Meta: Multiple IMAP connections using same mailbox are unsafe

[foxcpp]

Given 2 clients (A & B) connected and using the same mailbox. Deleting (expunging, in terms of IMAP) messages by client A can make client B act on wrong messages unless there is an explicit synchronization (e.g. CHECK, NOOP, IDLE) before action.

Clients that maintain persistent connections are likely to use IDLE and so will consume any pending updates before doing anything. This greatly reduces the chance of hitting this problem: both clients should do something at the same time and this is unlikely if the account is used only by a single person.

This should still be addressed in the long-term, since the current behavior breaks RFC 3501 by sending EXPUNGE updates when it is not allowed.

This is the meta-issue to link issues in different repositories and also to make it more visible for maddy users. There are changes need for all components (maddy, go-imap-sql backend, go-imap) to actually fix it.

References

• Lots of Server-side fixes and improvements emersion/go-imap#300 (comment)
• "Freeze" the view of sequence numbers on per-connection basis emersion/go-imap#318
• Delegate updates dispatching to the backend emersion/go-imap#323

[foxcpp]

Related work:

• Experimental maildir-based backend for v2 tests:
  https://github.com/foxcpp/go-imap-maildir

• Sequence number and updates logic extracted from go-imap-maildir for general use:
  https://github.com/foxcpp/go-imap-sequpdate

• go-imap v2 updates for backend interfaces:
  go-imap v2 Backend interface work  emersion/go-imap#374

[foxcpp]

Just merged go-imap-v2 into dev. We now have everything in place to consider this resolved.