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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
using System.Collections.Concurrent;
#nullable enable
namespace System.Buffers;
/// <summary>
/// Used to allocate and distribute re-usable blocks of memory.
/// </summary>
internal sealed class PinnedBlockMemoryPool : MemoryPool<byte>
{
/// <summary>
/// The size of a block. 4096 is chosen because most operating systems use 4k pages.
/// </summary>
private const int _blockSize = 4096;
/// <summary>
/// Max allocation block size for pooled blocks,
/// larger values can be leased but they will be disposed after use rather than returned to the pool.
/// </summary>
public override int MaxBufferSize { get; } = _blockSize;
/// <summary>
/// The size of a block. 4096 is chosen because most operating systems use 4k pages.
/// </summary>
public static int BlockSize => _blockSize;
/// <summary>
/// Thread-safe collection of blocks which are currently in the pool. A slab will pre-allocate all of the block tracking objects
/// and add them to this collection. When memory is requested it is taken from here first, and when it is returned it is re-added.
/// </summary>
private readonly ConcurrentQueue<MemoryPoolBlock> _blocks = new ConcurrentQueue<MemoryPoolBlock>();
/// <summary>
/// This is part of implementing the IDisposable pattern.
/// </summary>
private bool _isDisposed; // To detect redundant calls
private readonly object _disposeSync = new object();
/// <summary>
/// This default value passed in to Rent to use the default value for the pool.
/// </summary>
private const int AnySize = -1;
public override IMemoryOwner<byte> Rent(int size = AnySize)
{
if (size > _blockSize)
{
MemoryPoolThrowHelper.ThrowArgumentOutOfRangeException_BufferRequestTooLarge(_blockSize);
}
if (_isDisposed)
{
MemoryPoolThrowHelper.ThrowObjectDisposedException(MemoryPoolThrowHelper.ExceptionArgument.MemoryPool);
}
if (_blocks.TryDequeue(out var block))
{
// block successfully taken from the stack - return it
return block;
}
return new MemoryPoolBlock(this, BlockSize);
}
/// <summary>
/// Called to return a block to the pool. Once Return has been called the memory no longer belongs to the caller, and
/// Very Bad Things will happen if the memory is read of modified subsequently. If a caller fails to call Return and the
/// block tracking object is garbage collected, the block tracking object's finalizer will automatically re-create and return
/// a new tracking object into the pool. This will only happen if there is a bug in the server, however it is necessary to avoid
/// leaving "dead zones" in the slab due to lost block tracking objects.
/// </summary>
/// <param name="block">The block to return. It must have been acquired by calling Lease on the same memory pool instance.</param>
internal void Return(MemoryPoolBlock block)
{
#if BLOCK_LEASE_TRACKING
Debug.Assert(block.Pool == this, "Returned block was not leased from this pool");
Debug.Assert(block.IsLeased, $"Block being returned to pool twice: {block.Leaser}{Environment.NewLine}");
block.IsLeased = false;
#endif
if (!_isDisposed)
{
_blocks.Enqueue(block);
}
}
protected override void Dispose(bool disposing)
{
if (_isDisposed)
{
return;
}
lock (_disposeSync)
{
_isDisposed = true;
if (disposing)
{
// Discard blocks in pool
while (_blocks.TryDequeue(out _))
{
}
}
}
}
}
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