fix(database): Add foreign keys to *_mailbox_id fields in mail_accounts table

[DerDreschner]

The mail_accounts table includes several fields with references to the mail_mailboxes table. This relation wasn't declared via foreign keys previously, which means that mailboxes with a special meaning (e.g., Junk) can be deleted without removing them from the corresponding *_mailbox_id field. Inconsistent entries are being set to NULL before changing the database schema.

Testing

MariaDB [nextcloud]> SELECT drafts_mailbox_id, sent_mailbox_id, trash_mailbox_id,junk_mailbox_id,archive_mailbox_id,snooze_mailbox_id FROM oc_mail_accounts;
+-------------------+-----------------+------------------+-----------------+--------------------+-------------------+
| drafts_mailbox_id | sent_mailbox_id | trash_mailbox_id | junk_mailbox_id | archive_mailbox_id | snooze_mailbox_id |
+-------------------+-----------------+------------------+-----------------+--------------------+-------------------+
|                47 |              48 |               49 |              46 |                 45 |              NULL |
+-------------------+-----------------+------------------+-----------------+--------------------+-------------------+
1 row in set (0.000 sec)

MariaDB [nextcloud]> DELETE FROM oc_mail_mailboxes WHERE id=46;
Query OK, 1 row affected (0.015 sec)

MariaDB [nextcloud]> DELETE FROM oc_mail_mailboxes WHERE id=48;
Query OK, 1 row affected (0.007 sec)

MariaDB [nextcloud]> SELECT drafts_mailbox_id, sent_mailbox_id, trash_mailbox_id,junk_mailbox_id,archive_mailbox_id,snooze_mailbox_id FROM oc_mail_accounts;
+-------------------+-----------------+------------------+-----------------+--------------------+-------------------+
| drafts_mailbox_id | sent_mailbox_id | trash_mailbox_id | junk_mailbox_id | archive_mailbox_id | snooze_mailbox_id |
+-------------------+-----------------+------------------+-----------------+--------------------+-------------------+
|                47 |              48 |               49 |              46 |                 45 |              NULL |
+-------------------+-----------------+------------------+-----------------+--------------------+-------------------+
1 row in set (0.000 sec)

### MIGRATION RUN ###

MariaDB [nextcloud]> SELECT drafts_mailbox_id, sent_mailbox_id, trash_mailbox_id,junk_mailbox_id,archive_mailbox_id,snooze_mailbox_id FROM oc_mail_accounts;
+-------------------+-----------------+------------------+-----------------+--------------------+-------------------+
| drafts_mailbox_id | sent_mailbox_id | trash_mailbox_id | junk_mailbox_id | archive_mailbox_id | snooze_mailbox_id |
+-------------------+-----------------+------------------+-----------------+--------------------+-------------------+
|                47 |            NULL |               49 |            NULL |                 45 |              NULL |
+-------------------+-----------------+------------------+-----------------+--------------------+-------------------+
1 row in set (0.000 sec)

MariaDB [nextcloud]> SHOW CREATE TABLE oc_mail_accounts;

[...]

  PRIMARY KEY (`id`),
  KEY `mail_userid_index` (`user_id`),
  KEY `IDX_61A7E87F59A77CDC` (`drafts_mailbox_id`),
  KEY `IDX_61A7E87F75F54A4B` (`sent_mailbox_id`),
  KEY `IDX_61A7E87F1FA4CF4D` (`trash_mailbox_id`),
  KEY `IDX_61A7E87F5FDB8C8A` (`junk_mailbox_id`),
  KEY `IDX_61A7E87FA47FE329` (`archive_mailbox_id`),
  KEY `IDX_61A7E87F7587D725` (`snooze_mailbox_id`),
  CONSTRAINT `FK_61A7E87F1FA4CF4D` FOREIGN KEY (`trash_mailbox_id`) REFERENCES `oc_mail_mailboxes` (`id`) ON DELETE SET NULL,
  CONSTRAINT `FK_61A7E87F59A77CDC` FOREIGN KEY (`drafts_mailbox_id`) REFERENCES `oc_mail_mailboxes` (`id`) ON DELETE SET NULL,
  CONSTRAINT `FK_61A7E87F5FDB8C8A` FOREIGN KEY (`junk_mailbox_id`) REFERENCES `oc_mail_mailboxes` (`id`) ON DELETE SET NULL,
  CONSTRAINT `FK_61A7E87F7587D725` FOREIGN KEY (`snooze_mailbox_id`) REFERENCES `oc_mail_mailboxes` (`id`) ON DELETE SET NULL,
  CONSTRAINT `FK_61A7E87F75F54A4B` FOREIGN KEY (`sent_mailbox_id`) REFERENCES `oc_mail_mailboxes` (`id`) ON DELETE SET NULL,
  CONSTRAINT `FK_61A7E87FA47FE329` FOREIGN KEY (`archive_mailbox_id`) REFERENCES `oc_mail_mailboxes` (`id`) ON DELETE SET NULL
) ENGINE=InnoDB AUTO_INCREMENT=26 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+------------------
+
1 row in set (0.000 sec)

MariaDB [nextcloud]> 

This fixes #11243.

[DerDreschner]

Question: What's the best-practice with the hasTable() / hasColumn() / etc. checks? Is it even necessary here? I've done it because it's being done in older migrations as well, but I'm not sure why it's done there 🤔

[ChristophWurst]

The checks shouldn't be necessary ideally, but in case of a migration error it should be possible to re-run that migration.

Imagine a migration creates two tables: t1 and t2. Creating t2 failed (something temporary that the admin could resolve, or a patch is applied). The upgrade runs the same migration again, because the migration is not yet written to oc_migrations.
If the migration tries to create t1 no matter if it exists, it will now fail here. If the migration has a check it can skip creating t1 and creates t2.

This is why we add the table/column exists checks :)