A post named by its page is found as by its id

Pixelfed names one of our posts by the address of its page
(/@name/<post id>, the post's url) in its Like, Announce and their Undo,
so its likes were dropped as unknown objects. Before an activity is
handled, such a reference to a post of ours, as its object or the object
of the activity it undoes, is replaced by the post's id. Found by the
pasture's new Pixelfed peer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub.Tests/Federation/InboxGapTests.cs

@@ -42,6 +42,31 @@ namespace PrivaPub.Tests.Federation
 
 		PrivaPub.Models.Statistics.InteractionEvent Processed(string activity) => _harness.Ledger.Of("in").Last(e => e.Activity == activity);
 
+		// Pixelfed names our post by its page (/@name/<id>, the post's url) in its Like, Announce and their Undo: the post is
+		// found the same as by its id
+		[Fact]
+		public async Task A_like_naming_a_post_by_its_page_counts_and_its_undo_too()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var (_, alice) = await _harness.Persona("alice");
+			var fan = new RemoteActor(_harness.Peer, "fan");
+			var post = (await _harness.Statuses.Publish(alice, new StatusDraft { Text = "a picture" }, token)).Post;
+			var page = alice.PostHtmlUrl(post.ID);
+			Assert.NotEqual(post.ObjectURI, page);
+			var like = new JsonObject { ["id"] = NewId(fan, "likes"), ["type"] = "Like", ["actor"] = fan.Id, ["object"] = page };
+
+			await _harness.Deliver(fan, "/human-centipede", like);
+			Assert.Equal("accepted", Processed("Like").Outcome);
+			Assert.Equal(1, (await DB.Default.Find<Post>().OneAsync(post.ID, token)).FavouritesCount);
+			await _harness.Deliver(fan, "/human-centipede", Activity(fan, "Undo", like));
+			Assert.Equal(0, (await DB.Default.Find<Post>().OneAsync(post.ID, token)).FavouritesCount);
+
+			// a page that is no post of ours stays as it is
+			var elsewhere = new JsonObject { ["id"] = NewId(fan, "likes"), ["type"] = "Like", ["actor"] = fan.Id, ["object"] = alice.PostHtmlUrl("000000000000000000000000") };
+			await _harness.Deliver(fan, "/human-centipede", elsewhere);
+			Assert.Equal(("dropped", "unknown-object"), (Processed("Like").Outcome, Processed("Like").Reason));
+		}
+
 		[Fact]
 		public async Task An_actor_update_refreshes_the_account_from_its_origin_even_when_its_updated_is_older()
 		{

PrivaPub.Tests/Support/Harness.cs

@@ -71,7 +71,7 @@ namespace PrivaPub.Tests.Support
 				new BlockHandler(Db, Local)
 			};
 			((AnnounceHandler)Handlers.First(h => h is AnnounceHandler)).Relays = Handlers;
-			Processor = new InboxProcessor(Remote, Handlers, NullLogger<InboxProcessor>.Instance, Ledger);
+			Processor = new InboxProcessor(Remote, Handlers, NullLogger<InboxProcessor>.Instance, Ledger, Local);
 			Follows = new FollowService(Db, Local, Remote, Delivery, new KeyLocalizer<GenericRes>(), NullLogger<FollowService>.Instance);
 			Content = new ContentRenderer(Local, Remote);
 			Media = new MediaService(new StaticOptions<MediaOptions>(new MediaOptions { Root = Path.Combine(Path.GetTempPath(), $"privapub-media-{Guid.NewGuid():N}") }),

PrivaPub/Federation/Inbox/InboxProcessor.cs

@@ -26,10 +26,12 @@ namespace PrivaPub.Federation.Inbox
 		readonly IReadOnlyDictionary<string, IActivityHandler> _handlers;
 		readonly ILogger<InboxProcessor> _logger;
 		readonly IInteractionLedger _ledger;
+		readonly ILocalActorService _localActors;
 
 		public InboxProcessor(IRemoteActorService remoteActors, IEnumerable<IActivityHandler> handlers, ILogger<InboxProcessor> logger,
-			IInteractionLedger ledger = default)
+			IInteractionLedger ledger = default, ILocalActorService localActors = default)
 		{
+			_localActors = localActors;
 			_remoteActors = remoteActors;
 			_handlers = handlers.ToDictionary(h => h.Type, StringComparer.Ordinal);
 			_logger = logger;
@@ -65,6 +67,7 @@ namespace PrivaPub.Federation.Inbox
 			Arrival.Current = arrival;
 			try
 			{
+				await LocalReferences.Canonicalise(activity, _localActors?.BaseAddress, token);
 				await handler.Handle(activity, actor, token);
 			}
 			catch (Exception ex) when (ex is not OperationCanceledException)

PrivaPub/Federation/Inbox/LocalReferences.cs

@@ -0,0 +1,42 @@
+using System.Text.Json.Nodes;
+using System.Text.RegularExpressions;
+
+using MongoDB.Entities;
+
+using PostEntity = PrivaPub.Models.Post.Post;
+
+namespace PrivaPub.Federation.Inbox
+{
+	// Some servers name one of our posts by the address of its page instead of its id (Pixelfed likes /@name/<post id>, the
+	// post's `url`). Before an activity is handled, such a reference to a post of ours, as the activity's object or as the
+	// object of the activity it undoes, is replaced by the post's id, so that every handler finds the post the same way.
+	public static partial class LocalReferences
+	{
+		[GeneratedRegex("^/@[^/?#]+/([0-9a-f]{24})$")]
+		private static partial Regex PostPage();
+
+		public static async Task Canonicalise(JsonNode activity, string baseAddress, CancellationToken token)
+		{
+			if (activity is not JsonObject outer || string.IsNullOrEmpty(baseAddress))
+				return;
+			await Replace(outer, baseAddress, token);
+			if (outer["object"] is JsonObject inner)
+				await Replace(inner, baseAddress, token);
+		}
+
+		static async Task Replace(JsonObject node, string baseAddress, CancellationToken token)
+		{
+			if (node["object"] is not JsonValue value || !value.TryGetValue<string>(out var uri) || PostId(uri, baseAddress) is not { } id)
+				return;
+			var post = await DB.Default.Find<PostEntity>().Match(p => p.ID == id && !p.IsFederatedCopy).ExecuteFirstAsync(token);
+			if (post?.ObjectURI is { } canonical)
+				node["object"] = canonical;
+		}
+
+		// the id of the post whose page the address is, on our own site
+		public static string PostId(string uri, string baseAddress) =>
+			uri.StartsWith(baseAddress + "/@", StringComparison.OrdinalIgnoreCase) && PostPage().Match(uri[baseAddress.Length..]) is { Success: true } page
+				? page.Groups[1].Value
+				: default;
+	}
+}