[Bug]: preview:generate silently fails for files in external storage on NC 33.0.3 (returns success but no preview file is created)

[consonimovie-ship-it]

⚠️ 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

Preview generation completely fails for any file located in external storage mounts (files_external app), affecting both images and videos.

The occ preview:generate <fileId> command incorrectly reports "preview generated" but no preview file is actually written to disk. The same behavior affects occ preview:generate-all --path=..., which scans folders but generates nothing.

Impact:

• ~46,000 image and video files in external storage have no preview thumbnails in the Files app
• The Viewer app cannot open these files properly (clicking on an image without a preview causes the viewer to navigate back one folder level instead of displaying the file correctly)
• preview:pre-generate running via cron does NOT cover these files
• preview:generate-all reports "Scanning folder" but produces zero generated previews

Diagnostic data from my instance:

• Total files with image/video mimetypes in oc_filecache: ~74,200
• Files with generated previews on disk: 27,978 (~37%)
• The ~46,000 missing previews are all on external storage mounts
• Files uploaded to the user's home directory (not external storage) have previews generated correctly

Configurations already tested without success:

• Changed previewgenerator app config types from filesystem to filesystem external
• Verified previews: "1" is set on external storage mounts
• Verified filesystem permissions (www-data:www-data) on appdata preview folder
• Verified mimetype detection (image/jpeg correctly identified as ID 14)
• Ran occ db:add-missing-indices
• Cron runs as www-data (both container cron and host crontab as redundancy)

Note: Upgrade from 33.0.2 to 33.0.3 did NOT fix this issue. This appears related to #56510 (reported on 32.0.1, still unresolved), suggesting the bug spans multiple releases.

Steps to reproduce

1. Configure an external storage mount using the Local backend (e.g., a filesystem bind-mount pointing to a path outside the Nextcloud data directory)
2. Upload images or videos to that external storage, OR have existing files in the mounted path
3. Run as the web user inside the container:
   sudo docker exec -u www-data nextcloud php occ preview:generate <fileId>
4. Observe the output: preview generated
5. Check on disk if the preview was actually created:
   sudo find /path/to/data/appdata_<instanceid>/preview/ -type d -name "<fileId>"
6. Result: nothing is found — the preview folder for that fileId was never created

Equivalent attempts that also fail silently (return success, but write nothing to disk):

• occ preview:generate <full/path/to/file> (also returns "preview generated")
• occ preview:generate-all <user> (no per-file output for external storage files)
• occ preview:generate-all <user> --path="/<user>/files/<external_mount_name>/..." (only outputs "Scanning folder", never generates)
• Opening the file via the web Viewer (Viewer redirects to parent folder instead of displaying the file)
• HTTP request to /index.php/core/preview?fileId=<id> with valid app password authentication returns empty array []

Important: the exact same operations work correctly for files stored in the user's regular home directory (not external storage). Only files in external storage mounts exhibit this silent failure.

Expected behavior

The preview should be generated and stored in the appdata preview folder, exactly as happens for files in the user's home directory (non-external storage).

If preview generation fails for any reason (permission, codec, file access, etc.), the command should:

• Return a non-zero exit code
• Print a clear error message describing what went wrong
• Log the failure at default loglevel

The current silent failure (returning success message + zero exit code + no preview file on disk + no error in logs) makes the issue impossible to diagnose without deep inspection of the database and filesystem.

Nextcloud Server version

33

Operating system

Other

PHP engine version

PHP 8.4

Web server

Apache (supported)

Database engine version

PostgreSQL

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

Updated from a MINOR version (ex. 32.0.1 to 32.0.2)

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": {
        "htaccess.RewriteBase": "\/",
        "memcache.local": "\\OC\\Memcache\\APCu",
        "apps_paths": [
            {
                "path": "\/var\/www\/html\/apps",
                "url": "\/apps",
                "writable": false
            },
            {
                "path": "\/var\/www\/html\/custom_apps",
                "url": "\/custom_apps",
                "writable": true
            }
        ],
        "memcache.distributed": "\\OC\\Memcache\\Redis",
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "password": "***REMOVED SENSITIVE VALUE***",
            "port": 6379
        },
        "overwriteprotocol": "https",
        "upgrade.disable-web": true,
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "localhost",
            "192.168.x.x",
            "cloud.example.com"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "pgsql",
        "version": "33.0.3.2",
        "overwrite.cli.url": "https:\/\/cloud.example.com",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbtableprefix": "oc_",
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "maintenance": false,
        "filesystem_check_changes": 1,
        "unicode.normalization.form": "NFC",
        "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
        "forwarded_for_headers": [
            "HTTP_X_FORWARDED_FOR"
        ],
        "overwritehost": "cloud.example.com",
        "enabledPreviewProviders": [
            "OC\\Preview\\PNG",
            "OC\\Preview\\JPEG",
            "OC\\Preview\\GIF",
            "OC\\Preview\\BMP",
            "OC\\Preview\\XBitmap",
            "OC\\Preview\\Movie",
            "OC\\Preview\\MP4"
        ],
        "max_chunk_size": 104857600,
        "preview_max_x": "2048",
        "preview_max_y": "2048",
        "jpeg_quality": "80",
        "excluded_directories": [
            "private-folder"
        ],
        "loglevel": 2
    }
}

List of activated Apps

Enabled:
  - activity: 6.0.0
  - admincockpit: 1.3.0
  - app_api: 33.0.0
  - bruteforcesettings: 6.0.0
  - circles: 33.0.0
  - cloud_federation_api: 1.17.0
  - comments: 1.23.0
  - contactsinteraction: 1.14.1
  - dashboard: 7.13.0
  - dav: 1.36.0
  - external: 8.0.1
  - federatedfilesharing: 1.23.0
  - federation: 1.23.0
  - files: 2.5.0
  - files_downloadlimit: 5.1.0
  - files_external: 1.25.1
  - files_pdfviewer: 6.0.0
  - files_reminders: 1.6.0
  - files_sharing: 1.25.2
  - files_trashbin: 1.23.0
  - files_versions: 1.26.0
  - firstrunwizard: 6.0.0
  - logreader: 6.0.0
  - lookup_server_connector: 1.21.0
  - nextcloud_announcements: 5.0.0
  - notifications: 6.0.0
  - oauth2: 1.21.0
  - password_policy: 5.0.0
  - photos: 6.0.0
  - previewgenerator: 5.13.0
  - privacy: 5.0.0
  - profile: 1.2.0
  - provisioning_api: 1.23.0
  - recommendations: 6.0.0
  - related_resources: 4.0.0
  - richdocuments: 10.1.3
  - richdocumentscode: 25.4.904
  - serverinfo: 5.0.0
  - settings: 1.16.0
  - sharebymail: 1.23.0
  - support: 5.0.0
  - survey_client: 5.0.0
  - systemtags: 1.23.0
  - text: 7.0.1
  - theming: 2.8.0
  - twofactor_backupcodes: 1.22.0
  - twofactor_totp: 15.0.0
  - updatenotification: 1.23.0
  - user_status: 1.13.0
  - viewer: 6.0.0
  - weather_status: 1.13.0
  - webhook_listeners: 1.5.0
  - workflowengine: 2.15.0
Disabled:
  - admin_audit: 1.23.0
  - encryption: 2.21.0
  - suspicious_login: 11.0.0
  - testing: 1.23.0
  - twofactor_nextcloud_notification: 7.0.0
  - user_ldap: 1.24.0

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

The following exception is logged when accessing a file in external storage that should have a preview (preview entry exists in database but the actual preview file was never created on disk):

{"level":3,"time":"2026-05-29T18:51:24+00:00","user":"casaos","app":"index","method":"GET","url":"/core/preview?fileId=24713&x=128&y=128&mimeFallback=true&v=da7a04&a=0","message":"Unable to open preview stream at /var/www/html/data/appdata_INSTANCEID/preview/8/a/3/e/0/4/0/24713/256-256-crop.jpg","version":"33.0.3.2","exception":{"Exception":"OCP\\Files\\NotFoundException","Message":"Unable to open preview stream at /var/www/html/data/appdata_INSTANCEID/preview/8/a/3/e/0/4/0/24713/256-256-crop.jpg","File":"/var/www/html/lib/private/Preview/Storage/LocalPreviewStorage.php","Line":60}}

KEY FINDING: The preview entry was registered in the database (OC\Preview\Db\Preview object exists with an ID), but the actual preview file was never created in the appdata directory. The Nextcloud upgrade from 33.0.2 to 33.0.3 added a new repair step "Queue a job to move the preview" — this suggests previews were moved to a new location, but for files in external storage, the source files never existed in the first place.

When running `occ preview:generate <fileId>` for any file in external storage:
- Command returns "preview generated" 
- A new preview entry is created in the database
- BUT no file is created in /var/www/html/data/appdata_INSTANCEID/preview/<hash>/<fileId>/
- Subsequent requests for that preview fail with NotFoundException at LocalPreviewStorage.php:60

For files in the user's home directory (non-external storage), the same command works correctly and the preview file is properly written to disk.

Additional info

Container details

• Container image: bigbeartechworld/big-bear-nextcloud-with-smbclient:33.0.3 (digest sha256:f597ae02ebf0fc7640440273b273050f16b8437f566609dbf15230edbb20d9cf)
• This image is built on top of the official Nextcloud Apache image with smbclient added to support SMB-based external storage
• Upgraded from 33.0.2 to 33.0.3 specifically to test if this bug was fixed — it was not

External storage configuration

Two external storage mounts configured, both using the "Local" backend with bind-mounted volumes from the host (mount points anonymized):

Mount ID	Mount Point	Storage	Auth	Configuration
1	/MountA	Local	None	datadir: "/mnt/MountA"
2	/MountB	Local	None	datadir: "/mnt/MountB"

Both mounts have previews: "1" and filesystem_check_changes: "1" enabled, applicable to two regular users.

Database evidence

The preview registration entries DO exist in the database for affected files. Example query result for a specific fileId shows the file is properly indexed in oc_filecache with storage ID 3 (external storage):

fileid: 24713
storage: 3
path: .jpg

But there is no corresponding folder in /var/www/html/data/appdata_INSTANCEID/preview/ for this fileId.

Statistics from my instance

• Total files in oc_filecache with image/video mimetypes: 74,200
• Total preview folders on disk: 27,978 fileIds (~37% coverage)
• Smallest fileId with preview: 11,494
• Largest fileId with preview: 3,177,680
• Gap distribution: random — preview folders exist for some fileIds in any range, but the majority of older fileIds (lower numbers) are missing previews. This suggests preview:generate-all was attempted in the past, processed some files, but silently failed on most.

Reproducibility

100% reproducible on my instance. Every attempt to generate a preview for a file in external storage results in:

1. occ preview:generate returns success
2. No preview file is written to disk
3. Subsequent attempts to access that preview return NotFoundException from LocalPreviewStorage.php:60

Workarounds attempted (all failed)

• Setting previewgenerator app types to "filesystem external" (default is just "filesystem")
• Running occ files:scan --path="<external_storage_path>" followed by preview:generate
• Running occ preview:generate-all (only outputs "Scanning folder" for external storage paths, never generates)
• Running as www-data user inside container with docker exec -u www-data
• HTTP request to /index.php/core/preview with valid app password authentication

Related issues

• [Bug]: Preview Generation Fail - Local Storage #56510 — Same symptom reported on 32.0.1, still unresolved. Different version, same root cause appears to be involved.

Browser

This affects all clients/browsers (Chrome 148, Safari, Firefox, Nextcloud desktop sync client 33.0.5, Nextcloud iOS app). It is server-side, not client-side.

[nvedana]

Esta perfeito, resolveu meu problema, parabéns!

[nvedana]

Esta lindo e maravilhoso esse código!