SimpleX-Chat/docs/rfcs/2022-12-12-disappearing-messages.md

Disappearing messages

• ability to turn on/off disappearing messages feature per conversation, specify ttl

• use preferences framework, preference affects local deletion of both sent and received messages

• special chat item on change

• in direct chat - chat item can be interacted with to agree or disagree with preference change, updates preference accordingly

  • how does party that offered preference change learn about disagreement? (maybe just no preference update from contact is ok, since it's still not on if it's not mutual)

  • how does it learn about disagreement on ttl? (it's on already - so it works for both but differently if there's no agreement)

  • single updating chat item or per event? (probably per event is better since they can be spaced in time)

• in group - set by owner

• should it be allowed to be configured globally?

• change of setting shouldn't prevent previous disappearing messages from being deleted

Design

• add delete_at field to chat_items table, index idx_chat_items_delete_at

• add disappearingItems :: TMap ChatItemId (Async ()) to ChatController (use Weak ThreadId?)

• new background process that periodically scans for disappearing messages bound to be deleted during next 30 minutes:

  • add cleanupManager :: TVar (Async ()) to ChatController

  • periodically gets items to schedule for deletion based on delete_at field

  • for items to be deleted in next 30 minutes - add thread to disappearingItems - thread delays until deleteAt date, then deletes and sends CRChatItemDeleted to view

  • for items past current time - delete in bulk

  • race condition between bulk deletion of expired items on start and opening a chat with them - they should be removed from chat view once deleted - don't optimize for bulk deletion and create threads? create multiple CRs after bulk deletion? create single chat response with all ids?

• when chat item is deleted locally, either by user or via "delete for everyone" feature, kill thread and remove from map

• when MsgContent chat item is sent or marked read, add thread to disappearingItems based on chat preference

• UI shows timer based on chat item's createdAt date and deleteAt date

***

Preference agreement:

• new preference types?

  data DisappearingMessagesPreference = DisappearingMessagesPreference
  {
    allow :: FeatureAllowed,
    ttl :: Int
  }
  
  data DisappearingMessagesGroupPreference = DisappearingMessagesGroupPreference
  {
    enable :: GroupFeatureEnabled,
    ttl :: Int
  }
  
  -- requires changing functions and types using Preference and GroupPreference
  

• chat items to contain old and new preference value

***

Maybe agreement shouldn't be via preferences framework, but ad-hoc? For example:

• new protocol messages XMsgTtlOffer ttl, XMsgTtlAgree ttl, XMsgTtlOff

• for direct chats on XMsgTtlOffer contact disappearingMessages fields is updated to

• for direct chats on XMsgTtlAgree check ttl equals offered, then turn on

• for group chats only XMsgTtlAgree has to be sent, should only be accepted from owner

• XMsgTtlOff turns off unconditionally, for group chats should only be accepted from owner

• types:

  data DisappearingMessagesState
    = DMSOff
    | DMSOffered ttl
    | DMSAgreed ttl
  
  data Contact = Contact
  { ...
    disappearingMessagesState :: DisappearingMessagesState,
    ...
  }
  
  data GroupInfo = GroupInfo
  { ...
    disappearingMessagesState :: DisappearingMessagesState,
    ...
  }
  
  -- make part of ChatSettings?