一次寻找IBatisNet事务bug的过程

本文的上下文环境

操作系统：Win7 x64 Professional

开发工具：Visual Studio 2017   语言：C#

数据库ORM：IBatisNet 1.6.2

一、前言

这个项目的前端有Web端，公众号，微信小程序，后端是用WCF写的，部署成windows service。后端使用了IBatisNet这样的轻量级ORM框架，sql是写在xml里面的，每个模块都有一个xml文件。sql的返回值对应的实体类配置在同一个xml文件里，像这样。

二、问题出现

 这个项目一直运行的很正常，直到有一天发布了某一个功能之后，问题就出现了，异常日志经常看到这样的文字“SqlMap could not invoke BeginTransaction(). A Transaction is already started. Call CommitTransaction() or RollbackTransaction first”，一看代码，这个异常是从SqlMapper.BeginTransaction方法里面抛出的，字面意思就是启动事务，不能正常启动，需要首先调用提交或回滚事务。按着这个逻辑来说，应该是某一次事务忘记提交或回滚了。但是这个错误在本地开发环境复现不了，线上环境时不时的出现。如果本地能够reproduce，应该会比较好解决一些。

三、查看bug起因

我们项目中事务用了PostSharp里面的AOP编程，只要在需要事务的方法上面加个attribute就能实现事务支持。代码如下：

[Serializable]
    [AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
    public class AopTransactionMethodAttribute : MethodInterceptionAspect
    {
        //调用本函数时截取异常
        public override void OnInvoke(MethodInterceptionArgs args)
        {
            Mapper.Instance().BeginTransaction();
            try
            {
                //执行方法
                base.OnInvoke(args);
            }
            catch
            {
                Mapper.Instance().RollBackTransaction();

                throw;
            }

            TData result = args.ReturnValue as TData;

            if (result == null || result.Tag <= 0)
            {
                //回滚事务
                Mapper.Instance().RollBackTransaction();
            }
            else
            {
                //提交事务
                Mapper.Instance().CommitTransaction();
            }
        }
    }

这样就是很方便写代码，不用在每个操作数据库多表的地方都显式地调用事务。

一开始怀疑这个地方可能有问题，在try catch的基础上又加了一个try catch finally 用来彻底提交或回归事务。发布到线上之后，错误仍然时不时的出现。

由于只能在线上环境可以重现，考虑只能使用日志来记录执行过程了，我把事务的BeginTransaction，CommitTransaction，RollbackTransaction的执行全部记上日志，考虑业务会并发执行，日志肯定要加锁，并且把每次事务的CallStack和ThreadId给输出来，代码如下：

 public static readonly object LockTransaction = new object();
public ISqlMapSession BeginTransaction()
        {
            lock (LockTransaction)
            {
                string file = string.Format(AppDomain.CurrentDomain.BaseDirectory + "Log\\{0}\\{1}.log", DateTime.Now.ToString("yyyy-MM"), DateTime.Now.ToString("yyyy-MM-dd"));
                System.IO.StreamWriter sw = new System.IO.StreamWriter(file, true);
                sw.Write(DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss"));
                sw.Write(" Begin  Transaction Thread Id:" + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString().PadLeft(2, '0'));
                if (_sessionStore.LocalSession != null)
                {
                    sw.WriteLine();
                    StackTrace trace = new StackTrace(true);
                    for (int i = 0; i < trace.FrameCount; i++)
                    {
                        StackFrame frame = trace.GetFrame(i);
                        MethodBase method = frame.GetMethod();
                        if (method.DeclaringType != null)
                        {
                            sw.WriteLine(method.DeclaringType.FullName + " " + method.Name);
                        }
                        else
                        {
                            sw.WriteLine(method.Name);
                        }
                    }
                    sw.WriteLine(" SqlMap could not invoke BeginTransaction(). A Transaction is already started. Call CommitTransaction() or RollbackTransaction first");
                    //throw new DataMapperException("SqlMap could not invoke BeginTransaction(). A Transaction is already started. Call CommitTransaction() or RollbackTransaction first.");
                }
                ISqlMapSession session = CreateSqlMapSession();
                _sessionStore.Store(session);
                session.BeginTransaction();
                sw.WriteLine();
                sw.Close();
                return session;
            }
        }

public void CommitTransaction()
        {
            lock (LockTransaction)
            {
                string file = string.Format(AppDomain.CurrentDomain.BaseDirectory + "Log\\{0}\\{1}.log", DateTime.Now.ToString("yyyy-MM"), DateTime.Now.ToString("yyyy-MM-dd"));
                System.IO.StreamWriter sw = new System.IO.StreamWriter(file, true);
                sw.Write(DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss"));
                sw.Write(" Commit Transaction Thread Id:" + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString().PadLeft(2, '0'));
                if (_sessionStore.LocalSession == null)
                {
                    throw new DataMapperException("SqlMap could not invoke CommitTransaction(). No Transaction was started. Call BeginTransaction() first.");
                }
                try
                {
                    ISqlMapSession session = _sessionStore.LocalSession;
                    session.CommitTransaction();
                }
                finally
                {
                    _sessionStore.Dispose();
                }
                if (_sessionStore.LocalSession != null)
                {
                    sw.Write(" Commit Transaction Not Successfully " + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString().PadLeft(2, '0'));
                }
                else
                {
                    sw.Write(" Commit Transaction " + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString().PadLeft(2, '0'));
                }
                sw.WriteLine();
                sw.Close();
            }
        }

  public void RollBackTransaction()
        {
            lock (LockTransaction)
            {
                string file = string.Format(AppDomain.CurrentDomain.BaseDirectory + "Log\\{0}\\{1}.log", DateTime.Now.ToString("yyyy-MM"), DateTime.Now.ToString("yyyy-MM-dd"));
                System.IO.StreamWriter sw = new System.IO.StreamWriter(file, true);
                sw.Write(DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss"));
                sw.Write(" Roll   Transaction Thread Id:" + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString().PadLeft(2, '0'));
                if (_sessionStore.LocalSession == null)
                {
                    throw new DataMapperException("SqlMap could not invoke RollBackTransaction(). No Transaction was started. Call BeginTransaction() first.");
                }
                try
                {
                    ISqlMapSession session = _sessionStore.LocalSession;
                    session.RollBackTransaction();
                }
                finally
                {
                    _sessionStore.Dispose();
                }
                if (_sessionStore.LocalSession != null)
                {
                    sw.Write(" Roll Transaction Not Successfully " + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString().PadLeft(2, '0'));
                }
                else
                {
                    sw.Write(" Roll Transaction " + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString().PadLeft(2, '0'));
                }
                sw.WriteLine();
                sw.Close();
            }
        }

 结果出乎意料的是，事务一个BeginTransaction后面要么跟一个Rollback Transaction，要么跟一个Commit Transaction，这完全是正确的啊，心里顿时真不是滋味，什么破代码竟然没问题。

不过，问题总归是需要解决的，不然永远都是止步不前。

回到最开始错误的地方，因为在BeginTransaction的时候，_sessionStore.LocalSession != null，抛出了那个错误，我就需要调查这个LocalSession。

因为LocalSession对应的对象SqlMapSession里面没有标识对象唯一性的建，我就在SqlMapSession里面加了一个Id标识，用来确定这个LocalSession到底有没有被销毁。

public SqlMapSession(ISqlMapper sqlMapper)
        {
            _dataSource = sqlMapper.DataSource;
            _sqlMapper = sqlMapper;
            lock (IBatisNet.DataMapper.SqlMapper.LockTransaction)
            {
                id = long.Parse(DateTime.Now.ToString("yyyyMMddHHmmssfff"));
            }
        }

IBatisNet在保存session的时候，在Winform程序里面使用CallContextSessionStore这个类来获取、保存、销毁session。CallContextSessionStore类里面用CallContext.GetData(name)获取，CallContext.SetData(name, session)保存，CallContext.SetData(name, null)销毁。CallContext这个类支持在同一个线程内，访问的是同一个对象，也就是说，当前线程对对象进行储存到线程本地储存区，对象随着线程的销毁而销毁。

同样地，我把每个方法的调用输出在另外一个日志文件里面，输出在同一个文件会报文件拒绝访问，可能是由于在Dispose的时候，我去调用GetSession所致。代码如下：

public override ISqlMapSession LocalSession
        {
            get
            {
                lock (IBatisNet.DataMapper.SqlMapper.LockTransaction)
                {
                    var session = CallContext.GetData(sessionName) as SqlMapSession;
                    if (session != null)
                    {
                        string file = string.Format(AppDomain.CurrentDomain.BaseDirectory + "Log\\{0}\\{1}_1.log", DateTime.Now.ToString("yyyy-MM"), DateTime.Now.ToString("yyyy-MM-dd"));
                        System.IO.StreamWriter sw = new System.IO.StreamWriter(file, true);
                        sw.Write(DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss"));
                        sw.Write(" LocalSession Thread Id:" + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString().PadLeft(2, '0'));
                        sw.Write(" session Id:" + session.Id);
                        sw.WriteLine();
                        sw.Close();
                    }
                    return session;
                }
            }
        }

public override void Store(ISqlMapSession session)
        {
            lock (IBatisNet.DataMapper.SqlMapper.LockTransaction)
            {
                string file = string.Format(AppDomain.CurrentDomain.BaseDirectory + "Log\\{0}\\{1}_1.log", DateTime.Now.ToString("yyyy-MM"), DateTime.Now.ToString("yyyy-MM-dd"));
                System.IO.StreamWriter sw = new System.IO.StreamWriter(file, true);
                sw.Write(DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss"));
                sw.Write(" Store        Thread Id:" + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString().PadLeft(2, '0'));
                sw.Write(" session Id:" + session.Id);
                sw.WriteLine();

                StackTrace trace = new StackTrace(true);
                for (int i = 0; i < trace.FrameCount; i++)
                {
                    StackFrame frame = trace.GetFrame(i);
                    MethodBase method = frame.GetMethod();
                    if (method.DeclaringType != null)
                    {
                        sw.WriteLine(method.DeclaringType.FullName + " " + method.Name);
                    }
                    else
                    {
                        sw.WriteLine(method.Name);
                    }
                }

                sw.Close();
                CallContext.SetData(sessionName, session);
            }
        }

public override void Dispose()
        {
            lock (IBatisNet.DataMapper.SqlMapper.LockTransaction)
            {
                var session = CallContext.GetData(sessionName) as SqlMapSession;
                string file = string.Format(AppDomain.CurrentDomain.BaseDirectory + "Log\\{0}\\{1}_1.log", DateTime.Now.ToString("yyyy-MM"), DateTime.Now.ToString("yyyy-MM-dd"));
                System.IO.StreamWriter sw = new System.IO.StreamWriter(file, true);
                sw.Write(DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss"));
                sw.Write(" Dispose      Thread Id:" + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString().PadLeft(2, '0'));
                sw.Write(" session Id:" + session.Id);
                sw.WriteLine();
                sw.Close();

                CallContext.SetData(sessionName, null);
            }
        }

找到上一个日志文件出错的时间，再和这个日志文件对照，顿时发现有一个session id只有Store，没有Dispose。并且通过CallStack知道是哪个方法在调用和使用这个session，简直欣喜若狂啊：）

Finally，问题找到了，是一个业务方法不经意地调用了一个获取Sql的方法，来获取当前执行的sql，但是这个方法的写法有问题，session只有open，没有close。

四、总结

一沙一世界，一叶一菩提

用心去看，去体悟，哪怕再小的事物都可以成为一个独立的空间，哪怕是一粒沙子都可以是一个世界，容纳万物。问题在那里啊？就在那一叶上。