presence fixes

src/Argon.Api/Grains/FriendsGrain.cs

@@ -23,6 +23,16 @@ private async Task NotifyAsync<T>(Guid userId, T payload) where T : IArgonEvent
         await notifier.NotifySessionsAsync(sessions, payload);
     }
 
+    /// <summary>
+    /// Two people who have just become friends have each missed every status event the other has
+    /// ever fired, and presence is only pushed forward from here - without this both sides read as
+    /// offline until the other happens to change something.
+    /// </summary>
+    private Task ExchangePresenceAsync(Guid a, Guid b)
+        => Task.WhenAll(
+            GrainFactory.GetGrain<IUserGrain>(a).PushFriendPresenceAsync().AsTask(),
+            GrainFactory.GetGrain<IUserGrain>(b).PushFriendPresenceAsync().AsTask());
+
     public async Task<List<UserBlock>> GetBlockListAsync(int limit, int offset, CancellationToken ct = default)
     {
         var             meUserId = this.GetUserId();
@@ -158,6 +168,8 @@ await NotifyAsync(me,
             await NotifyAsync(target.Value,
                 new FriendRequestAcceptedEvent(me, ts));
 
+            await ExchangePresenceAsync(me, target.Value);
+
             return SendFriendStatus.AutoAccepted;
         }
 
@@ -290,6 +302,8 @@ await NotifyAsync(me,
         await NotifyAsync(fromUserId,
             new FriendRequestAcceptedEvent(me, ts));
 
+        await ExchangePresenceAsync(me, fromUserId);
+
         var chatId = ArgonId.New();
 
         var chatGrain = this.GrainFactory.GetGrain<IUserChatGrain>(chatId);

src/Argon.Api/Grains/UserGrain.cs

@@ -16,6 +16,8 @@ public class UserGrain(
     IDbContextFactory<ApplicationDbContext> context,
     IUserPresenceService presenceService,
     ILogger<IUserGrain> logger,
+    IUserSessionDiscoveryService sessionDiscovery,
+    IUserSessionNotifier notifier,
     AppHubServer appHubServer) : Grain, IUserGrain
 {
     private static readonly TimeSpan DisplayNameCooldown = TimeSpan.FromMinutes(10);
@@ -492,6 +494,89 @@ await Task.WhenAll(servers.Select(server =>
             GrainFactory
                .GetGrain<ISpaceGrain>(server)
                .SetUserStatus(userId, aggregatedStatus)));
+
+        await BroadcastStatusToFriendsAsync(userId, aggregatedStatus, ct);
+    }
+
+    /// <summary>
+    /// The mirror of the fan-out below: a session that has just connected has missed every status
+    /// event that fired before it existed, so friends who were already online would read as offline
+    /// until they next changed anything.
+    /// </summary>
+    /// <remarks>
+    /// One friend-id query and one batched presence read per session start. Only friends who are
+    /// actually online are sent - the client's own default for an unknown user is offline.
+    /// </remarks>
+    public async ValueTask PushFriendPresenceAsync(CancellationToken ct = default)
+    {
+        var userId = this.GetPrimaryKey();
+
+        await using var ctx = await context.CreateDbContextAsync(ct);
+
+        var friendIds = await ctx.Friends
+           .AsNoTracking()
+           .Where(x => x.UserId == userId)
+           .Select(x => x.FriendId)
+           .ToListAsync(ct);
+
+        if (friendIds.Count == 0)
+            return;
+
+        var sessions = await sessionDiscovery.GetUserSessionsAsync(userId, ct);
+        if (sessions.Count == 0)
+            return;
+
+        var statuses = await presenceService.BatchGetAggregatedStatusAsync(friendIds, ct);
+
+        foreach (var (friendId, status) in statuses)
+        {
+            if (status == UserStatus.Offline)
+                continue;
+
+            await notifier.NotifySessionsAsync(
+                sessions,
+                new UserChangedStatus(Guid.Empty, friendId, status, new IonArray<string>([""])),
+                ct);
+        }
+    }
+
+    /// <summary>
+    /// UserChangedStatus is only ever fired by SpaceGrain, to the members of that space - so a
+    /// friend you share no space with never learned that you came online, and their friends list
+    /// sat on whatever it last happened to cache (for someone just added: offline, forever).
+    /// </summary>
+    /// <remarks>
+    /// Only reached when the aggregate actually changed - the hysteresis check above already
+    /// swallowed heartbeats and reconnects - so this costs one friend-id query and one notify per
+    /// real transition. A friend who is also a space member receives the event twice; deduplicating
+    /// would cost a membership join on every transition, and the client keys the update on the user
+    /// id alone, so the second one is a no-op.
+    /// </remarks>
+    private async Task BroadcastStatusToFriendsAsync(Guid userId, UserStatus status, CancellationToken ct)
+    {
+        await using var ctx = await context.CreateDbContextAsync(ct);
+
+        var friendIds = await ctx.Friends
+           .AsNoTracking()
+           .Where(x => x.UserId == userId)
+           .Select(x => x.FriendId)
+           .ToListAsync(ct);
+
+        if (friendIds.Count == 0)
+            return;
+
+        var sessionsPerFriend = await Task.WhenAll(
+            friendIds.Select(friendId => sessionDiscovery.GetUserSessionsAsync(friendId, ct)));
+
+        var sessions = sessionsPerFriend.SelectMany(x => x).ToList();
+        if (sessions.Count == 0)
+            return;
+
+        // There is no space this is about; the client reads userId and status and ignores the rest.
+        await notifier.NotifySessionsAsync(
+            sessions,
+            new UserChangedStatus(Guid.Empty, userId, status, new IonArray<string>([""])),
+            ct);
     }
 
     private async ValueTask RecordViolationAsync(

src/Argon.Api/Grains/UserSessionGrain.cs

@@ -148,6 +148,7 @@ private async Task EnsureSessionStartedAsync(UserStatus? preferred)
         await presenceService.SetSessionOnlineAsync(_userId, SessionId);
         await presenceService.SetSessionStatusAsync(_userId, SessionId, activation.State.PreferredStatus.Value);
         await grainFactory.GetGrain<IUserGrain>(_userId).AggregateAndBroadcastStatusAsync();
+        await grainFactory.GetGrain<IUserGrain>(_userId).PushFriendPresenceAsync();
         await grainFactory.GetGrain<IUserGrain>(_userId).UpdateUserDeviceHistory();
 
         logger.LogInformation("Session {sid} started for user {userId}", SessionId, _userId);

src/Argon.Core/Grains/Interfaces/IUserGrain.cs

@@ -65,6 +65,14 @@ public interface IUserGrain : IGrainWithGuidKey
     [Alias(nameof(AggregateAndBroadcastStatusAsync))]
     ValueTask AggregateAndBroadcastStatusAsync(CancellationToken ct = default);
 
+    /// <summary>
+    /// Sends this user's sessions the current status of each of their friends.
+    /// Called by UserSessionGrain when a session starts: presence events only travel forward in
+    /// time, so a fresh session knows nothing about friends who came online before it connected.
+    /// </summary>
+    [Alias(nameof(PushFriendPresenceAsync))]
+    ValueTask PushFriendPresenceAsync(CancellationToken ct = default);
+
     [Alias(nameof(ResetPremiumProfileAsync))]
     ValueTask ResetPremiumProfileAsync(CancellationToken ct = default);