Enable bcc_self=1 every time we detect a sync message in our inbox

[link2xt]

This is an actionable replacement issue for #6432

If we detect a sync message, even if one of the devices has switched to bcc_self=0 due to some unknown bug, we will automatically recover back to bcc_self=1.

bcc_self now essentially means "multi-device setup" and is used to decide whether to send sync messages in should_send_sync_msgs.

I would not do the same on any outgoing message because it is possible that the server copies own messages to Sent like Gmail does and maybe changes Message-ID like outlook does, so seeing a message from self, especially unencrypted, is not a very clear indication that we have a multi-device setup, but sync message is unlikely to produce false positives.

[gerryfrancis]

I would not do the same on any outgoing message because it is possible that the server copies own messages to Sent like Gmail does and maybe changes Message-ID like outlook does

To be clear: This change applies to Chatmail accounts only, right?

[link2xt]

@gerryfrancis Not only. For non-chatmail accounts we also want this, because there user can disable bcc-self manually and we want to reenable it back if user has a second device which keeps sending sync messages as it is a mistake to disable bcc-self in multi-device setup.

[gerryfrancis]

@link2xt Understood, but please log the timestamp somewhere in the database when the user enabled/disabled bcc_self so we can ignore older sync messages from another Delta Chat client to avoid an (unwanted) overwrite of this setting. (Some users forget about their second device, especially when it has become damaged/lost.)

[link2xt]

Normally Delta Chat does not load old messages unless you restore a backup. But if you restore a backup, storing timestamp in the database will not help.