This function should only return temporary errors, e.g. database errors,
as precheck_imf() and dc_receive_imf::add_parts() treat them as such,
retrying the fetch on failure. When permanent errors, like missing
Message-ID, are bubbled up, they cause infinite fetch loop.
If precheck_imf fails to check if message with the same rfc724_mid
already exists, the same message may be downloaded twice. Instead,
abort the whole operation and retry later.
Sometimes message deletion job marks message as unlinked without
actually deleting it. It is possible if the message was already moved
into another folder, possibly by second device, but not detected there
yet. It should be detected later in the other folder, and the
server_uid in the database should be set.
Since introduction of add_imap_deletion_jobs() any expired message
record will be marked as unlinked eventually, because another message
deletion job will be scheduled even if update_server_uid() resurrects
the message once.
Currently only trashed or hidden messages are deleted by
DeleteMsgOnImap, so it is safe to remove database records.
It is planned to delete messages on IMAP server after
user-configurable time to cleanup the server even for messages
displayed in chats. For such messages, we unlink them from the
Message-ID, but keep the database record to display them.
In addition to moving the message into trash chat, this function
removes message text to make sure the message does not remain in the
database. Only the information necessary to delete message from the
server and avoid redownloading it should be kept, such as Message-Id
and IMAP UID.
eg. android relies on that and fallsback to wrong aspect ratios otherwise.
also, this makes job.rs a little more readable
as some things are moved to message.rs :)
