Add proactive selective auto user downloading

This amounts to being a workaround, as Discord.Net's option is still broken for large bots.

Commands.cs

@@ -15,6 +15,7 @@ internal class Commands {
     const string ErrInvalidZone = ":x: Not a valid zone name."
         + " To find your time zone, refer to: <https://kevinnovak.github.io/Time-Zone-Picker/>.";
     const string ErrTargetUserNotFound = ":x: Unable to find the target user.";
+    const string ErrNoUserCache = ":warning: Please try the command again.";
     const int MaxSingleLineLength = 750;
     const int MaxSingleOutputLength = 900;
 
@@ -102,6 +103,11 @@ internal class Commands {
     }
 
     private async Task CmdList(SocketTextChannel channel, SocketGuildUser sender, SocketMessage message) {
+        if (!await AreUsersDownloadedAsync(channel.Guild).ConfigureAwait(false)) {
+            await channel.SendMessageAsync(ErrNoUserCache).ConfigureAwait(false);
+            return;
+        }
+
         var wspl = message.Content.Split(' ', 2, StringSplitOptions.RemoveEmptyEntries);
         if (wspl.Length == 2) {
             // Has parameter - do specific user lookup
@@ -206,6 +212,11 @@ internal class Commands {
             await channel.SendMessageAsync(":x: You must specify a time zone to apply to the user.").ConfigureAwait(false);
             return;
         }
+
+        if (!await AreUsersDownloadedAsync(channel.Guild).ConfigureAwait(false)) {
+            await channel.SendMessageAsync(ErrNoUserCache).ConfigureAwait(false);
+            return;
+        }
         var targetuser = ResolveUserParameter(channel.Guild, wspl[1]);
         if (targetuser == null) {
             await channel.SendMessageAsync(ErrTargetUserNotFound).ConfigureAwait(false);
@@ -236,6 +247,11 @@ internal class Commands {
             await channel.SendMessageAsync(":x: You must specify a user for whom to remove time zone data.").ConfigureAwait(false);
             return;
         }
+
+        if (!await AreUsersDownloadedAsync(channel.Guild).ConfigureAwait(false)) {
+            await channel.SendMessageAsync(ErrNoUserCache).ConfigureAwait(false);
+            return;
+        }
         var targetuser = ResolveUserParameter(channel.Guild, wspl[1]);
         if (targetuser == null) {
             await channel.SendMessageAsync(ErrTargetUserNotFound).ConfigureAwait(false);
@@ -304,9 +320,6 @@ internal class Commands {
     /// Given parameter input, attempts to find the corresponding SocketGuildUser.
     /// </summary>
     private static SocketGuildUser? ResolveUserParameter(SocketGuild guild, string input) {
-        // if (!guild.HasAllMembers) await guild.DownloadUsersAsync().ConfigureAwait(false);
-        // TODO selective user downloading - see here when implementing
-
         // Try interpreting as ID
         var match = _userMention.Match(input);
         string idcheckstr = match.Success ? match.Groups[1].Value : input;
@@ -333,5 +346,24 @@ internal class Commands {
 
         return null;
     }
+
+    /// <summary>
+    /// Checks if the member cache for the specified guild needs to be filled, and sends a request if needed.
+    /// </summary>
+    /// <remarks>
+    /// Due to a quirk in Discord.Net, the user cache cannot be filled until the command handler is no longer executing,
+    /// regardless of if the request runs on its own thread.
+    /// </remarks>
+    /// <returns>
+    /// True if the guild's members are already downloaded. If false, the command handler must notify the user.
+    /// </returns>
+    private static async Task<bool> AreUsersDownloadedAsync(SocketGuild guild) {
+        if (guild.HasAllMembers) return true;
+        else {
+            // Event handler hangs if awaited normally or used with Task.Run
+            await Task.Factory.StartNew(guild.DownloadUsersAsync).ConfigureAwait(false);
+            return false;
+        }
+    }
     #endregion
 }

Database.cs

@@ -8,6 +8,7 @@ namespace WorldTime;
 /// </summary>
 internal class Database {
     private const string UserDatabase = "userdata";
+    private const string CutoffInterval = "INTERVAL '30 days'"; // TODO make configurable?
 
     private readonly string _connectionString;
 
@@ -38,6 +39,25 @@ internal class Database {
         await c.ExecuteNonQueryAsync().ConfigureAwait(false);
     }
 
+    /// <summary>
+    /// Checks if a given guild contains at least one user data entry with recent enough activity.
+    /// </summary>
+    public async Task<bool> HasAnyAsync(SocketGuild guild) {
+        using var db = await OpenConnectionAsync().ConfigureAwait(false);
+        using var c = db.CreateCommand();
+        c.CommandText = $@"
+SELECT true FROM {UserDatabase}
+WHERE
+    guild_id = @Gid
+    AND last_active >= now() - {CutoffInterval}
+LIMIT 1
+";
+        c.Parameters.Add("@Gid", NpgsqlDbType.Bigint).Value = (long)guild.Id;
+        await c.PrepareAsync().ConfigureAwait(false);
+        using var r = await c.ExecuteReaderAsync().ConfigureAwait(false);
+        return await r.ReadAsync().ConfigureAwait(false);
+    }
+
     /// <summary>
     /// Gets the number of unique time zones in the database.
     /// </summary>
@@ -51,7 +71,8 @@ internal class Database {
     /// <summary>
     /// Updates the last activity field for the specified guild user, if existing in the database.
     /// </summary>
-    public async Task UpdateLastActivityAsync(SocketGuildUser user) {
+    /// <returns>True if a value was updated, implying that the specified user exists in the database.</returns>
+    public async Task<bool> UpdateLastActivityAsync(SocketGuildUser user) {
         using var db = await OpenConnectionAsync().ConfigureAwait(false);
         using var c = db.CreateCommand();
         c.CommandText = $"UPDATE {UserDatabase} SET last_active = now() " +
@@ -59,7 +80,7 @@ internal class Database {
         c.Parameters.Add("@Gid", NpgsqlDbType.Bigint).Value = (long)user.Guild.Id;
         c.Parameters.Add("@Uid", NpgsqlDbType.Bigint).Value = (long)user.Id;
         await c.PrepareAsync().ConfigureAwait(false);
-        await c.ExecuteNonQueryAsync().ConfigureAwait(false);
+        return await c.ExecuteNonQueryAsync().ConfigureAwait(false) > 0;
     }
 
     // TODO remove data from users with very distant last activity. how long ago?
@@ -124,7 +145,7 @@ internal class Database {
 SELECT zone, user_id FROM {UserDatabase}
 WHERE
     guild_id = @Gid
-    AND last_active >= now() - INTERVAL '30 days' -- TODO make configurable?
+    AND last_active >= now() - {CutoffInterval}
 ORDER BY RANDOM() -- Randomize results for display purposes";
         c.Parameters.Add("@Gid", NpgsqlDbType.Bigint).Value = (long)guildId;
         await c.PrepareAsync().ConfigureAwait(false);

WorldTime.cs

@@ -44,7 +44,6 @@ internal class WorldTime : IDisposable {
             LogLevel = LogSeverity.Info,
             DefaultRetryMode = RetryMode.RetryRatelimit,
             MessageCacheSize = 0, // disable message cache
-            AlwaysDownloadUsers = true,
             GatewayIntents = GatewayIntents.Guilds | GatewayIntents.GuildMembers | GatewayIntents.GuildMessages
         });
         DiscordClient.Log += DiscordClient_Log;
@@ -150,7 +149,14 @@ internal class WorldTime : IDisposable {
 
     private Task DiscordClient_ShardReady(DiscordSocketClient arg) => arg.SetGameAsync(Commands.CommandPrefix + "help");
 
+    /// <summary>
+    /// Non-specific handler for incoming events.
+    /// </summary>
     private async Task DiscordClient_MessageReceived(SocketMessage message) {
+        if (message.Author.IsWebhook) return;
+        if (message.Type != MessageType.Default) return;
+        if (message.Channel is not SocketTextChannel channel) return;
+
         /*
          * From https://support-dev.discord.com/hc/en-us/articles/4404772028055:
          * "You will still receive the events and can call the same APIs, and you'll get other data about a message like
@@ -159,13 +165,17 @@ internal class WorldTime : IDisposable {
          * 
          * Assuming this stays true, it will be possible to maintain legacy behavior after this bot loses full access to incoming messages.
          */
-
+        // Attempt to update user's last_seen column
         // POTENTIAL BUG: If user does a list command, the list may be processed before their own time's refreshed, and they may be skipped.
-        if (message.Author.IsWebhook) return;
-        if (message.Type != MessageType.Default) return;
-        if (message.Channel is not SocketTextChannel) return;
+        var hasMemberHint = await Database.UpdateLastActivityAsync((SocketGuildUser)message.Author).ConfigureAwait(false);
 
-        await Database.UpdateLastActivityAsync((SocketGuildUser)message.Author).ConfigureAwait(false);
+        // Proactively fill guild user cache if the bot has any data for the respective guild
+        // Can skip an extra query if the last_seen update is known to have been successful, otherwise query for any users
+        var guild = channel.Guild;
+        if (!guild.HasAllMembers && (hasMemberHint || await Database.HasAnyAsync(guild).ConfigureAwait(false))) {
+            // Event handler hangs if awaited normally or used with Task.Run
+            await Task.Factory.StartNew(guild.DownloadUsersAsync).ConfigureAwait(false);
+        }
     }
     #endregion
 }