PluralKit/PluralKit.Bot/Services/WebhookExecutorService.cs

using System;
using System.Net.Http;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using App.Metrics;
using App.Metrics.Logging;
using Discord;
using Discord.Net;
using Discord.Webhook;
using Microsoft.Extensions.Caching.Memory;
using Serilog;

namespace PluralKit.Bot
{
    public class WebhookExecutorService: IDisposable
    {
        private WebhookCacheService _webhookCache;
        private IMemoryCache _cache;
        private ILogger _logger;
        private IMetrics _metrics;
        private HttpClient _client;

        public WebhookExecutorService(IMemoryCache cache, IMetrics metrics, WebhookCacheService webhookCache, ILogger logger)
        {
            _cache = cache;
            _metrics = metrics;
            _webhookCache = webhookCache;
            _logger = logger.ForContext<WebhookExecutorService>();
            _client = new HttpClient();
        }

        public async Task<ulong> ExecuteWebhook(ITextChannel channel, string name, string avatarUrl, string content, IAttachment attachment)
        {
            _logger.Debug("Invoking webhook in channel {Channel}", channel.Id);
            
            // Get a webhook, execute it
            var webhook = await _webhookCache.GetWebhook(channel);
            var id = await ExecuteWebhookInner(webhook, name, avatarUrl, content, attachment);
            
            // Log the relevant metrics
            _metrics.Measure.Meter.Mark(BotMetrics.MessagesProxied);
            _logger.Information("Invoked webhook {Webhook} in channel {Channel}", webhook.Id,
                channel.Id);
            
            return id;
        }

        private async Task<ulong> ExecuteWebhookInner(IWebhook webhook, string name, string avatarUrl, string content,
            IAttachment attachment, bool hasRetried = false)
        {
            var client = await GetClientFor(webhook);

            try
            {
                // If we have an attachment, use the special SendFileAsync method
                if (attachment != null)
                    using (var attachmentStream = await _client.GetStreamAsync(attachment.Url))
                    using (_metrics.Measure.Timer.Time(BotMetrics.WebhookResponseTime))
                        return await client.SendFileAsync(attachmentStream, attachment.Filename, content,
                            username: FixClyde(name),
                            avatarUrl: avatarUrl);

                // Otherwise, send normally
                return await client.SendMessageAsync(content, username: name, avatarUrl: avatarUrl);
            }
            catch (HttpException e)
            {
                // If we hit an error, just retry (if we haven't already)
                if (e.DiscordCode == 10015 && !hasRetried) // Error 10015 = "Unknown Webhook"
                {
                    _logger.Warning(e, "Error invoking webhook {Webhook} in channel {Channel}", webhook.Id, webhook.ChannelId);
                    return await ExecuteWebhookInner(await _webhookCache.InvalidateAndRefreshWebhook(webhook), name, avatarUrl, content, attachment, hasRetried: true);
                }

                throw;
            }
        }
        
        private async Task<DiscordWebhookClient> GetClientFor(IWebhook webhook)
        {
            _logger.Debug("Looking for client for webhook {Webhook} in cache", webhook.Id);
            return await _cache.GetOrCreateAsync($"_webhook_client_{webhook.Id}",
                (entry) => MakeCachedClientFor(entry, webhook));
        }

        private Task<DiscordWebhookClient> MakeCachedClientFor(ICacheEntry entry, IWebhook webhook) {
            _logger.Information("Client for {Webhook} not found in cache, creating", webhook.Id);
            
            // Define expiration for the client cache
            // 10 minutes *without a query* and it gets yoten
            entry.SlidingExpiration = TimeSpan.FromMinutes(10);

            // IMemoryCache won't automatically dispose of its values when the cache gets evicted
            // so we register a hook to do so here.
            entry.RegisterPostEvictionCallback((key, value, reason, state) => (value as IDisposable)?.Dispose());
            
            // DiscordWebhookClient has a sync network call in its constructor (!!!)
            // and we want to punt that onto a task queue, so we do that.
            return Task.Run(async () =>
            {
                try
                {
                    return new DiscordWebhookClient(webhook);
                }
                catch (InvalidOperationException)
                {
                    // TODO: does this leak stuff inside the DiscordWebhookClient created above?
                    
                    // Webhook itself was found in cache, but has been deleted on the channel
                    // We request a new webhook instead
                    return new DiscordWebhookClient(await _webhookCache.InvalidateAndRefreshWebhook(webhook));
                }

            });
        }
        
        private string FixClyde(string name)
        {
            // Check if the name contains "Clyde" - if not, do nothing
            var match = Regex.Match(name, "clyde", RegexOptions.IgnoreCase);
            if (!match.Success) return name;

            // Put a hair space (\u200A) between the "c" and the "lyde" in the match to avoid Discord matching it
            // since Discord blocks webhooks containing the word "Clyde"... for some reason. /shrug
            return name.Substring(0, match.Index + 1) + '\u200A' + name.Substring(match.Index + 1);
        }

        public void Dispose()
        {
            _client.Dispose();
        }
    }
}