NFR: better error reporting for failed preview generation on external storage if 'server-side encryption' is enabled

Feature Request

Preview generation fails on external storage if 'enable server-side encryption' is selected even if there is no encryption app enabled.

Current log entries look like this:

{gallery} Exception: Preview generation has failed

It would be great if either an on-screen message or log entry indicated the reason that preview generation had failed.

Steps to reproduce

Check 'enable server-side encryption' in Admin->encryption with no 'Encryption' app enabled
Browse to an external storage location containing images and open 'gallery'

Expected behaviour

Image previews appear or system provides a useful error message or log entry
Clicking on an image thumbnail (generic icons in this case) opens the image in slideshow view in the browser

Actual behaviour

No previews appear
Images are only available for download (not for online preview)
The logged error is unhelpful

Additional Notes and Workaround

'Enable server-side encryption' (in Admin->Encryption) could not be unchecked from the GUI. Clicking the box produced no response, possibly because no encryption app was enabled.

Disabling encryption using occ from the command line restored proper Gallery operation - both preview generation and image slideshow behavior: sudo -u www-data php /var/www/nextcloud/occ encryption:disable

Server configuration detail

Operating system: Linux 4.4.0-101-generic #124-Ubuntu SMP Fri Nov 10 18:29:59 UTC 2017 x86_64

Webserver: Apache/2.4.18 (Ubuntu) (apache2handler)

Database: mysql 5.7.20

PHP version: 7.0.22-0ubuntu0.16.04.1 Modules loaded: Core, date, libxml, openssl, pcre, zlib, filter, hash, Reflection, SPL, session, standard, apache2handler, mysqlnd, PDO, xml, apcu, calendar, ctype, curl, dom, mbstring, fileinfo, ftp, gd, gettext, iconv, igbinary, json, exif, mcrypt, mysqli, pdo_mysql, Phar, posix, readline, redis, shmop, SimpleXML, sockets, sysvmsg, sysvsem, sysvshm, tokenizer, wddx, xmlreader, xmlwriter, xsl, zip, Zend OPcache

Nextcloud version: 12.0.3 - 12.0.3.3

Updated from an older Nextcloud/ownCloud or fresh install:

Where did you install Nextcloud from:

installed Nextcloud 10 from zip
upgraded from 10 -> 11 -> 11.0.1 -> 11.0.2 -> 11.0.3 -> 12.0.0 -> 12.0.1 -> 12.0.2 -> 12.0.3

Signing status

Array

List of activated apps

``` Enabled: - activity: 2.5.2 - admin_audit: 1.2.0 - audioplayer: 2.2.1 - comments: 1.2.0 - dav: 1.3.0 - federatedfilesharing: 1.2.0 - federation: 1.2.0 - files: 1.7.2 - files_external: 1.3.0 - files_pdfviewer: 1.1.1 - files_sharing: 1.4.0 - files_texteditor: 2.4.1 - files_trashbin: 1.2.0 - files_versions: 1.5.0 - files_videoplayer: 1.1.0 - firstrunwizard: 2.1 - gallery: 17.0.0 - issuetemplate: 0.3.0 - logreader: 2.0.0 - lookup_server_connector: 1.0.0 - nextcloud_announcements: 1.1 - notifications: 2.0.0 - oauth2: 1.0.5 - password_policy: 1.2.2 - previewgenerator: 1.0.7 - provisioning_api: 1.2.0 - serverinfo: 1.2.0 - sharebymail: 1.2.0 - survey_client: 1.0.0 - systemtags: 1.2.0 - theming: 1.3.0 - twofactor_backupcodes: 1.1.1 - twofactor_totp: 1.3.1 - updatenotification: 1.2.0 - workflowengine: 1.2.0 Disabled: - encryption - user_external - user_ldap ```

Configuration (config/config.php)

``` { "instanceid": "ocfuo1azzqcx", "passwordsalt": "***REMOVED SENSITIVE VALUE***", "secret": "***REMOVED SENSITIVE VALUE***", "trusted_domains": [ "192.168.200.167", "cloud.mmsionline.us" ], "datadirectory": "\/cloud\/data", "tempdirectory": "\/cloud\/tmp", "overwrite.cli.url": "https:\/\/192.168.200.167", "dbtype": "mysql", "version": "12.0.3.3", "dbname": "nextcloud", "dbhost": "localhost", "dbport": "", "dbtableprefix": "oc_", "dbuser": "***REMOVED SENSITIVE VALUE***", "dbpassword": "***REMOVED SENSITIVE VALUE***", "log_type": "syslog", "logfile": "", "logtimezone": "UTC", "installed": true, "memcache.local": "\\OC\\Memcache\\APCu", "memcache.locking": "\\OC\\Memcache\\Redis", "filelocking.enabled": "true", "redis": { "host": "\/var\/run\/redis\/redis.sock", "port": 0, "timeout": 0 }, "loglevel": 2, "mail_smtpmode": "smtp", "mail_from_address": "mmccarn-nextcloud", "mail_domain": "mmsionline.us", "mail_smtpauthtype": "LOGIN", "mail_smtpauth": 1, "mail_smtphost": "office.mmsionline.us", "mail_smtpport": "25", "mail_smtpname": "***REMOVED SENSITIVE VALUE***", "mail_smtppassword": "***REMOVED SENSITIVE VALUE***", "preview_max_scale_factor": 1, "enabledPreviewProviders": { "0": "OC\\Preview\\PNG", "1": "OC\\Preview\\JPEG", "2": "OC\\Preview\\GIF", "11": "OC\\Preview\\Illustrator", "12": "OC\\Preview\\Postscript", "13": "OC\\Preview\\Photoshop", "14": "OC\\Preview\\TIFF" }, "asset-pipline.enabled": true, "maintenance": false, "theme": "", "mail_smtpsecure": "tls" } ```

Are you using external storage, if yes which one: local/smb/sftp/...

Are you using encryption: no

nextcloud / gallery