This is how the current kdepim-runtime/resources/imap/retrieveitemstask.cpp looks like:


  } else if ( messageCount > realMessageCount && messageCount > 0 ) {
    // The amount on the server is bigger than that we have in the cache
    // that probably means that there is new mail. Fetch missing.

    kDebug( 5327 ) << "Fetch missing: " << messageCount << " But: " << realMessageCount;

    KIMAP::FetchJob *fetch = new KIMAP::FetchJob( m_session );
    fetch->setSequenceSet( KIMAP::ImapSet( realMessageCount+1, messageCount ) );


If I understand this correctly and it really sends ``FETCH oldExists+1:*`` over the wire, then it's broken -- it will fail to notice any deletions among the old messages, for example.

Goging further, the next branch apparently assumes that UIDs are assigned on a consecutive manner, i.e. after 123, there will always be 124. This is wrong; the UIDNEXT as provided by the IMAP server is a lower bound on what the next visible UID might be. It is perfectly safe to have messages with assigned UIDs 10, 15, 21 etc from the very beginning (not to mention a possible removal on the server side from a concurrent client while Akonadi is offline).

  } else if ( messageCount == realMessageCount && oldNextUid != nextUid
           && oldNextUid != 0 && !firstTime && messageCount > 0 ) {
    // amount is right but uidnext is different.... something happened
    // behind our back...
    kDebug( 5327 ) << "UIDNEXT check failed, refetching mailbox";

    qint64 startIndex = 1;
    // one scenario we can recover from is that an equal amount of mails has been deleted and added while we were not looking
    // the amount has to be less or equal to (nextUid - oldNextUid) due to strictly ascending UIDs
    // so, we just have to reload the last (nextUid - oldNextUid) mails if the uidnext values seem sane
    if ( oldNextUid < nextUid && oldNextUid != 0 && !firstTime )
      startIndex = qMax( 1ll, messageCount - ( nextUid - oldNextUid ) );


This is wrong -- if the UIDNEXT was 10 previosuly and is now 11, the code will do a FETCH 8:*. However, there is no guaranteee whatsoever that the removal was not for, let's say, message with seq. number 1.

This is based just on reading the synchronization code, I have not performed any real tests, but it looks like testretrieveitemstask.cpp does not have a test coverage for this situation.

There's quite a lot of testcases for this in Trojita; please feel free to get in touch if you cannot find them.

Reproducible: Always