feat: cache Discord DM channels in database

PluralKit.Bot/Services/PrivateChannelService.cs

@@ -0,0 +1,63 @@
+using Serilog;
+
+using Myriad.Cache;
+using Myriad.Gateway;
+using Myriad.Rest;
+
+using PluralKit.Core;
+
+namespace PluralKit.Bot;
+
+public class PrivateChannelService
+{
+    private readonly ILogger _logger;
+    private readonly ModelRepository _repo;
+    private readonly DiscordApiClient _rest;
+
+    private static Dictionary<ulong, ulong> _channelsCache = new();
+    public PrivateChannelService(ILogger logger, ModelRepository repo, DiscordApiClient rest)
+    {
+        _logger = logger;
+        _repo = repo;
+        _rest = rest;
+    }
+
+    public async Task TrySavePrivateChannel(MessageCreateEvent evt)
+    {
+        if (evt.GuildId != null) return;
+        if (_channelsCache.TryGetValue(evt.Author.Id, out _)) return;
+
+        await SaveDmChannel(evt.Author.Id, evt.ChannelId);
+    }
+
+    public async Task<ulong> GetOrCreateDmChannel(ulong userId)
+    {
+        if (_channelsCache.TryGetValue(userId, out var cachedChannelId))
+            return cachedChannelId;
+
+        var channelId = await _repo.GetDmChannel(userId);
+        if (channelId == null)
+        {
+            var channel = await _rest.CreateDm(userId);
+            channelId = channel.Id;
+        }
+
+        // spawn off saving the channel as to not block the current thread
+        _ = SaveDmChannel(userId, channelId.Value);
+
+        return channelId.Value;
+    }
+
+    private async Task SaveDmChannel(ulong userId, ulong channelId)
+    {
+        try
+        {
+            _channelsCache.Add(userId, channelId);
+            await _repo.UpdateAccount(userId, new() { DmChannel = channelId });
+        }
+        catch (Exception e)
+        {
+            _logger.Error(e, "Failed to save DM channel {ChannelId} for user {UserId}", channelId, userId);
+        }
+    }
+}