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/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;
+	}
+}