Problems with encryption being enabled

Steps to reproduce

1. Enable encryption
2. Enable antivirus plugin
3. tail the log

Expected behaviour

Scanning should work, no errors should occur

Actual behaviour

The antivirus plugin spams the log with messages like:

  "reqId": "Vsym1t5rzVkM7MvbdWpB",
  "level": 4,
  "time": "2020-03-21T12:31:59+00:00",
  "remoteAddr": "",
  "user": "--",
  "app": "files_antivirus",
  "method": "",
  "url": "--",
  "message": "OCA\\Files_Antivirus\\BackgroundJob\\BackgroundScanner::run, exception: Can not decrypt this file, probably this is a shared file. Please ask the file owner to reshare the file with you.",
  "userAgent": "--",
  "version": "18.0.2.2"

Server configuration

Not sure if that's only 18.x related or happened before

PHP version:
7.3

Nextcloud version: (see Nextcloud admin page)
18.0.2.2

Where did you install Nextcloud from:
manual / tarball

List of activated apps:

% php occ app:list
Enabled:
  - accessibility: 1.4.0
  - activity: 2.11.0
  - admin_audit: 1.8.0
  - announcementcenter: 3.7.0
  - bruteforcesettings: 1.5.0
  - calendar: 2.0.2
  - cloud_federation_api: 1.1.0
  - comments: 1.8.0
  - contacts: 3.2.0
  - dav: 1.14.0
  - encryption: 2.6.0
  - external: 3.5.0
  - federatedfilesharing: 1.8.0
  - files: 1.13.1
  - files_accesscontrol: 1.8.1
  - files_antivirus: 2.2.1
  - files_external: 1.9.0
  - files_pdfviewer: 1.7.0
  - files_retention: 1.7.0
  - files_rightclick: 0.15.2
  - files_sharing: 1.10.1
  - files_trashbin: 1.8.0
  - files_versions: 1.11.0
  - files_videoplayer: 1.7.0
  - firstrunwizard: 2.7.0
  - logreader: 2.3.0
  - lookup_server_connector: 1.6.0
  - nextcloud_announcements: 1.7.0
  - notifications: 2.6.0
  - oauth2: 1.6.0
  - password_policy: 1.8.0
  - photos: 1.0.0
  - privacy: 1.2.0
  - provisioning_api: 1.8.0
  - recommendations: 0.6.0
  - serverinfo: 1.8.0
  - settings: 1.0.0
  - sharebymail: 1.8.0
  - socialsharing_email: 2.0.0
  - support: 1.1.0
  - survey_client: 1.6.0
  - systemtags: 1.8.0
  - text: 2.0.0
  - theming: 1.9.0
  - twofactor_backupcodes: 1.7.0
  - twofactor_totp: 4.1.3
  - updatenotification: 1.8.0
  - viewer: 1.2.0
  - workflowengine: 2.0.0
Disabled:
  - federation
  - user_ldap

Nextcloud configuration:

% php occ config:list system
{
    "system": {
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "my.nextcloud.com"
        ],
        "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
        "forwarded_for_headers": [
            "HTTP_X_FORWARDED"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "overwrite.cli.url": "http:\/\/my.nextcloud.com",
        "dbtype": "mysql",
        "version": "18.0.2.2",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpmode": "sendmail",
        "mail_smtpauthtype": "LOGIN",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "enable_previews": true,
        "overwriteprotocol": "https",
        "maintenance": false,
        "theme": "",
        "loglevel": 2,
        "knowledgebaseenabled": true,
        "mysql.utf8mb4": true,
        "memcache.local": "\\OC\\Memcache\\APCu"
    }
}

antivirus is using the clamd UNIX socket, which worked fine actually.

Start at files_antivirus's BackgroundScanner::run and trace how encryption-related shared files are handled when the clamd UNIX socket is used. Reproduce with encryption and the antivirus plugin enabled, then tail the log while scanning. Done means valid scanning no longer repeatedly logs decryption exceptions for these files.