Ignore already known UIDs when looking for new ones to satisfy mail_msg_mb_uid_idx

[ChristophWurst]

Fixes issues like #5108, #5019 and #4341

Basically if one sync attempt fails mid way after it had already inserted messages of one chunk (our level of atomicity for transacted and rolled back inserts) then the next sync will fail until the eternity. This patch discards duplicates when we determine new UIDs. Therefore the duplicates are avoided.

[ChristophWurst]

/backport to stable1.10

[st3iny]

Code looks good. How could I test this?

[ChristophWurst]

Code looks good. How could I test this?

Throw an arbitrary exception after

mail/lib/Db/MessageMapper.php

Line 347 in 06edd02

$this->db->commit();

. Then send a message to your inbox and sync again. Your error will show up, but the new message is stored in the DB. Then remove the exception and try again. On master you are in an unrecoverable state. On this branch the code will be able to continue.

[ChristophWurst]

FYI deplyed this on my personal instance that was stuck before and it revived my inbox from the dead 🙌

[miaulalala]

LGTM

[ramezrafla]

@ChristophWurst
Thanks so much for this change - a mailbox that was locked also is released now and functional.
Well done!