sqlite事务嵌套

sqlite本身的事务并不支持嵌套，而savepoint方法虽然支持嵌套，但是，使用起来比较复杂。下面，提供一种针对线程数据库连接的事务嵌套模式，如果是在单线程中操作数据库，则可以进一步简化实现 （采用静态变量保存事务计数即可）。

在代码中，使用了windows下的线程槽的概念，其是针对每个线程保留单独的一份数据。

#include "sqlite3.h"
/**
 * @class LockHelp
 * @brief a lock class
 */
class CLockHelp
{
public:
 CLockHelp()
 {
  ::InitializeCriticalSection(&m_CriticalSection);
 }
 ~CLockHelp()
 {
  ::DeleteCriticalSection(&m_CriticalSection);
 }

 /// lock
 void lock()
 {
  ::EnterCriticalSection(&m_CriticalSection);
 }

 /// unlock
 void unlock()
 {
  ::LeaveCriticalSection(&m_CriticalSection);
 }
private:
 CLockHelp(const NLock&);
 CLockHelp& operator=(const NLock&);

private:
 CRITICAL_SECTION m_CriticalSection;
};

/**
 * @class CTransaction
 * @brief 启动、回滚、提交事务操作
 */
class CTransaction
{
public:
    CTransaction(sqlite *pConnection);
    ~CTransaction(void);

public:
    /**
     * @brief   获取m_dwTLSRollbackTrans，表明数据库操作出错
     * @retval  TLS中的值
     */
    void SetRollbackCounter();

private:
    /**
     * @brief   获取TLS中的值
     * @param   dwIdx: TLS索引
     * @retval  TLS中的值
     */
    INT64 GetCounter(DWORD dwIdx);

    /**
     * @brief   设置TLS中的值
     * @param   dwIdx: TLS索引
     * @param   nCount: 待设定的值
     */
    void SetCounter(DWORD dwIdx,INT64 nCount);

private:
    /// 线程局部存储变量
    /// 是否调用BeginTransaction()
    static DWORD m_dwTLSBeginTransIdx;

    /// 是否调用RollbackTransaction()
    static DWORD m_dwTLSRollbackTransIdx;
    
    /// 线程锁
    static CLockHelp m_CriticalLock;

    /// CTableHandler对象
    sqlite3 *m_pConnection;
};


#include "Transaction.h"

DWORD CTransaction::m_dwTLSBeginTransIdx = ::TlsAlloc();
DWORD CTransaction::m_dwTLSRollbackTransIdx = ::TlsAlloc();
CLockHelp CTransaction::m_CriticalLock;

CTransaction::CTransaction(CTableHandler *pTableHandler)
: m_pTableHandler(pTableHandler)
{
    // 加锁
    m_CriticalLock.lock();

    INT64 nBeginTrans(GetCounter(m_dwTLSBeginTransIdx));
    if (nBeginTrans == 0)
    {
        if (!m_pTableHandler->BeginTransaction())
        {
            // 设置为0
            SetCounter(m_dwTLSBeginTransIdx,nBeginTrans);
            return;
        }
    }

    SetCounter(m_dwTLSBeginTransIdx,nBeginTrans + 1);
}

CTransaction::~CTransaction(void)
{
    INT64 nEndTrans(GetCounter(m_dwTLSBeginTransIdx));
    if (nEndTrans == 0)
    {
        // 启动事务失败
        INT64 nRollbackTrans(GetCounter(m_dwTLSRollbackTransIdx));
        if (nRollbackTrans == 1)
        {
            SetCounter(m_dwTLSRollbackTransIdx,0);
        }

        // 解锁
        m_CriticalLock.unlock();
        return;
    }
    
    if (nEndTrans == 1)
    {
        INT64 nRollbackTrans(GetCounter(m_dwTLSRollbackTransIdx));
        if (nRollbackTrans == 1)
        {
           m_pTableHandler->RollbackTransaction();
           SetCounter(m_dwTLSRollbackTransIdx,0);
        }
        else
        {
            m_pTableHandler->CommitTransaction();
        }
    }

    SetCounter(m_dwTLSBeginTransIdx,nEndTrans - 1);

    // 解锁
    m_CriticalLock.unlock();
}

void CTransaction::SetRollbackCounter()
{
     SetCounter(m_dwTLSRollbackTransIdx,1);
}

INT64 CTransaction::GetCounter(DWORD dwIdx)
{
    return reinterpret_cast<INT64>(::TlsGetValue(dwIdx));
}

void CTransaction::SetCounter(DWORD dwIdx,INT64 nCount)
{
    ::TlsSetValue(dwIdx,reinterpret_cast<void *>(nCount));
}

（编辑：李大同）

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