/// <summary>
/// 获取缓冲项
/// </summary>
/// <param name="pos"></param>
/// <param name="flags"></param>
/// <returns></returns>
public BufferEntry GetEntry(PagePosition pos, BufferSeekFlags flags = BufferSeekFlags.None)
{
//获取或者新分配(创建)一个Buffer
var buffer = GetOrCreateEntry(pos, flags);
if (buffer == null)
{
return(buffer);
}
if (buffer.State == BufferState.NoneLoading)
{
//新buffer,未加载,加载页面,此时一定是自己获取了写锁
LoadBufferPage(buffer);
lock (SyncRoot)
{
AddLRUEntry(buffer);
return(buffer);
}
}
return(buffer);
}
/// <summary>
/// 获取或创建页面缓冲
/// </summary>
/// <param name="pos"></param>
/// <param name="flags"></param>
/// <returns></returns>
public BufferEntry GetOrCreateEntry(PagePosition pos, BufferSeekFlags flags)
{
//缓冲区锁
Monitor.Enter(SyncRoot);
//对应页的buffer不存在,创建,获取写锁(很快)阻塞读,释放缓冲区锁,返回去读取
if (!Buffers.TryGetValue(pos, out var buffer))
{
buffer = CreateEntry(pos);
buffer.Count++;
buffer.Latch.EnterWriteScope();
Buffers[pos] = buffer;
Monitor.Exit(SyncRoot);
return(buffer);
}
//buffer对应的页是无须加载的状态,增加引用计数,移动LRU,返回
if (buffer.State != BufferState.NoneLoading)
{
buffer.Count++;
if (!flags.HasFlag(BufferSeekFlags.NoLRU))
{
MoveLRUEntry(buffer);
}
Monitor.Exit(SyncRoot);
return(buffer);
}
//不等待页面加载,不增加引用计数
if (flags.HasFlag(BufferSeekFlags.NoWait))
{
Monitor.Exit(SyncRoot);
return(null);
}
//页面未加载,释放缓冲区锁
buffer.Count++;
Monitor.Exit(SyncRoot);
//等待加载完成
buffer.Latch.EnterReadScope();
buffer.Latch.ExitReadScope();
//加缓冲区锁,移动LRUList
Monitor.Enter(SyncRoot);
if (!flags.HasFlag(BufferSeekFlags.NoLRU))
{
MoveLRUEntry(buffer);
}
Monitor.Exit(SyncRoot);
return(buffer);
}