[Bug]: Querying for empty strings using search methods of UserConfig returns wrong results

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

• The default value for the indexed field in preferences table is set to '' in https://github.com/nextcloud/server/blob/b33fdaf0858e48a29d732c49407cbe6a9d0420d7/core/Migrations/Version31000Date20240814184402.php#L41
• This means if you have configvalue entries like '' you will always get wrong results as the query in https://github.com/nextcloud/server/blob/b33fdaf0858e48a29d732c49407cbe6a9d0420d7/lib/private/Config/UserConfig.php#L516 checks for indexed = '' on top of checking for configvalue field
• The workaround for this would be to set the flag value to avoid indexed queries for all these rows
• But shouldn't there be a more sensible NULL default instead?

Steps to reproduce

1. Set a preference using occ user:setting username1 some-app some-key ''
2. Set a preference using occ user:setting username2 some-app some-key some-value
3. Use IUserConfig::searchUsersByValueString('some-app', 'some-key', '');
4. It will return both results

Expected behavior

• The method should return only one result
• The sensible default for the indexed field should be NULL as it is a NULLABLE field

Nextcloud Server version

31

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.3

Web server

Nginx

Database engine version

MariaDB

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

List of activated Apps

Nextcloud Signing status

Nextcloud Logs

Additional info

No response

Start with core/Migrations/Version31000Date20240814184402.php and lib/private/Config/UserConfig.php, especially the indexed default and search query referenced in the issue. Reproduce with the two occ user:setting commands, then call IUserConfig::searchUsersByValueString for an empty value. Done means the empty-value search returns only the matching user and the indexed default is addressed.