[Feature] Opt-in public read receipts&configuration policy

Public read receipts and read receipt invariants

A public read receipt is a marker that signifies the acknowledgement of a message which is visible to others. Public read receipts need to be forwards-only to be admissible as a legal acknowledgement. Each channel should maintain its own read state. Sending a message shall implicitly advance the sender's public read receipt to that message, unless the whole channel/guild has its read receipts disabled.

There is another attribute which should be kept. It is called the notification cursor. This denotes the last-notified event in the channel. This also needs to be forwards-only, to prevent re-pinging a user when a certain channel is marked unread afterwards.

The private read marker should stay as-is, however it should be separated from the two above.

https://github.com/fosscord/fosscord-server/blob/da030aaff2ca77395d0ca7a39e5ca4f7abfc23cd/util/src/entities/ReadState.ts#L9

/policies/:id/read-receipts

Enable/disable read receipt capability.

GET to retrieve, PUT to replace, DELETE to reset to defaults.

Defaults are 1 for guilds, 2 for DMs, and 0 for bot users.

This policy should be applied as follows:

1. Read the policy of the user.
2. Read the policy of the channel.
3. Read the policy of the custom routing, if exists.
4. Use the smallest of the values obtained in steps 1 through 3.

Permissions: A user who can send messages can change own policy into a larger value if not locked. An administrator can change policies of which the users he controls, so can a user can always change its own policy to a smaller value. A non-administrative user cannot read another user's policy value to prevent nagging to enable the read receipts.

Related:

https://github.com/fosscord/fosscord-server/blob/da030aaff2ca77395d0ca7a39e5ca4f7abfc23cd/util/src/entities/ReadState.ts#L8

Sub-tasks

• Separation of private read marker, notification cursor and public read receipt to allow for forwards-only invariant in latter two
  • Initial work done at 43b02a4da5331dd710e7e56ae25ba0abbe726f9a
  • Actually enforce the forwards-only constraint on public receipts and ping cursors
• Configuration policy
• MESSAGE_ACK routing for public read receipts

Start with util/src/entities/ReadState.ts and the initial work referenced in commit 43b02a4da5331dd710e7e56ae25ba0abbe726f9a. Trace how private markers, notification cursors, and public receipts are represented, then review the requested /policies/:id/read-receipts operations and MESSAGE_ACK routing. Done means the forwards-only constraints, policy rules, and public receipt routing described here are implemented.