backend/pkg/library/cassandra/lock.go

package cassandra

import (
	"context"
	"errors"
	"fmt"
	"time"

	"github.com/gocql/gocql"
	"github.com/scylladb/gocqlx/v2/qb"
)

const (
	defaultLockTTLSec = 30
	defaultLockRetry  = 3
	lockBaseDelay     = 100 * time.Millisecond
)

// LockOption 用來設定 TryLock 的 TTL 行為
type LockOption func(*lockOptions)

type lockOptions struct {
	ttlSeconds int // TTL，單位秒；<=0 代表不 expire
}

// WithLockTTL 設定鎖的 TTL
func WithLockTTL(d time.Duration) LockOption {
	return func(o *lockOptions) {
		o.ttlSeconds = int(d.Seconds())
	}
}

// WithNoLockExpire 永不自動解鎖
func WithNoLockExpire() LockOption {
	return func(o *lockOptions) {
		o.ttlSeconds = 0
	}
}

// TryLock 嘗試在表上插入一筆唯一鍵（IF NOT EXISTS）作為鎖
// 預設 30 秒 TTL，可透過 option 調整或取消 TTL
func (r *repository[T]) TryLock(ctx context.Context, doc T, opts ...LockOption) error {
	// 組合 option
	options := &lockOptions{ttlSeconds: defaultLockTTLSec}
	for _, opt := range opts {
		opt(options)
	}

	// 建 TTL 子句
	builder := qb.Insert(r.table).
		Unique(). // IF NOT EXISTS
		Columns(r.metadata.Columns...)

	if options.ttlSeconds > 0 {
		ttl := time.Duration(options.ttlSeconds) * time.Second
		builder = builder.TTL(ttl)
	}
	stmt, names := builder.ToCql()

	// 執行 CAS
	q := r.db.session.Query(stmt, names).BindStruct(doc).
		WithContext(ctx).
		WithTimestamp(time.Now().UnixNano() / 1e3).
		SerialConsistency(gocql.Serial)

	applied, err := q.ExecCASRelease()
	if err != nil {
		return ErrInvalidInput.WithTable(r.table).WithError(err)
	}

	if !applied {
		return NewError(ErrCodeConflict, "acquire lock failed").WithTable(r.table)
	}
	return nil
}

// UnLock 釋放鎖，其實就是 Delete
func (r *repository[T]) UnLock(ctx context.Context, doc T) error {
	var lastErr error

	for i := 0; i < defaultLockRetry; i++ {
		builder := qb.Delete(r.table).Existing()

		// 動態添加 WHERE 條件（使用 Partition Key）
		for _, key := range r.metadata.PartKey {
			builder = builder.Where(qb.Eq(key))
		}
		stmt, names := builder.ToCql()
		q := r.db.session.Query(stmt, names).BindStruct(doc).
			WithContext(ctx).
			WithTimestamp(time.Now().UnixNano() / 1e3).
			SerialConsistency(gocql.Serial)

		applied, err := q.ExecCASRelease()
		if err == nil && applied {
			return nil
		}

		if err != nil {
			lastErr = fmt.Errorf("unlock error: %w", err)
		} else if !applied {
			lastErr = fmt.Errorf("unlock not applied: row not found or not visible yet")
		}

		time.Sleep(lockBaseDelay * time.Duration(1<<i)) // 100ms → 200ms → 400ms
	}

	return ErrInvalidInput.WithTable(r.table).WithError(
		fmt.Errorf("unlock failed after %d retries: %w", defaultLockRetry, lastErr),
	)
}

// IsLockFailed 檢查錯誤是否為獲取鎖失敗
func IsLockFailed(err error) bool {
	var e *Error
	if errors.As(err, &e) {
		return e.Code == ErrCodeConflict && e.Message == "acquire lock failed"
	}
	return false
}