jobtrackingapp/JobTrackerApi/Services/CurrentUserService.cs

using System.Security.Claims;

namespace JobTrackerApi.Services;

public interface ICurrentUserService
{
    string? UserId { get; }
}

public sealed class CurrentUserService : ICurrentUserService
{
    private readonly IHttpContextAccessor _http;
    private string? _backgroundUserId;

    public CurrentUserService(IHttpContextAccessor http)
    {
        _http = http;
    }

    public string? UserId => _backgroundUserId ?? LocalAuthIdentity.GetRequiredUserId(_http.HttpContext?.User);

    public IDisposable UseBackgroundUser(string userId)
    {
        if (string.IsNullOrWhiteSpace(userId)) throw new ArgumentException("A background owner is required.", nameof(userId));
        if (_http.HttpContext is not null) throw new InvalidOperationException("Background owner scope cannot replace an HTTP request identity.");

        var previous = _backgroundUserId;
        _backgroundUserId = userId;
        return new Reset(() => _backgroundUserId = previous);
    }

    private sealed class Reset(Action reset) : IDisposable
    {
        private Action? _reset = reset;
        public void Dispose() => Interlocked.Exchange(ref _reset, null)?.Invoke();
    }
}