M3: what each handler did with an activity

Arrival carries a verdict that handlers set with one line before an existing return
(Arrival.Drop, Reject, Accept, About), so no handler signature changes. InboxProcessor
records it as an 'in' event, with the time taken, the wait since the inbox accepted it, the
attempt, the audience (from the post's visibility, or else the activity's addressing), the
local actor kind, the object's age for updates, deletes and reactions, and the FEP features
of a delivered object. It also records types nothing handles (unknown-type), actors that
cannot be loaded (deferred) and handler failures.

Drop reasons: fetch-failed, unparseable, misattributed, duplicate, deleted, not-addressed,
not-followed, not-public, not-visible, not-deleted, unknown-object, unknown-recipient,
cross-origin, unsupported. Accepted sub-reasons: stored, poll-vote, edit, refresh,
actor-refresh, actor-delete, removed, tombstone-only, auto-accepted, pending,
follow-answer, quote-answer, quote-granted, undone, reaction, reported. Rejected: blocked,
ignored, quote-refused.

A circle's traffic is private and kindless, and a stranger posting into one is just
not-addressed, so the event store cannot reveal a circle.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ELjqpznMFMNrJoJUj6K5p2

PrivaPub/Federation/Inbox/Handlers/UndoHandler.cs

@@ -37,20 +37,26 @@ namespace PrivaPub.Federation.Inbox.Handlers
 			var innerType = inner is JsonObject ? Value(inner, "type") : default;
 			var innerId = Id(inner);
 
+			Arrival.About(innerType);
+			var undone = false;
 			if (innerType is null or "Follow")
-				await UndoFollow(inner, innerId, actor, token);
+				undone |= await UndoFollow(inner, innerId, actor, token);
 			if (innerType is null or "Like")
-				await UndoLike(inner, innerId, actor, token);
+				undone |= await UndoLike(inner, innerId, actor, token);
 			if (innerType is null or "Announce")
-				await UndoAnnounce(innerId, actor, token);
+				undone |= await UndoAnnounce(innerId, actor, token);
 			if (innerType is null or "Dislike")
-				await UndoDislike(inner, innerId, actor, token);
+				undone |= await UndoDislike(inner, innerId, actor, token);
 			if (innerType is null or "Like" or "EmojiReact")
-				await _reactions.Withdraw(actor, innerId, inner is JsonObject ? Id(inner["object"]) : default,
+				undone |= await _reactions.Withdraw(actor, innerId, inner is JsonObject ? Id(inner["object"]) : default,
 					inner is JsonObject ? Value(inner, "_misskey_reaction") ?? Value(inner, "content") : default, token);
+			if (undone)
+				Arrival.Accept("undone");
+			else
+				Arrival.Drop("unknown-object");
 		}
 
-		async Task UndoDislike(JsonNode inner, string innerId, ForeignAvatar actor, CancellationToken token)
+		async Task<bool> UndoDislike(JsonNode inner, string innerId, ForeignAvatar actor, CancellationToken token)
 		{
 			var downvote = innerId == default
 				? default
@@ -63,16 +69,18 @@ namespace PrivaPub.Federation.Inbox.Handlers
 					: await DB.Default.Find<Downvote>().Match(d => d.PostId == post.ID && d.ActorURI == actor.ActorURI).ExecuteFirstAsync(token);
 			}
 			if (downvote == default)
-				return;
+				return false;
 			await DB.Default.DeleteAsync<Downvote>(downvote.ID);
 			await DB.Default.Update<PostEntity>().MatchID(downvote.PostId).Modify(b => b.Inc(p => p.DownvotesCount, -1)).ExecuteAsync(token);
+			return true;
 		}
 
-		async Task UndoFollow(JsonNode inner, string innerId, ForeignAvatar actor, CancellationToken token)
+		async Task<bool> UndoFollow(JsonNode inner, string innerId, ForeignAvatar actor, CancellationToken token)
 		{
 			var followers = await _dbEntities.Followers.Match(f => f.ActorURI == actor.ActorURI).ExecuteAsync(token);
 			var targetUri = inner is JsonObject ? Id(inner["object"]) : default;
 			var target = targetUri == default ? default : await _localActors.FindByUri(targetUri, token);
+			var undone = false;
 			foreach (var follower in followers)
 			{
 				var matchesActivity = innerId != default && follower.FollowActivityURI == innerId;
@@ -83,10 +91,12 @@ namespace PrivaPub.Federation.Inbox.Handlers
 				await DB.Default.DeleteAsync<Follower>(follower.ID);
 				if (follower.LocalActorKind == LocalActorKind.Group)
 					await RemoveForeignMember(follower.LocalActorId, actor.ActorURI, token);
+				undone = true;
 			}
+			return undone;
 		}
 
-		async Task UndoLike(JsonNode inner, string innerId, ForeignAvatar actor, CancellationToken token)
+		async Task<bool> UndoLike(JsonNode inner, string innerId, ForeignAvatar actor, CancellationToken token)
 		{
 			var favourite = innerId == default
 				? default
@@ -99,23 +109,25 @@ namespace PrivaPub.Federation.Inbox.Handlers
 					: await _dbEntities.Favourites.Match(f => f.PostId == post.ID && f.ActorURI == actor.ActorURI).ExecuteFirstAsync(token);
 			}
 			if (favourite == default)
-				return;
+				return false;
 			await DB.Default.DeleteAsync<Favourite>(favourite.ID);
 			await DB.Default.Update<PostEntity>().MatchID(favourite.PostId).Modify(b => b.Inc(p => p.FavouritesCount, -1)).ExecuteAsync(token);
+			return true;
 		}
 
-		async Task UndoAnnounce(string innerId, ForeignAvatar actor, CancellationToken token)
+		async Task<bool> UndoAnnounce(string innerId, ForeignAvatar actor, CancellationToken token)
 		{
 			if (innerId == default)
-				return;
+				return false;
 			var reblog = await _dbEntities.Posts
 				.Match(p => p.ObjectURI == innerId && p.ActorURI == actor.ActorURI && p.ReblogOfPostId != null)
 				.ExecuteFirstAsync(token);
 			if (reblog == default)
-				return;
+				return false;
 			await DB.Default.DeleteAsync<PostEntity>(reblog.ID);
 			await DB.Default.DeleteAsync<TimelineEntry>(e => e.PostId == reblog.ID);
 			await DB.Default.Update<PostEntity>().MatchID(reblog.ReblogOfPostId).Modify(b => b.Inc(p => p.ReblogsCount, -1)).ExecuteAsync(token);
+			return true;
 		}
 	}
 }