reduce max comment depth to 50

Cargo.lock

@@ -17,8 +17,7 @@ checksum = "8f27d075294830fcab6f66e320dab524bc6d048f4a151698e153205559113772"
 [[package]]
 name = "activitypub_federation"
 version = "0.5.8"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b86eea7a032da501fe07b04a83c10716ea732c45e6943d7f045bc053aca03f2a"
+source = "git+https://github.com/LemmyNet/activitypub-federation-rust.git?branch=nested-comments-stack-overflow#4b94af9ea0580226aaa10a4abb004d0b8ca10eae"
 dependencies = [
  "activitystreams-kinds",
  "actix-web",

Cargo.toml

@@ -100,7 +100,7 @@ lemmy_db_views = { version = "=0.19.6-beta.7", path = "./crates/db_views" }
 lemmy_db_views_actor = { version = "=0.19.6-beta.7", path = "./crates/db_views_actor" }
 lemmy_db_views_moderator = { version = "=0.19.6-beta.7", path = "./crates/db_views_moderator" }
 lemmy_federate = { version = "=0.19.6-beta.7", path = "./crates/federate" }
-activitypub_federation = { version = "0.5.8", default-features = false, features = [
+activitypub_federation = { git = "https://github.com/LemmyNet/activitypub-federation-rust.git", branch = "nested-comments-stack-overflow", default-features = false, features = [
   "actix-web",
 ] }
 diesel = "2.1.6"

api_tests/src/comment.spec.ts

@@ -860,9 +860,7 @@ test("Dont send a comment reply to a blocked community", async () => {
 });
 
 /// Fetching a deeply nested comment can lead to stack overflow as all parent comments are also
-/// fetched recursively. Ensure that it works properly. Stack size depends on the OS so this
-/// may fail in some cases. In release builds Lemmy is more optimized and can handle the max depth
-/// of 100.
+/// fetched recursively. Ensure that it works properly.
 test("Fetch a deeply nested comment", async () => {
   let lastComment;
   for (let i = 0; i < 50; i++) {

crates/api_crud/src/comment/create.rs

@@ -29,11 +29,9 @@ use lemmy_db_schema::{
 use lemmy_db_views::structs::{LocalUserView, PostView};
 use lemmy_utils::{
   error::{LemmyErrorExt, LemmyErrorType, LemmyResult},
-  utils::{mention::scrape_text_for_mentions, validation::is_valid_body_field},
+  utils::{mention::scrape_text_for_mentions, validation::is_valid_body_field}, MAX_COMMENT_DEPTH_LIMIT,
 };
 
-const MAX_COMMENT_DEPTH_LIMIT: usize = 100;
-
 #[tracing::instrument(skip(context))]
 pub async fn create_comment(
   data: Json<CreateComment>,

crates/apub/src/protocol/objects/note.rs

@@ -20,7 +20,7 @@ use lemmy_db_schema::{
   source::{community::Community, post::Post},
   traits::Crud,
 };
-use lemmy_utils::{error::LemmyResult, LemmyErrorType};
+use lemmy_utils::{error::{LemmyError, LemmyResult}, LemmyErrorType, MAX_COMMENT_DEPTH_LIMIT};
 use serde::{Deserialize, Serialize};
 use serde_with::skip_serializing_none;
 use url::Url;
@@ -57,13 +57,24 @@ impl Note {
     &self,
     context: &Data<LemmyContext>,
   ) -> LemmyResult<(ApubPost, Option<ApubComment>)> {
-    // Fetch parent comment chain in a box, otherwise it can cause a stack overflow.
+    // We use recursion here to fetch the entire comment chain up to the top-level parent. This is
+    // necessary because we need to know the post and parent comment in order to insert a new
+    // comment. However it can also lead to stack overflow when fetching many comments recursively.
+    // To avoid this we check the request count against max comment depth, which based on testing
+    // can be handled without risking stack overflow. This is not a perfect solution, because in
+    // some cases we have to fetch user profiles too, and reach the limit after only 25 comments
+    // or so.
+    // A cleaner solution would be converting the recursion into a loop, but that is tricky.
+    // Use a lower request limit here. Otherwise we can run into stack overflow due to recursion.
+    if context.request_count() > MAX_COMMENT_DEPTH_LIMIT as u32 {
+      Err(LemmyErrorType::MaxCommentDepthReached)?;
+    }
     let parent = self.in_reply_to.dereference(context).await?;
     match parent {
       PostOrComment::Post(p) => Ok((p.clone(), None)),
       PostOrComment::Comment(c) => {
         let post_id = c.post_id;
-        let post = Post::read(&mut context.pool(), post_id)
+        let post = Box::pin(Post::read(&mut context.pool(), post_id))
           .await?
           .ok_or(LemmyErrorType::CouldntFindPost)?;
         Ok((post.into(), Some(c.clone())))

crates/utils/src/lib.rs

@@ -29,6 +29,8 @@ pub const CACHE_DURATION_FEDERATION: Duration = Duration::from_secs(60);
 
 pub const CACHE_DURATION_API: Duration = Duration::from_secs(1);
 
+pub const MAX_COMMENT_DEPTH_LIMIT: usize = 50;
+
 #[macro_export]
 macro_rules! location_info {
   () => {