURNING balance
 	`, amount, userID).Scan(&newBalance)
 	if errors.Is(err, sql.ErrNoRows) {
-		return 0, service.ErrUserNotFound
+		return 0, false, service.ErrUserNotFound
 	}
 	if err != nil {
-		return 0, err
+		return 0, false, err
 	}
-	return newBalance, nil
+	return newBalance, false, nil
 }
 
 func incrementUsageBillingAPIKeyQuota(ctx context.Context, tx *sql.Tx, apiKeyID int64, amount float64) (bool, error) {

backend/internal/repository/usage_billing_repo_unit_test.go

@@ -0,0 +1,119 @@
+//go:build unit
+
+package repository
+
+import (
+	"context"
+	"database/sql"
+	"testing"
+
+	"github.com/DATA-DOG/go-sqlmock"
+	"github.com/stretchr/testify/require"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+)
+
+const (
+	conditionalBalanceDeductSQL = `(?s)UPDATE users\s+SET balance = balance - \$1,\s+updated_at = NOW\(\)\s+WHERE id = \$2 AND deleted_at IS NULL AND balance >= \$1\s+RETURNING balance`
+	overdraftBalanceDeductSQL   = `(?s)UPDATE users\s+SET balance = balance - \$1,\s+updated_at = NOW\(\)\s+WHERE id = \$2 AND deleted_at IS NULL\s+RETURNING balance`
+)
+
+func TestDeductUsageBillingBalance_UsesSufficientBalanceGuard(t *testing.T) {
+	ctx := context.Background()
+	db, mock, err := sqlmock.New()
+	require.NoError(t, err)
+	defer func() { _ = db.Close() }()
+
+	mock.ExpectBegin()
+	tx, err := db.BeginTx(ctx, nil)
+	require.NoError(t, err)
+	mock.ExpectQuery(conditionalBalanceDeductSQL).
+		WithArgs(2.5, int64(42)).
+		WillReturnRows(sqlmock.NewRows([]string{"balance"}).AddRow(7.5))
+	mock.ExpectCommit()
+
+	newBalance, sufficient, err := deductUsageBillingBalance(ctx, tx, 42, 2.5)
+	require.NoError(t, err)
+	require.True(t, sufficient)
+	require.InDelta(t, 7.5, newBalance, 0.000001)
+	require.NoError(t, tx.Commit())
+	require.NoError(t, mock.ExpectationsWereMet())
+}
+
+func TestDeductUsageBillingBalance_RecordsOverdraftWhenGuardMisses(t *testing.T) {
+	ctx := context.Background()
+	db, mock, err := sqlmock.New()
+	require.NoError(t, err)
+	defer func() { _ = db.Close() }()
+
+	mock.ExpectBegin()
+	tx, err := db.BeginTx(ctx, nil)
+	require.NoError(t, err)
+	mock.ExpectQuery(conditionalBalanceDeductSQL).
+		WithArgs(10.0, int64(42)).
+		WillReturnError(sql.ErrNoRows)
+	mock.ExpectQuery(overdraftBalanceDeductSQL).
+		WithArgs(10.0, int64(42)).
+		WillReturnRows(sqlmock.NewRows([]string{"balance"}).AddRow(-5.0))
+	mock.ExpectCommit()
+
+	newBalance, sufficient, err := deductUsageBillingBalance(ctx, tx, 42, 10)
+	require.NoError(t, err)
+	require.False(t, sufficient)
+	require.InDelta(t, -5.0, newBalance, 0.000001)
+	require.NoError(t, tx.Commit())
+	require.NoError(t, mock.ExpectationsWereMet())
+}
+
+func TestApplyUsageBillingEffects_FlagsBalanceOverdraft(t *testing.T) {
+	ctx := context.Background()
+	db, mock, err := sqlmock.New()
+	require.NoError(t, err)
+	defer func() { _ = db.Close() }()
+
+	mock.ExpectBegin()
+	tx, err := db.BeginTx(ctx, nil)
+	require.NoError(t, err)
+	mock.ExpectQuery(conditionalBalanceDeductSQL).
+		WithArgs(10.0, int64(42)).
+		WillReturnError(sql.ErrNoRows)
+	mock.ExpectQuery(overdraftBalanceDeductSQL).
+		WithArgs(10.0, int64(42)).
+		WillReturnRows(sqlmock.NewRows([]string{"balance"}).AddRow(-5.0))
+	mock.ExpectCommit()
+
+	result := &service.UsageBillingApplyResult{Applied: true}
+	err = (&usageBillingRepository{}).applyUsageBillingEffects(ctx, tx, &service.UsageBillingCommand{
+		UserID:      42,
+		BalanceCost: 10,
+	}, result)
+	require.NoError(t, err)
+	require.NotNil(t, result.NewBalance)
+	require.InDelta(t, -5.0, *result.NewBalance, 0.000001)
+	require.True(t, result.BalanceOverdrafted)
+	require.NoError(t, tx.Commit())
+	require.NoError(t, mock.ExpectationsWereMet())
+}
+
+func TestDeductUsageBillingBalance_ReturnsUserNotFoundWhenNoUserUpdated(t *testing.T) {
+	ctx := context.Background()
+	db, mock, err := sqlmock.New()
+	require.NoError(t, err)
+	defer func() { _ = db.Close() }()
+
+	mock.ExpectBegin()
+	tx, err := db.BeginTx(ctx, nil)
+	require.NoError(t, err)
+	mock.ExpectQuery(conditionalBalanceDeductSQL).
+		WithArgs(10.0, int64(42)).
+		WillReturnError(sql.ErrNoRows)
+	mock.ExpectQuery(overdraftBalanceDeductSQL).
+		WithArgs(10.0, int64(42)).
+		WillReturnError(sql.ErrNoRows)
+	mock.ExpectRollback()
+
+	_, _, err = deductUsageBillingBalance(ctx, tx, 42, 10)
+	require.ErrorIs(t, err, service.ErrUserNotFound)
+	require.NoError(t, tx.Rollback())
+	require.NoError(t, mock.ExpectationsWereMet())
+}

backend/internal/repository/user_repo.go

@@ -757,6 +757,17 @@ func (r *userRepository) UpdateBalance(ctx context.Context, id int64, amount flo
 func (r *userRepository) DeductBalance(ctx context.Context, id int64, amount float64) error {
 	client := clientFromContext(ctx, r.client)
 	n, err := client.User.Update().
+		Where(dbuser.IDEQ(id), dbuser.BalanceGTE(amount)).
+		AddBalance(-amount).
+		Save(ctx)
+	if err != nil {
+		return err
+	}
+	if n > 0 {
+		return nil
+	}
+
+	n, err = client.User.Update().
 		Where(dbuser.IDEQ(id)).
 		AddBalance(-amount).
 		Save(ctx)