Question

以下是我在iOS xamarin应用程序中使用的一些数据访问代码的简化表示：

public async Task<List<Foo>> GetAllFoos()
{
    var foos = new List<Foo>();
    using(CommandWrapper command = GetCommandWrapper("SELECT Id, Name, Rank FROM Foos"))//this creates SqliteCommand object and puts it in wrapper object
    {
        using(ReaderWrapper reader = ExecuteReaderAsync(_Connection, command))
        {
            while(reader.Read())
            {
                var foo = ConstructFoo(reader);//construct foo with column values read from reader
                foos.Add(foo);
            }
        }
    }
    return foos;
}

private SemaphoreSlim _commandLockSemaphore = new SemaphoreSlim(1, 1);//only 1 thread at once

private async ExecuteReaderAsync(Mono.Data.Sqlite.SqliteConnection connection, CommandWrapper command)
{
    if(!(await _SemaphoreLock.WaitAsync(_TimeoutTimespan))) throw new Exception("timeout");

    //lock now held

    Mono.Data.Sqlite.SqliteCommand sqliteCommand = command.UnderlyingCommand;

    try
    {
        sqliteCommand.Connection = connection;
        IDataReader rawReader = await sqliteCommand.ExecuteReaderAsync();
    }
    catch(Exception)
    {
        //emergency lock release
        _SemaphoreLock.Release();
        throw;
    }
    return new ReaderWrapper(){DataReader = rawReader, SemaphoreLock = _SemaphoreLock};
}

internal class ReaderWrapper : IDisposable
{
    internal IDataReader DataReader{get; set;}
    internal SemaphoreSlim SemaphoreLock{get; set;}

    //... [read methods]

    public void Dispose()
    {
        DataReader.Dispose();
        SemaphoreLock.Release();
    }
}

由于我们知道Sqlite不支持连接上的多个线程，因此我们将SemaphoreSlim放在适当的位置，以确保连接的使用和任何命令的执行仅发生在单个线程中。 SemaphoreSlim随后在阅读器疲惫不堪后被释放。

但是，当我有多个线程调用GetAllFoos时，应用程序崩溃时会出现以下情况：

2015-08-19 14:33:22.296 MyCoolIosApp[8421:5311923] critical: Stacktrace:
2015-08-19 14:33:22.296 MyCoolIosApp[8421:5311923] critical:   at  
2015-08-19 14:33:22.297 MyCoolIosApp[8421:5311923] critical:   at (wrapper managed-to-native) Mono.Data.Sqlite.UnsafeNativeMethods.sqlite3_prepare (intptr,intptr,int,intptr&,intptr&) 
2015-08-19 14:33:22.297 MyCoolIosApp[8421:5311923] critical:   at Mono.Data.Sqlite.SQLite3.Prepare (Mono.Data.Sqlite.SqliteConnection,string,Mono.Data.Sqlite.SqliteStatement,uint,string&) [0x00044] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLite3.cs:268
2015-08-19 14:33:22.298 MyCoolIosApp[8421:5311923] critical:   at Mono.Data.Sqlite.SqliteCommand.BuildNextCommand () [0x00019] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLiteCommand.cs:230
2015-08-19 14:33:22.298 MyCoolIosApp[8421:5311923] critical:   at Mono.Data.Sqlite.SqliteCommand.GetStatement (int) [0x0000b
] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLiteCommand.cs:264
2015-08-19 14:33:22.299 MyCoolIosApp[8421:5311923] critical:   at Mono.Data.Sqlite.SqliteDataReader.NextResult () [0x000cc] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLiteDataReader.cs:914
2015-08-19 14:33:22.299 MyCoolIosApp[8421:5311923] critical:   at Mono.Data.Sqlite.SqliteDataReader..ctor (Mono.Data.Sqlite.SqliteCommand,System.Data.CommandBehavior) [0x00051] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLiteDataReader.cs:89
2015-08-19 14:33:22.300 MyCoolIosApp[8421:5311923] critical:   at Mono.Data.Sqlite.SqliteCommand.ExecuteReader (System.Data.CommandBehavior) [0x00006] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLiteCommand.cs:539
2015-08-19 14:33:22.300 MyCoolIosApp[8421:5311923] critical:   at Mono.Data.Sqlite.SqliteCommand.ExecuteReader () [0x00000] in //Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.4.46/src/mono/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLiteCommand.cs:551
...

根据各种讨论，该错误是由iOS设备上的Sqlite上的线程太多引起的。我可以证实这一点，因为如果我有一对线程尝试在while循环中无休止地调用GetAllFoos，它会100％崩溃。当我重新安排锁定时（例如，在构造和处理命令时添加一个简单的锁定语句），它解决了问题：

public async Task<List<Foo>> GetAllFoos()
{
    var foos = new List<Foo>();
    lock(_someStaticObject)//why is this needed
    {
        using(CommandWrapper command = GetCommandWrapper("SELECT Id, Name, Rank FROM Foos"))//this creates SqliteCommand object and puts it in wrapper object
        {
            using(ReaderWrapper reader = ExecuteReaderAsync(_Connection, command))
            {
                while(reader.Read())
                {
                    var foo = ConstructFoo(reader);//construct foo with column values read from reader
                    foos.Add(foo);
                }
            }
        }
    }
    return foos;
}

就我所知，SqliteCommand上的Dispose方法（可能还有构造函数）导致了并发问题。

在Mono中SqliteCommand对象的构造函数和Dispose方法是否为线程安全？或者我是否需要考虑其中一个或两个是锁定目的的关键部分？

Answer 1

使用Mono的好处在于它是开源的，所以让我们回答你的问题“Mono中SqliteCommand对象线程安全的构造函数和Dispose方法吗？”

如果我们快速浏览一下：

https://github.com/mono/mono/blob/master/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLiteCommand.cs#L108

在这里：

https://github.com/mono/mono/blob/master/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLiteCommand.cs#L153

...然后我们可以强调回答“不，构造函数和Mono的SQLiteCommand的Dispose都不是线程安全的”。

就此而言，假设任何地方都没有线程安全是正确的，除非它通过文档明确说明。即便如此，它有时也是谎言。

（现在微软的东西也是开源的，所以我们可以用同样的方式在Mono之外回答这些问题。是的！）

Mono中SqliteCommand对象的构造函数和Dispose方法是否为线程安全？

1 个答案: