Support note placeholders

What feature would you like implemented?

Implement support for empty "placeholder" notes that indicate a deleted, missing, or unavailable part of a thread. Placeholders have no content or author associated, but can be used as a FOREIGN KEY reference to support partial threads.

This is extremely non-trivial, but is essential to fix one of Misskey's most long-standing federation issues.

Why should we add this feature?

Misskey has long stood out as the only mainstream fedi backend to require a full thread before loading a post. This limitation appears in many ways:

• Replies cannot be loaded if the parent instance blocks the local instance
• Replies cannot be loaded if the parent post is inaccessible (#728)
• Misskey users cannot be added to an existing DM chain (extension of the above issue)
• Threads are erased if a parent post is deleted

Challenges:

• Misskey / Sharkey encodes a timestamp into note private keys, but we don't know the timestamp of a placeholder.
• Foreign-key constraints are in effect.
• This intersects with ActivityPub's Tombstone specification, so we will need to support that as well.
• Inaccessible notes may become accessible later, and we will need to "upgrade" the placeholder.
• To maintain soft-fork status, we need a working "down" migration to remove this support.

Version

2024.9.1-rc

Instance

enby.life

Contribution Guidelines By submitting this issue, you agree to follow our Contribution Guidelines

• I agree to follow this project's Contribution Guidelines
• I have searched the issue tracker for similar requests, and this is not a duplicate.