SocialPub/PrivaPub/Federation/Actors/LocalActorService.cs

using Microsoft.Extensions.Options;

using MongoDB.Driver;
using MongoDB.Entities;

using PrivaPub.Models;
using PrivaPub.Models.Federation;
using PrivaPub.Models.Group;
using PrivaPub.Models.User;
using PrivaPub.StaticServices;

using System.Security.Cryptography;

using GroupEntity = PrivaPub.Models.Group.Group;

namespace PrivaPub.Federation.Actors
{
	public class LocalActor
	{
		public string Id { get; init; }
		public LocalActorKind Kind { get; init; }
		public string UserName { get; init; }
		public string Name { get; init; }
		public string Summary { get; init; }
		public string PictureURL { get; init; }
		public string ThumbnailURL { get; init; }
		public string PrivateKeyPem { get; init; }
		public string PublicKeyPem { get; init; }
		public string SigningKey { get; init; }//a persona's Ed25519 seed (FEP-521a), for the proofs its activities carry (FEP-8b32)
		public bool Discoverable { get; init; } = true;
		public bool ManuallyApprovesFollowers { get; init; }
		public bool IsFederated { get; init; } = true;
		public bool IsCircle { get; init; }
		public bool PostingRestrictedToModerators { get; init; }
		public bool IsBot { get; init; }
		public bool Indexable { get; init; }
		public AvatarSettings Settings { get; init; } = new();
		public DateTime Published { get; init; }
		public string BaseAddress { get; init; }
		public IReadOnlyDictionary<string, string> Fields { get; init; } = new Dictionary<string, string>();

		public string Uri => $"{BaseAddress}/peasants/{UserName}";
		public string KeyId => $"{Uri}#main-key";
		public string AssertionKeyId => $"{Uri}#ed25519-key";
		public string Inbox => $"{Uri}/mouth";
		public string Outbox => $"{Uri}/anus";
		public string Followers => $"{Uri}/groupies";
		public string Following => $"{Uri}/stalking";
		public string Featured => $"{Uri}/trophies";
		public string FeaturedTags => $"{Uri}/tattoos";
		public string Wall => $"{Uri}/graffiti";
		public bool HasWall => Kind == LocalActorKind.Person && !IsCircle && IsFederated;
		// the server's own actors (the instance actor, the reporter): nobody follows, mentions or looks them up as accounts
		public bool IsServerActor => Kind is LocalActorKind.Application or LocalActorKind.Reporter;
		public string Flock => $"{Uri}/flock";
		public string Wardens => $"{Uri}/wardens";
		public string SharedInbox => $"{BaseAddress}/human-centipede";
		public string Domain => new Uri(BaseAddress).Authority;
		public string Handle => $"{UserName}@{Domain}";
		public string HtmlUrl => $"{BaseAddress}/@{UserName}";
		public string PostUri(string postId) => $"{Uri}/scribbles/{postId}";
		public string PostHtmlUrl(string postId) => $"{HtmlUrl}/{postId}";
		public string ActivityUri(string activityId) => $"{Uri}/grunts/{activityId}";
		public string ConversationUri(string conversationId) => $"{Uri}/whispers/{conversationId}";
	}

	public interface ILocalActorService
	{
		string BaseAddress { get; }
		Task<LocalActor> FindByUserName(string userName, CancellationToken token);
		Task<GoneActor> Gone(string userName, CancellationToken token);
		Task<LocalActor> FindById(LocalActorKind kind, string id, CancellationToken token);
		Task<LocalActor> FindByUri(string actorUri, CancellationToken token);
		Task<LocalActor> FindByAddress(string address, CancellationToken token);
		Task<LocalActor> GetInstanceActor(CancellationToken token);
		Task<LocalActor> GetReporterActor(CancellationToken token);
		Task<bool> IsUserNameTaken(string userName, CancellationToken token);
		Task<bool> TryReserveUserName(string userName, LocalActorKind kind, string ownerId, CancellationToken token);
		LocalActor FromAvatar(Avatar avatar);
		LocalActor FromGroup(GroupEntity group);
	}

	// a persona or group that was deleted: its name stays reserved, and its documents answer 410 with this
	public sealed record GoneActor(string Uri, string FormerType, DateTime DeletedAt);

	public class LocalActorService : ILocalActorService
	{
		public const string InstanceUserName = "privapub";
		// the anonymous Service that carries this server's reports to Lemmy, which takes no report from an Application
		// (owner decision 2026-10-06): one actor for the server, never one per persona
		public const string ReporterUserName = "privapub_reports";

		public static bool IsServerActorName(string userName) =>
			string.Equals(userName, InstanceUserName, StringComparison.OrdinalIgnoreCase) || string.Equals(userName, ReporterUserName, StringComparison.OrdinalIgnoreCase);

		static readonly HashSet<string> ReservedByInstance = new(StringComparer.Ordinal)
		{
			InstanceUserName, ReporterUserName, "admin", "administrator", "root", "system", "support", "help", "moderator", "mod",
			"abuse", "postmaster", "webmaster", "hostmaster", "security", "noreply", "no_reply", "null", "undefined"
		};

		readonly DbEntities _dbEntities;
		readonly IOptionsMonitor<AppConfiguration> _appConfiguration;
		InstanceActor _instanceActor;
		ReporterActor _reporterActor;

		public LocalActorService(DbEntities dbEntities, IOptionsMonitor<AppConfiguration> appConfiguration)
		{
			_dbEntities = dbEntities;
			_appConfiguration = appConfiguration;
		}

		public string BaseAddress => _appConfiguration.CurrentValue.BackendBaseAddress?.TrimEnd('/');

		public async Task<LocalActor> FindByUserName(string userName, CancellationToken token)
		{
			if (string.IsNullOrEmpty(userName))
				return default;
			userName = userName.ToLowerInvariant();
			if (userName == InstanceUserName)
				return await GetInstanceActor(token);
			if (userName == ReporterUserName)
				return await GetReporterActor(token);

			var avatar = await _dbEntities.Avatars
				.Match(a => a.UserName == userName && !a.DeletionAt.HasValue)
				.ExecuteFirstAsync(token);
			if (avatar != default)
				return FromAvatar(avatar);

			var group = await _dbEntities.Groups
				.Match(g => g.UserName == userName && !g.DeletionAt.HasValue)
				.ExecuteFirstAsync(token);
			return group == default ? default : FromGroup(group);
		}

		public async Task<GoneActor> Gone(string userName, CancellationToken token)
		{
			if (string.IsNullOrEmpty(userName))
				return default;
			userName = userName.ToLowerInvariant();
			var avatar = await _dbEntities.Avatars.Match(a => a.UserName == userName && a.DeletionAt.HasValue).ExecuteFirstAsync(token);
			if (avatar != default)
				return new GoneActor($"{BaseAddress}/peasants/{avatar.UserName}", "Person", avatar.DeletionAt.Value);
			var group = await _dbEntities.Groups.Match(g => g.UserName == userName && g.DeletionAt.HasValue).ExecuteFirstAsync(token);
			return group == default ? default : new GoneActor($"{BaseAddress}/peasants/{group.UserName}", "Group", group.DeletionAt.Value);
		}

		public async Task<LocalActor> FindById(LocalActorKind kind, string id, CancellationToken token)
		{
			switch (kind)
			{
				case LocalActorKind.Person:
					var avatar = await _dbEntities.Avatars.MatchID(id).ExecuteFirstAsync(token);
					return avatar == default ? default : FromAvatar(avatar);
				case LocalActorKind.Group:
					var group = await _dbEntities.Groups.MatchID(id).ExecuteFirstAsync(token);
					return group == default ? default : FromGroup(group);
				case LocalActorKind.Reporter:
					return await GetReporterActor(token);
				default:
					return await GetInstanceActor(token);
			}
		}

		public Task<LocalActor> FindByUri(string actorUri, CancellationToken token)
		{
			var prefix = $"{BaseAddress}/peasants/";
			if (string.IsNullOrEmpty(actorUri) || !actorUri.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
				return Task.FromResult<LocalActor>(default);
			var userName = actorUri[prefix.Length..].Split('/', '#', '?')[0];
			return FindByUserName(userName, token);
		}

		// an actor id, or the profile page its document names as `url`: Mbin addresses its private messages to that page
		public Task<LocalActor> FindByAddress(string address, CancellationToken token)
		{
			var profile = $"{BaseAddress}/@";
			if (string.IsNullOrEmpty(address) || !address.StartsWith(profile, StringComparison.OrdinalIgnoreCase))
				return FindByUri(address, token);
			var userName = address[profile.Length..];
			return userName.Length == 0 || userName.IndexOfAny(['/', '#', '?']) >= 0 ? Task.FromResult<LocalActor>(default) : FindByUserName(userName, token);
		}

		public async Task<LocalActor> GetInstanceActor(CancellationToken token)
		{
			var instance = _instanceActor ??= await LoadInstanceActor(token);

			return new LocalActor
			{
				Id = instance.ID,
				Kind = LocalActorKind.Application,
				UserName = InstanceUserName,
				Name = "PrivaPub",
				Summary = "The instance actor of this PrivaPub server; it signs the requests no other actor speaks for.",
				PrivateKeyPem = instance.PrivateKey,
				PublicKeyPem = instance.PublicKey,
				Discoverable = false,
				Published = instance.CreationDate,
				BaseAddress = BaseAddress
			};
		}

		public async Task<LocalActor> GetReporterActor(CancellationToken token)
		{
			var reporter = _reporterActor ??= await LoadReporterActor(token);

			return new LocalActor
			{
				Id = reporter.ID,
				Kind = LocalActorKind.Reporter,
				UserName = ReporterUserName,
				Name = $"Reports from {new Uri(BaseAddress).Host}",
				Summary = "It carries this PrivaPub server's reports to the servers that take them only from a person or a service; "
					+ "it never names who made them.",
				PrivateKeyPem = reporter.PrivateKey,
				PublicKeyPem = reporter.PublicKey,
				Discoverable = false,
				Published = reporter.CreationDate,
				BaseAddress = BaseAddress
			};
		}

		async Task<ReporterActor> LoadReporterActor(CancellationToken token)
		{
			var reporter = await _dbEntities.ReporterActors.Sort(r => r.CreationDate, Order.Ascending).ExecuteFirstAsync(token);
			if (reporter != default)
				return reporter;
			var (privateKey, publicKey) = Keys.NewKeyPair();
			reporter = new ReporterActor { PrivateKey = privateKey, PublicKey = publicKey };
			await DB.Default.SaveAsync(reporter, token);
			return reporter;
		}

		async Task<InstanceActor> LoadInstanceActor(CancellationToken token)
		{
			var instance = await _dbEntities.InstanceActors.Sort(i => i.CreationDate, Order.Ascending).ExecuteFirstAsync(token);
			if (instance != default)
				return instance;
			var (privateKey, publicKey) = Keys.NewKeyPair();
			instance = new InstanceActor { PrivateKey = privateKey, PublicKey = publicKey };
			await DB.Default.SaveAsync(instance, token);
			return instance;
		}

		public async Task<bool> IsUserNameTaken(string userName, CancellationToken token)
		{
			userName = userName?.ToLowerInvariant();
			if (string.IsNullOrEmpty(userName) || ReservedByInstance.Contains(userName))
				return true;
			return await DB.Default.Find<ReservedName>().Match(r => r.Name == userName).ExecuteAnyAsync(token);
		}

		public async Task<bool> TryReserveUserName(string userName, LocalActorKind kind, string ownerId, CancellationToken token)
		{
			userName = userName?.ToLowerInvariant();
			if (string.IsNullOrEmpty(userName) || ReservedByInstance.Contains(userName))
				return false;
			try
			{
				await DB.Default.SaveAsync(new ReservedName { Name = userName, OwnerKind = kind, OwnerId = ownerId }, token);
				return true;
			}
			catch (MongoWriteException ex) when (ex.WriteError?.Category == ServerErrorCategory.DuplicateKey)
			{
				return false;
			}
		}

		public LocalActor FromAvatar(Avatar avatar) => new()
		{
			Id = avatar.ID,
			Kind = LocalActorKind.Person,
			UserName = avatar.UserName,
			Name = string.IsNullOrEmpty(avatar.Name) ? avatar.UserName : avatar.Name,
			Summary = avatar.Biography,
			PictureURL = avatar.PictureURL,
			ThumbnailURL = avatar.ThumbnailURL,
			PrivateKeyPem = avatar.PrivateKey,
			PublicKeyPem = avatar.PublicKey,
			SigningKey = avatar.SigningKey,
			Published = avatar.PublishedOn,
			Fields = avatar.Fields ?? new Dictionary<string, string>(),
			ManuallyApprovesFollowers = avatar.Settings?.IsLocked == true,
			Discoverable = avatar.Settings?.IsDiscoverable != false,
			IsBot = avatar.Settings?.IsBot == true,
			Indexable = avatar.Settings?.IsIndexable == true,
			Settings = avatar.Settings ?? new AvatarSettings(),
			BaseAddress = BaseAddress
		};

		public LocalActor FromGroup(GroupEntity group) => new()
		{
			Id = group.ID,
			Kind = LocalActorKind.Group,
			UserName = group.UserName,
			Name = string.IsNullOrEmpty(group.Name) ? group.UserName : group.Name,
			Summary = group.Description,
			PictureURL = group.PictureURL,
			ThumbnailURL = group.ThumbnailURL,
			PrivateKeyPem = group.PrivateKey,
			PublicKeyPem = group.PublicKey,
			Discoverable = group.Kind == GroupKind.Community && group.IsDiscoverable,
			ManuallyApprovesFollowers = group.Kind == GroupKind.Circle || group.ManuallyApprovesMembers,
			IsCircle = group.Kind == GroupKind.Circle,
			PostingRestrictedToModerators = group.PostingPolicy == PostingPolicy.Moderators,
			Published = group.PublishedOn,
			BaseAddress = BaseAddress
		};
	}

	public static class Keys
	{
		public static (string PrivateKeyPem, string PublicKeyPem) NewKeyPair()
		{
			using var rsa = RSA.Create(2048);
			return (rsa.ExportRSAPrivateKeyPem(), rsa.ExportSubjectPublicKeyInfoPem());
		}

		public static string ToSubjectPublicKeyInfoPem(string publicKeyPem)
		{
			if (string.IsNullOrEmpty(publicKeyPem) || publicKeyPem.Contains("BEGIN PUBLIC KEY"))
				return publicKeyPem;
			using var rsa = RSA.Create();
			rsa.ImportFromPem(publicKeyPem);
			return rsa.ExportSubjectPublicKeyInfoPem();
		}
	}
}