From 2523f8a4d1734026e7d778ccebf477972e019305 Mon Sep 17 00:00:00 2001
From: Kopper <kopper@w.on-t.work>
Date: Mon, 9 Sep 2024 17:41:15 +0300
Subject: [PATCH] [backend/federation] Expose replies collection

Exposes the replies collection for local notes, allowing remote
instances to backfill replies when fetching our posts.
---
 .../Federation/ActivityPubController.cs       | 28 +++++++++++++++++++
 .../Federation/ActivityPub/NoteRenderer.cs    |  7 +++--
 .../ActivityStreams/Types/ASNote.cs           |  4 +++
 3 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/Iceshrimp.Backend/Controllers/Federation/ActivityPubController.cs b/Iceshrimp.Backend/Controllers/Federation/ActivityPubController.cs
index 298015c5..74f04989 100644
--- a/Iceshrimp.Backend/Controllers/Federation/ActivityPubController.cs
+++ b/Iceshrimp.Backend/Controllers/Federation/ActivityPubController.cs
@@ -74,6 +74,34 @@ public class ActivityPubController(
 		                  .Compact();
 	}
 
+	[HttpGet("/notes/{id}/replies")]
+	[AuthorizedFetch]
+	[OverrideResultType<ASOrderedCollection>]
+	[ProducesResults(HttpStatusCode.OK)]
+	[ProducesErrors(HttpStatusCode.NotFound)]
+	public async Task<JObject> GetNoteReplies(string id)
+	{
+		var actor = HttpContext.GetActor();
+		var note = await db.Notes
+		                   .EnsureVisibleFor(actor)
+		                   .FirstOrDefaultAsync(p => p.Id == id) ??
+		           throw GracefulException.NotFound("Note not found");
+
+		var replies = await db.Notes.Where(p => p.ReplyId == id)
+		                   .OrderByDescending(p => p.Id)
+		                   .ToListAsync();
+
+		var rendered = replies.Select(noteRenderer.RenderLite).ToList();
+		var res = new ASOrderedCollection
+		{
+			Id         = $"{note.GetPublicUri(config.Value)}/replies",
+			TotalItems = (ulong)rendered.Count,
+			Items      = rendered.Cast<ASObject>().ToList()
+		};
+
+		return res.Compact();
+	}
+
 	[HttpGet("/users/{id}")]
 	[AuthorizedFetch]
 	[MediaTypeRouteFilter("application/activity+json", "application/ld+json")]
diff --git a/Iceshrimp.Backend/Core/Federation/ActivityPub/NoteRenderer.cs b/Iceshrimp.Backend/Core/Federation/ActivityPub/NoteRenderer.cs
index 402aa3fe..82b19f19 100644
--- a/Iceshrimp.Backend/Core/Federation/ActivityPub/NoteRenderer.cs
+++ b/Iceshrimp.Backend/Core/Federation/ActivityPub/NoteRenderer.cs
@@ -27,8 +27,9 @@ public class NoteRenderer(IOptions<Config.InstanceSection> config, MfmConverter
 		if (note.IsPureRenote)
 			throw GracefulException.BadRequest("Refusing to render pure renote as ASNote");
 
-		var id     = note.GetPublicUri(config.Value);
-		var userId = note.User.GetPublicUri(config.Value);
+		var id      = note.GetPublicUri(config.Value);
+		var userId  = note.User.GetPublicUri(config.Value);
+		var replies = new ASOrderedCollection($"{id}/replies");
 		var replyId = note.Reply != null
 			? new ASObjectBase(note.Reply.Uri ?? note.Reply.GetPublicUri(config.Value))
 			: null;
@@ -160,6 +161,7 @@ public class NoteRenderer(IOptions<Config.InstanceSection> config, MfmConverter
 					UpdatedAt    = note.UpdatedAt,
 					Sensitive    = sensitive,
 					InReplyTo    = replyId,
+					Replies      = replies,
 					Cc           = cc,
 					To           = to,
 					Tags         = tags,
@@ -190,6 +192,7 @@ public class NoteRenderer(IOptions<Config.InstanceSection> config, MfmConverter
 			UpdatedAt    = note.UpdatedAt,
 			Sensitive    = sensitive,
 			InReplyTo    = replyId,
+			Replies      = replies,
 			Cc           = cc,
 			To           = to,
 			Tags         = tags,
diff --git a/Iceshrimp.Backend/Core/Federation/ActivityStreams/Types/ASNote.cs b/Iceshrimp.Backend/Core/Federation/ActivityStreams/Types/ASNote.cs
index cfa33bae..1a07de69 100644
--- a/Iceshrimp.Backend/Core/Federation/ActivityStreams/Types/ASNote.cs
+++ b/Iceshrimp.Backend/Core/Federation/ActivityStreams/Types/ASNote.cs
@@ -87,6 +87,10 @@ public class ASNote : ASObject
 	[JC(typeof(ASAttachmentConverter))]
 	public List<ASAttachment>? Attachments { get; set; }
 
+	[J($"{Constants.ActivityStreamsNs}#replies")]
+	[JC(typeof(ASOrderedCollectionConverter))]
+	public ASOrderedCollection? Replies { get; set; }
+
 	public Note.NoteVisibility GetVisibility(User actor)
 	{
 		if (actor.IsLocalUser) throw new Exception("Can't get recipients for local actor");