[Bug]: Performance issue when connecting to MySQL over IPv6 via AAAA DNS record

⚠️ 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

When trying to connect to a MySQL instance that is over a DNS that only resolve to an AAAA records (not impacted when setting dbhost to the IPv6 address directly), there is a huge delay on endpoints that requires database access (I always see at least 5 seconds, like it there was somethign timing out after 5s and then it connects correctly)

Screenshot of the impacted calls (when showing the main dashboard at /index.php/apps/dashboard/)

Steps to reproduce

1. Setup a nextcloud instance, connect to the database servers via a DNS that has only a AAAA records (assuming the mysql server is also listening on the IPv6 address and that the IPv6 network is working fine)
2. Try almost any page or endpoint (most of them are impacted)
3. Change the dbhost to the IPv6 address directly to compare the performance

Expected behavior

There should be no negative performance impact when connecting on IPv6 via a DNS AAAA record.

Installation method

Community Manual installation with Archive

Nextcloud Server version

28

Operating system

RHEL/CentOS

PHP engine version

PHP 8.3

Web server

Apache (supported)

Database engine version

MySQL

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

Fresh Nextcloud Server install

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

{
    "system": {
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "***REMOVED SENSITIVE VALUE***"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "28.0.3.2",
        "overwrite.cli.url": "***REMOVED SENSITIVE VALUE***",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "mysql.utf8mb4": true,
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "filelocking.enabled": true,
        "memcache.local": "\\OC\\Memcache\\APCu",
        "memcache.distributed": "\\OC\\Memcache\\Redis",
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "port": 6379,
            "timeout": 0,
            "password": "***REMOVED SENSITIVE VALUE***"
        },
        "maintenance_window_start": 1
    }
}

List of activated Apps

Enabled:
  - activity: 2.20.0
  - calendar: 4.6.6
  - circles: 28.0.0-dev
  - cloud_federation_api: 1.11.0
  - comments: 1.18.0
  - contacts: 5.5.3
  - contactsinteraction: 1.9.0
  - dashboard: 7.8.0
  - dav: 1.29.1
  - federatedfilesharing: 1.18.0
  - federation: 1.18.0
  - files: 2.0.0
  - files_pdfviewer: 2.9.0
  - files_reminders: 1.1.0
  - files_sharing: 1.20.0
  - files_trashbin: 1.18.0
  - files_versions: 1.21.0
  - firstrunwizard: 2.17.0
  - logreader: 2.13.0
  - lookup_server_connector: 1.16.0
  - mail: 3.5.7
  - nextcloud_announcements: 1.17.0
  - notes: 4.9.2
  - notifications: 2.16.0
  - oauth2: 1.16.3
  - password_policy: 1.18.0
  - photos: 2.4.0
  - privacy: 1.12.0
  - provisioning_api: 1.18.0
  - recommendations: 2.0.0
  - related_resources: 1.3.0
  - richdocuments: 8.3.2
  - richdocumentscode: 23.5.904
  - serverinfo: 1.18.0
  - settings: 1.10.1
  - sharebymail: 1.18.0
  - spreed: 18.0.5
  - support: 1.11.0
  - survey_client: 1.16.0
  - systemtags: 1.18.0
  - text: 3.9.1
  - theming: 2.3.0
  - twofactor_backupcodes: 1.17.0
  - updatenotification: 1.18.0
  - user_status: 1.8.1
  - viewer: 2.2.0
  - weather_status: 1.8.0
  - workflowengine: 2.10.0
Disabled:
  - admin_audit: 1.18.0
  - bruteforcesettings: 2.8.0
  - encryption: 2.16.0
  - files_external: 1.20.0
  - suspicious_login: 6.0.0
  - twofactor_totp: 10.0.0-beta.2
  - user_ldap: 1.19.0

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

<empty>

Additional info

No delay on any other dns resolution

No source file or test is identified in the report. Start by tracing the database connection path for an AAAA-only dbhost and compare it with the literal IPv6 configuration described in the reproduction steps. Done means reproducing the comparison and confirming that database-backed endpoints no longer incur the reported delay.