Remove semaphore in SelectiveAutoUserDownload

Also updated style for this file.
Additionally, reworded the corresponding error message to something friendlier.

BackgroundServices/SelectiveAutoUserDownload.cs

@@ -1,86 +1,61 @@
 using BirthdayBot.Data;
+using Discord;
 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 
-namespace BirthdayBot.BackgroundServices
+namespace BirthdayBot.BackgroundServices;
-{
-    /// <summary>
-    /// A type of workaround to the issue of user information not being cached for guilds that
-    /// have user information existing in the bot's database. This service runs frequently and
-    /// determines guilds in which user data must be downloaded, and proceeds to request it.
-    /// </summary>
-    class SelectiveAutoUserDownload : BackgroundService
-    {
-        private static readonly SemaphoreSlim _updateLock = new(2);
 
-        private readonly HashSet<ulong> _fetchRequests = new();
+/// <summary>
+/// Rather than use up unnecessary resources by auto-downloading the user list in -every-
+/// server we're in, this service checks if fetching the user list is warranted for each
+/// guild before proceeding to request it.
+/// </summary>
+class SelectiveAutoUserDownload : BackgroundService {
+    private readonly HashSet<ulong> _fetchRequests = new();
 
-        public SelectiveAutoUserDownload(ShardInstance instance) : base(instance) { }
+    public SelectiveAutoUserDownload(ShardInstance instance) : base(instance) { }
 
-        public override async Task OnTick(CancellationToken token)
+    public override async Task OnTick(CancellationToken token) {
-        {
+        IEnumerable<ulong> requests;
-            IEnumerable<ulong> requests;
+        lock (_fetchRequests) {
-            lock (_fetchRequests)
+            requests = _fetchRequests.ToArray();
-            {
+            _fetchRequests.Clear();
-                requests = _fetchRequests.ToArray();
-                _fetchRequests.Clear();
-            }
-
-            foreach (var guild in ShardInstance.DiscordClient.Guilds)
-            {
-                if (ShardInstance.DiscordClient.ConnectionState != Discord.ConnectionState.Connected)
-                {
-                    Log("Client no longer connected. Stopping early.");
-                    return;
-                }
-
-                // Determine if there is action to be taken...
-                if (guild.HasAllMembers) continue;
-                if (requests.Contains(guild.Id) || await GuildUserAnyAsync(guild.Id, token).ConfigureAwait(false))
-                {
-                    await guild.DownloadUsersAsync().ConfigureAwait(false);
-                    await Task.Delay(500, CancellationToken.None).ConfigureAwait(false);
-                }
-            }
         }
 
-        /// <summary>
+        foreach (var guild in ShardInstance.DiscordClient.Guilds) {
-        /// Determines if the user database contains any entries corresponding to this guild.
+            if (ShardInstance.DiscordClient.ConnectionState != ConnectionState.Connected) {
-        /// </summary>
+                Log("Client no longer connected. Stopping early.");
-        /// <returns>True if any entries exist.</returns>
+                return;
-        private static async Task<bool> GuildUserAnyAsync(ulong guildId, CancellationToken token)
-        {
-            try
-            {
-                await _updateLock.WaitAsync(token).ConfigureAwait(false);
             }
-            catch (Exception ex) when (ex is OperationCanceledException or ObjectDisposedException)
-            {
-                // Calling thread does not expect the exception that SemaphoreSlim throws...
-                throw new TaskCanceledException();
-            }
-            try
-            {
-                using var db = await Database.OpenConnectionAsync().ConfigureAwait(false);
-                using var c = db.CreateCommand();
-                c.CommandText = $"select count(*) from {GuildUserConfiguration.BackingTable} where guild_id = @Gid";
-                c.Parameters.Add("@Gid", NpgsqlTypes.NpgsqlDbType.Bigint).Value = (long)guildId;
-                await c.PrepareAsync(CancellationToken.None).ConfigureAwait(false);
-                var r = (long)await c.ExecuteScalarAsync(token).ConfigureAwait(false);
-                return r != 0;
-            }
-            finally
-            {
-                _updateLock.Release();
-            }
-        }
 
-        public void RequestDownload(ulong guildId)
+            // Determine if there is action to be taken...
-        {
+            if (guild.HasAllMembers) continue;
-            lock (_fetchRequests) _fetchRequests.Add(guildId);
+            if (requests.Contains(guild.Id) || await GuildUserAnyAsync(guild.Id).ConfigureAwait(false)) {
+                await guild.DownloadUsersAsync().ConfigureAwait(false);
+                // Must delay after a download request. Seems to hang indefinitely otherwise.
+                await Task.Delay(300, CancellationToken.None).ConfigureAwait(false);
+            }
         }
     }
+
+    /// <summary>
+    /// Determines if the user database contains any entries corresponding to this guild.
+    /// </summary>
+    /// <returns>True if any entries exist.</returns>
+    private static async Task<bool> GuildUserAnyAsync(ulong guildId) {
+        using var db = await Database.OpenConnectionAsync().ConfigureAwait(false);
+        using var c = db.CreateCommand();
+        c.CommandText = $"select true from {GuildUserConfiguration.BackingTable} where guild_id = @Gid limit 1";
+        c.Parameters.Add("@Gid", NpgsqlTypes.NpgsqlDbType.Bigint).Value = (long)guildId;
+        await c.PrepareAsync(CancellationToken.None).ConfigureAwait(false);
+        using var r = await c.ExecuteReaderAsync(CancellationToken.None).ConfigureAwait(false);
+        return r.Read();
+    }
+
+    public void RequestDownload(ulong guildId) {
+        lock (_fetchRequests) _fetchRequests.Add(guildId);
+    }
 }

UserInterface/CommandsCommon.cs

@@ -22,7 +22,7 @@ namespace BirthdayBot.UserInterface
         public const string ParameterError = ":x: Invalid usage. Refer to how to use the command and try again.";
         public const string NoParameterError = ":x: This command does not accept any parameters.";
         public const string InternalError = ":x: An internal bot error occurred. The bot maintainer has been notified of the issue.";
-        public const string UsersNotDownloadedError = ":x: Currently unavailable. Please try again in a few minutes.";
+        public const string UsersNotDownloadedError = ":eight_spoked_asterisk: Still catching up... Please try the command again in a few minutes.";
 
         public delegate Task CommandHandler(ShardInstance instance, GuildConfiguration gconf,
                                             string[] param, SocketTextChannel reqChannel, SocketGuildUser reqUser);