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// Software is furnished to do so, subject to the following conditions:
// 
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#nullable enable

using System;
using System.ComponentModel;
using System.Runtime.InteropServices;
using System.Runtime.Versioning;
using Vanara.PInvoke;
using Vanara.Security.AccessControl;
using static Vanara.PInvoke.AdvApi32;
using static Vanara.PInvoke.Kernel32;

namespace Duplicati.Library.WindowsModules;

/// <summary>
/// A scope that enables the SeBackupPrivilege for the current process.
/// This privilege is required to read files that are not accessible
/// due to access control restrictions, such as files owned by other users.
/// </summary>
[SupportedOSPlatform("windows")]
public sealed class SeBackupPrivilegeScope : IDisposable
{
    /// <summary>
    /// The token handle for the current process with the SeBackupPrivilege enabled.
    /// </summary>
    private readonly SafeHTOKEN _token;
    /// <summary>
    /// The original state of the SeBackupPrivilege before enabling it.
    /// </summary>
    private readonly TOKEN_PRIVILEGES _originalState;
    /// <summary>
    /// Indicates whether the scope has been disposed.
    /// </summary>
    private bool _disposed;

    /// <summary>
    /// Initializes a new instance of the <see cref="SeBackupPrivilegeScope"/> class,
    /// and enables the SeBackupPrivilege for the current process.
    /// </summary>
    public SeBackupPrivilegeScope()
    {
        const TokenAccess Access = TokenAccess.TOKEN_ADJUST_PRIVILEGES | TokenAccess.TOKEN_QUERY;

        if (!OpenProcessToken(GetCurrentProcess(), Access, out _token))
            throw Win32Error.GetLastError().GetException()
               ?? new Win32Exception(Marshal.GetLastWin32Error());

        _originalState = _token.AdjustPrivilege(SystemPrivilege.Backup, PrivilegeAttributes.SE_PRIVILEGE_ENABLED);

        // Even if AdjustPrivilege "succeeds", we must verify it worked:
        if (Win32Error.GetLastError() == Win32Error.ERROR_NOT_ALL_ASSIGNED)
        {
            _token.Dispose();
            throw new InvalidOperationException("SeBackupPrivilege is not available or cannot be enabled.");
        }
    }

    /// <inheritdoc/>
    public void Dispose()
    {
        if (_disposed)
            return;

        try
        {
            _token.AdjustPrivileges(_originalState);
        }
        finally
        {
            _token.Dispose();
            _disposed = true;
        }
    }
}