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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;
using System.Buffers;
using System.Runtime.InteropServices;
using System.Text;
internal static partial class Interop
{
internal static partial class Sys
{
/// <summary>
/// Takes a path to a symbolic link and attempts to place the link target path into the buffer. If the buffer is too
/// small, the path will be truncated. No matter what, the buffer will not be null terminated.
/// </summary>
/// <param name="path">The path to the symlink</param>
/// <param name="buffer">The buffer to hold the output path</param>
/// <param name="bufferSize">The size of the buffer</param>
/// <returns>
/// Returns the number of bytes placed into the buffer on success; bufferSize if the buffer is too small; and -1 on error.
/// </returns>
[LibraryImport(Libraries.SystemNative, EntryPoint = "SystemNative_ReadLink", SetLastError = true)]
private static partial int ReadLink(ref byte path, ref byte buffer, int bufferSize);
/// <summary>
/// Takes a path to a symbolic link and returns the link target path.
/// </summary>
/// <param name="path">The path to the symlink.</param>
/// <returns>Returns the link to the target path on success; and null otherwise.</returns>
internal static string? ReadLink(ReadOnlySpan<char> path)
{
const int StackBufferSize = 256;
// Use an initial buffer size that prevents disposing and renting
// a second time when calling ConvertAndTerminateString.
using var converter = new ValueUtf8Converter(stackalloc byte[StackBufferSize]);
Span<byte> spanBuffer = stackalloc byte[StackBufferSize];
byte[]? arrayBuffer = null;
ref byte pathReference = ref MemoryMarshal.GetReference(converter.ConvertAndTerminateString(path));
while (true)
{
int error = 0;
try
{
int resultLength = ReadLink(ref pathReference, ref MemoryMarshal.GetReference(spanBuffer), spanBuffer.Length);
if (resultLength < 0)
{
// error
error = Marshal.GetLastPInvokeError();
return null;
}
else if (resultLength < spanBuffer.Length)
{
// success
return Encoding.UTF8.GetString(spanBuffer.Slice(0, resultLength));
}
}
finally
{
if (arrayBuffer != null)
{
ArrayPool<byte>.Shared.Return(arrayBuffer);
}
if (error > 0)
{
Marshal.SetLastPInvokeError(error);
}
}
// Output buffer was too small, loop around again and try with a larger buffer.
arrayBuffer = ArrayPool<byte>.Shared.Rent(spanBuffer.Length * 2);
spanBuffer = arrayBuffer;
}
}
}
}
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