WatchedTogether/Jellyfin.Plugin.WatchedTogether/Services/DynamicGroupService.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Jellyfin.Data;
using Jellyfin.Database.Implementations.Entities;
using Jellyfin.Database.Implementations.Enums;
using Jellyfin.Plugin.WatchedTogether.Configuration;
using MediaBrowser.Controller.Library;
using MediaBrowser.Model.Cryptography;
using Microsoft.Extensions.Logging;

namespace Jellyfin.Plugin.WatchedTogether.Services;

/// <summary>
/// Creates a shared account the first time someone logs in as "alice+bob".
/// </summary>
/// <remarks>
/// <para>
/// Jellyfin only routes a login to the providers with a null resolved user when <em>no</em> local
/// user matches the typed name. A real account named "alice+bob" therefore always wins, and this
/// code never sees it - the ambiguity between a group and a same-named user resolves in favour of
/// the real user automatically.
/// </para>
/// <para>
/// Every named member must exist and none may be a shared account, so an unrelated username
/// containing the separator simply fails to resolve and is rejected.
/// </para>
/// </remarks>
public class DynamicGroupService : IDynamicGroupService
{
    private readonly IUserManager _userManager;
    private readonly IProvisioningService _provisioningService;
    private readonly ICryptoProvider _cryptoProvider;
    private readonly ILogger<DynamicGroupService> _logger;

    /// <summary>
    /// Initializes a new instance of the <see cref="DynamicGroupService"/> class.
    /// </summary>
    /// <param name="userManager">The user manager.</param>
    /// <param name="provisioningService">The provisioning service.</param>
    /// <param name="cryptoProvider">The crypto provider.</param>
    /// <param name="logger">The logger.</param>
    public DynamicGroupService(
        IUserManager userManager,
        IProvisioningService provisioningService,
        ICryptoProvider cryptoProvider,
        ILogger<DynamicGroupService> logger)
    {
        _userManager = userManager;
        _provisioningService = provisioningService;
        _cryptoProvider = cryptoProvider;
        _logger = logger;
    }

    /// <inheritdoc />
    public async Task<DynamicGroupResult?> TryCreateFromLoginAsync(string enteredUsername, string password)
    {
        var config = Plugin.Instance?.Configuration;
        if (config is null || !config.EnableDynamicGroups)
        {
            return null;
        }

        if (string.IsNullOrWhiteSpace(enteredUsername))
        {
            return null;
        }

        var separator = string.IsNullOrEmpty(config.NameSeparator) ? "+" : config.NameSeparator;

        var parts = enteredUsername
            .Split(separator, StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries)
            .ToList();

        // Needs at least two names to be a group at all.
        if (parts.Count < 2)
        {
            return null;
        }

        // Every part must name a real, non-shared, enabled user. Anything else means this is not a
        // group login, so fall through and let the attempt fail normally.
        var members = new List<User>(parts.Count);
        foreach (var part in parts)
        {
            var member = _userManager.GetUserByName(part);
            if (member is null)
            {
                _logger.LogDebug("Dynamic group login rejected: no user named {Part}", part);
                return null;
            }

            if (config.Groups.Any(g => g.SharedUserId == member.Id))
            {
                _logger.LogWarning(
                    "Dynamic group login rejected: {Part} is itself a shared account",
                    part);
                return null;
            }

            if (member.HasPermission(PermissionKind.IsDisabled))
            {
                _logger.LogWarning("Dynamic group login rejected: {Part} is disabled", part);
                return null;
            }

            if (members.Any(m => m.Id == member.Id))
            {
                _logger.LogWarning("Dynamic group login rejected: {Part} named more than once", part);
                return null;
            }

            members.Add(member);
        }

        // The password must belong to one of the named members. Without this any visitor could
        // conjure a shared account out of two usernames they happened to know.
        //
        // Members are tried in the order they were typed and the loop stops at the first match, so
        // whoever types their own name first has their password checked first. Verification is a
        // deliberately slow hash comparison, so the ordering is worth having.
        var matched = members.FirstOrDefault(m => VerifyPassword(m, password));
        if (matched is null)
        {
            _logger.LogWarning(
                "Dynamic group login for {Username} rejected: no named member's password matched",
                enteredUsername);
            return null;
        }

        var memberIds = members.Select(m => m.Id).ToList();

        // The same people in a different order are the same group: someone typing "john+jane" must
        // land on the existing "jane+john" account rather than creating a second one. Jellyfin only
        // reaches this code when no account matches the typed name, so without this check every
        // ordering would spawn its own account.
        var existing = FindGroupWithSameMembers(config, memberIds);
        if (existing is not null)
        {
            if (existing.IsDisabled)
            {
                _logger.LogWarning(
                    "Login for {Username} rejected: the matching group is disabled",
                    enteredUsername);
                return null;
            }

            var existingUser = _userManager.GetUserById(existing.SharedUserId);
            if (existingUser is null)
            {
                _logger.LogWarning(
                    "Group for {Username} references a shared account that no longer exists",
                    enteredUsername);
                return null;
            }

            _logger.LogInformation(
                "Login as {Entered} resolved to the existing shared account {Username}",
                enteredUsername,
                existingUser.Username);

            return new DynamicGroupResult(existing, existingUser.Username);
        }

        // Passing no name lets provisioning generate the canonical alphabetically-sorted one, so
        // the account is named the same whichever order the members were typed in.
        // The account is limited to the libraries all named members share, so creating one at the
        // login screen cannot grant anybody access they did not already have.
        var group = await _provisioningService.CreateGroupAsync(memberIds, null).ConfigureAwait(false);

        var sharedUser = _userManager.GetUserById(group.SharedUserId);
        if (sharedUser is null)
        {
            return null;
        }

        _logger.LogInformation(
            "Created shared account {Username} on demand for {MemberCount} members",
            sharedUser.Username,
            members.Count);

        return new DynamicGroupResult(group, sharedUser.Username);
    }

    /// <summary>
    /// Finds a configured group whose members are exactly the given set, ignoring order.
    /// </summary>
    /// <param name="config">The plugin configuration to search.</param>
    /// <param name="memberIds">The member identifiers to match.</param>
    /// <returns>The matching group, or <c>null</c> if no group has that membership.</returns>
    private static SharedGroup? FindGroupWithSameMembers(
        PluginConfiguration config,
        IReadOnlyList<Guid> memberIds)
    {
        var wanted = memberIds.ToHashSet();

        return config.Groups.FirstOrDefault(g =>
            g.MemberUserIds.Count == wanted.Count && wanted.SetEquals(g.MemberUserIds));
    }

    /// <summary>
    /// Verifies a submitted password against a member's live stored hash.
    /// </summary>
    /// <param name="member">The member to check.</param>
    /// <param name="password">The submitted password.</param>
    /// <returns><c>true</c> if the password matches.</returns>
    private bool VerifyPassword(User member, string password)
    {
        if (string.IsNullOrEmpty(member.Password))
        {
            return false;
        }

        try
        {
            return _cryptoProvider.Verify(PasswordHash.Parse(member.Password), password);
        }
        catch (Exception ex) when (ex is FormatException or ArgumentException)
        {
            // Never log the hash or the submitted password.
            _logger.LogError(ex, "Could not parse the stored password hash for member {MemberId}", member.Id);
            return false;
        }
    }
}