QBMapper::insertOrUpdate can lead to NotNullConstraintViolationException

Code in question:

https://github.com/nextcloud/server/blob/d78449c01cf1cc608eca7a4d6ae675f1bc3e8aeb/lib/public/AppFramework/Db/QBMapper.php#L150
and
https://github.com/nextcloud/server/blob/d78449c01cf1cc608eca7a4d6ae675f1bc3e8aeb/lib/public/AppFramework/Db/QBMapper.php#L111

Problem:

1. Have a DB Scheme that does not allow null for some fields:
   e.g: [userId, key, value], where userId and key are not allowed to be null.
2. Have this line in the DB: ['uid123', 'key1', 'value2']
3. $entity = $mapper->findByUid('uid123')
4. $entity->setValue('new-value-456')
5. $mapper->insertOrUpdate($entity)
6. 💥

Reason:

• QBMapper::insertOrUpdate tries to insert it. If it throws a UniqueConstraintViolationException it performs an update instead.
• QBMapper::insert does not insert all fields, but only the updated fields.
• In our example, it would only insert value: new-value-456, because that's the only field we altered, although userId and key are set in the entity, leading to a NotNullConstraintViolationException.

Possible Solutions:

1. Instead of using the updated fields, use array_keys($entity->getFieldTypes())
   https://github.com/nextcloud/server/blob/d78449c01cf1cc608eca7a4d6ae675f1bc3e8aeb/lib/public/AppFramework/Db/QBMapper.php#L114

• will create bigger insert statements, possibly inserting lots of null values.
• insert is widely used, might do 💥 very easily.

2. Deprecate insertOrUpdate, it's barely used anyway (only once in the server right now)
   https://github.com/nextcloud/server/search?q=insertOrUpdate&unscoped_q=insertOrUpdate

Read lib/public/AppFramework/Db/QBMapper.php around the linked insertOrUpdate and insert lines, then trace how updated fields are selected. Reproduce the findByUid, setValue, and insertOrUpdate scenario against a schema with non-null userId and key fields. Done means the operation no longer raises NotNullConstraintViolationException while its insert-or-update behavior remains defined.