[Bug]: FilesMetadataException - json cannot exceed 100000 characters long

⚠️ 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

Saw this exception in the log:

{
    "reqId": "LKZ6NZIAZIqyK75Gy13j",
    "level": 3,
    "time": "2023-12-14T23:50:46+01:00",
    "remoteAddr": "",
    "user": "--",
    "app": "core",
    "method": "",
    "url": "--",
    "message": "{\"Exception\":\"OCP\\\\FilesMetadata\\\\Exceptions\\\\FilesMetadataException\",\"Message\":\"json cannot exceed 100000 characters long\",\"Code\":0,\"Trace\":[{\"file\":\"/var/www/nextcloud/lib/private/FilesMetadata/FilesMetadataManager.php\",\"line\":116,\"function\":\"saveMetadata\",\"class\":\"OC\\\\FilesMetadata\\\\FilesMetadataManager\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/core/BackgroundJobs/GenerateMetadataJob.php\",\"line\":105,\"function\":\"refreshMetadata\",\"class\":\"OC\\\\FilesMetadata\\\\FilesMetadataManager\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/core/BackgroundJobs/GenerateMetadataJob.php\",\"line\":101,\"function\":\"scanFolder\",\"class\":\"OC\\\\Core\\\\BackgroundJobs\\\\GenerateMetadataJob\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/core/BackgroundJobs/GenerateMetadataJob.php\",\"line\":101,\"function\":\"scanFolder\",\"class\":\"OC\\\\Core\\\\BackgroundJobs\\\\GenerateMetadataJob\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/core/BackgroundJobs/GenerateMetadataJob.php\",\"line\":90,\"function\":\"scanFolder\",\"class\":\"OC\\\\Core\\\\BackgroundJobs\\\\GenerateMetadataJob\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/core/BackgroundJobs/GenerateMetadataJob.php\",\"line\":81,\"function\":\"scanFilesForUser\",\"class\":\"OC\\\\Core\\\\BackgroundJobs\\\\GenerateMetadataJob\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/lib/public/BackgroundJob/Job.php\",\"line\":81,\"function\":\"run\",\"class\":\"OC\\\\Core\\\\BackgroundJobs\\\\GenerateMetadataJob\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/lib/public/BackgroundJob/TimedJob.php\",\"line\":102,\"function\":\"start\",\"class\":\"OCP\\\\BackgroundJob\\\\Job\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/lib/public/BackgroundJob/TimedJob.php\",\"line\":92,\"function\":\"start\",\"class\":\"OCP\\\\BackgroundJob\\\\TimedJob\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/cron.php\",\"line\":152,\"function\":\"execute\",\"class\":\"OCP\\\\BackgroundJob\\\\TimedJob\",\"type\":\"->\"}],\"File\":\"/var/www/nextcloud/lib/private/FilesMetadata/FilesMetadataManager.php\",\"Line\":179,\"message\":\"Error while running background job (class: OC\\\\Core\\\\BackgroundJobs\\\\GenerateMetadataJob, arguments: )\",\"exception\":{},\"CustomMessage\":\"Error while running background job (class: OC\\\\Core\\\\BackgroundJobs\\\\GenerateMetadataJob, arguments: )\"}",
    "userAgent": "--",
    "version": "28.0.0.11"
}

Steps to reproduce

No idea, happened in the background job.

Expected behavior

Should not fail.

Installation method

Community Manual installation with Archive

Nextcloud Server version

28

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.2

Web server

Apache (supported)

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

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

Start by reading lib/private/FilesMetadata/FilesMetadataManager.php, especially saveMetadata, then trace how core/BackgroundJobs/GenerateMetadataJob.php reaches it. The report provides no reproduction steps, so investigate the background-job path and the 100000-character exception. Done should mean the metadata job no longer fails for the reported condition, with coverage for the resulting behavior.