Edge case when backfilling topics
When it comes to backfilling topics, NodeBB utilises what is called "context collection backfill" instead of reply-chain traversal. In a nutshell, instead of crawling up and down the reply tree one-by-one — kind of like what Mastodon does currently — it checks for a canonical source-of-truth in the context property.
NodeBB supports both strategies, since not all objects contain context.
I found a fun little edge case where if I start a topic, a reply is received, and that reply points to my instance, it won't catch any replies made in between.[...]
In other words:
localA starts topic → remoteB replies → remoteC replies to B only → remoteB replies to C and A
In this scenario, because my instance A is left out of the third activity, I don't know about it. When the fourth activity appears, the context is checked, C's reply isn't in it, and I proceed without knowing about it. Oops!
In this case, when incoming replies reference a context that is same-origin to your own instance, you should not use context collection backfill, because:
- You already have the entire context, so it's unnecessarily duplication of work
- You won't catch any replies made out-of-band unless you do reply-tree traversal
Tagging @silverpill@mitra.social because this is related to FEP f228.
