shoc-backend/Api.SeaHavenIndustries/HostedServices/PasswordResetEmailDelivery.cs

using System.Threading.Channels;
using SeaHaven.Services.Interfaces;

namespace Api.SeaHavenIndustries.HostedServices
{
    public static class PasswordResetEmailDelivery
    {
        /// <summary>
        /// Registers the process-wide reset email queue and the background service that
        /// drains it. Both must be singletons: the request and the sender share one channel.
        /// </summary>
        public static IServiceCollection AddPasswordResetEmailDelivery(this IServiceCollection services)
        {
            services.AddSingleton<PasswordResetEmailChannel>();
            services.AddSingleton<IPasswordResetEmailQueue>(provider => provider.GetRequiredService<PasswordResetEmailChannel>());
            services.AddHostedService<PasswordResetEmailSenderHostedService>();
            return services;
        }
    }

    public sealed record PasswordResetEmail(string EmailTo, string Subject, string Body);

    public sealed class PasswordResetEmailChannel : IPasswordResetEmailQueue
    {
        public const int Capacity = 1000;

        private readonly Channel<PasswordResetEmail> _channel = Channel.CreateBounded<PasswordResetEmail>(
            new BoundedChannelOptions(Capacity)
            {
                FullMode = BoundedChannelFullMode.DropWrite,
                SingleReader = true
            });
        private readonly ILogger<PasswordResetEmailChannel> _logger;
        private int _pending;

        public PasswordResetEmailChannel(ILogger<PasswordResetEmailChannel> logger)
        {
            _logger = logger;
        }

        public ChannelReader<PasswordResetEmail> Reader => _channel.Reader;

        /// <summary>Emails accepted and not yet handed to the mail provider.</summary>
        public int Pending => Volatile.Read(ref _pending);

        public bool TryEnqueue(string emailTo, string subject, string body)
        {
            Interlocked.Increment(ref _pending);
            if (_channel.Writer.TryWrite(new PasswordResetEmail(emailTo, subject, body)))
                return true;

            Interlocked.Decrement(ref _pending);
            _logger.LogWarning("Password reset email queue is full; an email was dropped.");
            return false;
        }

        public void MarkHandled() => Interlocked.Decrement(ref _pending);
    }

    public sealed class PasswordResetEmailSenderHostedService : BackgroundService
    {
        private readonly PasswordResetEmailChannel _channel;
        private readonly IServiceScopeFactory _scopeFactory;
        private readonly ILogger<PasswordResetEmailSenderHostedService> _logger;

        public PasswordResetEmailSenderHostedService(
            PasswordResetEmailChannel channel,
            IServiceScopeFactory scopeFactory,
            ILogger<PasswordResetEmailSenderHostedService> logger)
        {
            _channel = channel;
            _scopeFactory = scopeFactory;
            _logger = logger;
        }

        protected override async Task ExecuteAsync(CancellationToken stoppingToken)
        {
            await foreach (var email in _channel.Reader.ReadAllAsync(stoppingToken))
            {
                try
                {
                    await using var scope = _scopeFactory.CreateAsyncScope();
                    var sender = scope.ServiceProvider.GetRequiredService<IEmailSender>();
                    if (!await sender.SendEmailAsync(email.EmailTo, email.Subject, email.Body))
                        _logger.LogWarning("Password reset email was not accepted by the mail provider.");
                }
                catch (Exception ex)
                {
                    // The message can echo the recipient or the body, so only the type is logged.
                    _logger.LogError("Password reset email failed with {ExceptionType}.", ex.GetType().FullName);
                }
                finally
                {
                    _channel.MarkHandled();
                }
            }
        }
    }
}