From 1ef5be89173ac728cc6f9dd4f8f6d041fc068a31 Mon Sep 17 00:00:00 2001
From: Laura Hausmann <laura@hausmann.dev>
Date: Tue, 4 Jun 2024 19:31:46 +0200
Subject: [PATCH] [backend/core] Correctly handle remote AP redirects to local
 notes

---
 .../ActivityPub/ActivityFetcherService.cs     |  2 +-
 .../Core/Middleware/ErrorHandlerMiddleware.cs |  6 +++++
 .../Core/Services/NoteService.cs              | 24 +++++++++++++------
 3 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/Iceshrimp.Backend/Core/Federation/ActivityPub/ActivityFetcherService.cs b/Iceshrimp.Backend/Core/Federation/ActivityPub/ActivityFetcherService.cs
index 9f98eae2..13724278 100644
--- a/Iceshrimp.Backend/Core/Federation/ActivityPub/ActivityFetcherService.cs
+++ b/Iceshrimp.Backend/Core/Federation/ActivityPub/ActivityFetcherService.cs
@@ -87,7 +87,7 @@ public class ActivityFetcherService(
 	{
 		var requestHost = new Uri(url).Host;
 		if (requestHost == config.Value.WebDomain || requestHost == config.Value.AccountDomain)
-			throw GracefulException.UnprocessableEntity("Refusing to fetch activity from local domain");
+			throw new LocalFetchException(url);
 
 		if (await fedCtrlSvc.ShouldBlockAsync(requestHost))
 		{
diff --git a/Iceshrimp.Backend/Core/Middleware/ErrorHandlerMiddleware.cs b/Iceshrimp.Backend/Core/Middleware/ErrorHandlerMiddleware.cs
index 5d193337..a42a3e1e 100644
--- a/Iceshrimp.Backend/Core/Middleware/ErrorHandlerMiddleware.cs
+++ b/Iceshrimp.Backend/Core/Middleware/ErrorHandlerMiddleware.cs
@@ -201,6 +201,12 @@ public class AuthFetchException(HttpStatusCode statusCode, string message, strin
 		new(HttpStatusCode.NotFound, HttpStatusCode.NotFound.ToString(), message);
 }
 
+public class LocalFetchException(string uri)
+	: GracefulException(HttpStatusCode.UnprocessableEntity, "Refusing to fetch activity from local domain")
+{
+	public string Uri => uri;
+}
+
 public enum ExceptionVerbosity
 {
 	[SuppressMessage("ReSharper", "UnusedMember.Global")]
diff --git a/Iceshrimp.Backend/Core/Services/NoteService.cs b/Iceshrimp.Backend/Core/Services/NoteService.cs
index b183fcfd..2197f982 100644
--- a/Iceshrimp.Backend/Core/Services/NoteService.cs
+++ b/Iceshrimp.Backend/Core/Services/NoteService.cs
@@ -1029,19 +1029,29 @@ public class NoteService(
 			throw GracefulException.UnprocessableEntity("Refusing to resolve circular threads");
 		_resolverHistory.Add(uri);
 
-		var note = uri.StartsWith($"https://{config.Value.WebDomain}/notes/")
-			? await db.Notes.IncludeCommonProperties()
-			          .FirstOrDefaultAsync(p => p.Id ==
-			                                    uri.Substring($"https://{config.Value.WebDomain}/notes/".Length))
-			: await db.Notes.IncludeCommonProperties()
-			          .FirstOrDefaultAsync(p => p.Uri == uri);
+		if (uri.StartsWith($"https://{config.Value.WebDomain}/notes/"))
+		{
+			var id = uri[$"https://{config.Value.WebDomain}/notes/".Length..];
+			return await db.Notes.IncludeCommonProperties().FirstOrDefaultAsync(p => p.Id == id);
+		}
+
+		var note = await db.Notes.IncludeCommonProperties().FirstOrDefaultAsync(p => p.Uri == uri);
 
 		if (note != null) return note;
 
 		if (!fetchedNote?.VerifiedFetch ?? false)
 			fetchedNote = null;
 
-		fetchedNote ??= user != null ? await fetchSvc.FetchNoteAsync(uri, user) : await fetchSvc.FetchNoteAsync(uri);
+		try
+		{
+			fetchedNote ??=
+				user != null ? await fetchSvc.FetchNoteAsync(uri, user) : await fetchSvc.FetchNoteAsync(uri);
+		}
+		catch (LocalFetchException e) when (e.Uri.StartsWith($"https://{config.Value.WebDomain}/notes/"))
+		{
+			var id = e.Uri[$"https://{config.Value.WebDomain}/notes/".Length..];
+			return await db.Notes.IncludeCommonProperties().FirstOrDefaultAsync(p => p.Id == id);
+		}
 
 		if (fetchedNote == null)
 		{