A user reported to me that after they left a group, they were implicitly readded, but there's no any
readdition message, so currently it looks in the chat like leaving it has no effect, just new
messages continue to arrive. The readdition probably happened because some member didn't receive the
user's self-removal message, anyway, at least there must be a message that the user is readded, even
if it isn't known by whom.
I.e. add the "Messages are guaranteed to be end-to-end encrypted from now on." message and mark the
chat as protected again because no user action is required in this case. There are a couple of
problems though:
- If the program crashes earlier than the protection is restored, the chat remains
protection-broken. But this problem already exists because `ChatId::set_protection()` is never
retried.
- If multiple old unverified messages are received, protection messages added in between don't
annihilate, so they clutter the chat.
If a message from an old contact's setup is received, the outdated Autocrypt header isn't applied,
so the contact verification preserves. But the chat protection breaks because the old message is
sorted to the bottom as it mustn't be sorted over the protection info message (which is `InNoticed`
moreover). Would be nice to preserve the chat protection too e.g. add a "protection broken" message,
then the old message and then a new "protection enabled" message, but let's record the current
behaviour first.
Received messages shouldn't mingle with just sent ones and appear somewhere in the middle of the
chat, so we go after the newest non fresh message.
But if a received outgoing message is older than some `InSeen` message, better sort the received
message purely by timestamp (this is an heuristic in order not to break the Gmail-like case
simulated by `verified_chats::test_old_message_4()`). We could place the received message just
before that `InSeen` message, but anyway the user may not notice it.
At least this fixes outgoing messages sorting for shared accounts where messages from other devices
should be sorted the same way as incoming ones.
Otherwise it's impossible to remove a member with missing key from a protected group. In the worst
case a removed member will be added back due to the group membership consistency algo.
Let's add a 1-minute tolerance to `Params::MemberListTimestamp`.
This adds to the group membership consistency algo the following properties:
- If remote group membership changes were made by two members in parallel, both of them are applied,
no matter in which order the messages are received.
- If we remove a member locally, only explicit remote member additions/removals made in parallel are
allowed, but not the synchronisation of the member list from "To". Before, if somebody managed to
reply earlier than receiving our removal of a member, we added it back which doesn't look good.
If the Inbox is fetched before the Sentbox (as done currently), messages from the Sentbox will
correctly mingle with the Inbox messages in the end. So, this commit changes message ordering only
if we already have processed outgoing messages, e.g. if we just sent them in the chat as described
in #4621. Otherwise new incoming messages are displayed somewhere in the middle of the chat which
doesn't look usable.
Correctly handle messages with old timestamps for verified chats:
* They must not be sorted over a protection-changed info message
* If they change the protection, then they must not be sorted over existing other messages, because then the protection-changed info message would also be above these existing messages.
This PR fixes this:
1. Even seen messages can't be sorted into already-noticed messages anymore. **This also changes DC's behavior in the absence of verified 1:1 chats**. Before this PR, messages that are marked as seen when they are downloaded will always be sorted by their timestamp, even if it's very old.
2. protection-changed info messages are always sorted to the bottom.
**Edit:**
3. There is an exception to rule 1: Outgoing messages are still allowed to be sorted purely by their timestamp, and don't influence old messages. This is to the problem described at [*].
Together, these rules also make sure that the protection-changed info message is always right above the message causing the change.
[*] If we receive messages from two different folders, e.g. `Sent` and `Inbox`, then this will lead to wrong message ordering in many cases. I need to think about this more, or maybe someone else has an idea. One new idea that came to my mind is:
* Always sort noticed messages under the newest info message (this PR sorts them under the newest noticed message, master sorts them purely by their sent timestamp)
* Always sort unnoticed messages under the newest noticed message (that's the same behavior as in this PR and on master)
* Always sort protection-changed info messages to the bottom (as in this PR)
However, after a talk with @link2xt we instead decided to add rule 3. (see above) because it seemed a little bit easier.
