[Bug]: Weird state in appdata, incorrect return type handling

⚠️ 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

Likely related to: https://github.com/nextcloud/server/issues/38761

After running tests locally, I end up with a wiped appdata from time to time.

Visiting the app store greets me with a friendly:

{
  "reqId": "YvlS81wDtRf9uV5nQ1v5",
  "level": 3,
  "time": "2023-11-21T16:27:23+00:00",
  "remoteAddr": "172.19.0.1",
  "user": "admin",
  "app": "index",
  "method": "GET",
  "url": "/index.php/settings/apps",
  "message": "Exception thrown: OCP\\Files\\GenericFileException",
  "userAgent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/119.0.0.0 Safari/537.36",
  "version": "28.0.0.6",
  "exception": {
    "Exception": "OCP\\Files\\GenericFileException",
    "Message": "",
    "Code": 0,
    "Trace": [
      {
        "file": "/var/www/html/lib/private/Files/SimpleFS/SimpleFile.php",
        "line": 74,
        "function": "getContent",
        "class": "OC\\Files\\Node\\File",
        "type": "->",
        "args": []
      },
      {
        "file": "/var/www/html/lib/private/App/AppStore/Fetcher/Fetcher.php",
        "line": 166,
        "function": "getContent",
        "class": "OC\\Files\\SimpleFS\\SimpleFile",
        "type": "->",
        "args": []
      },
      {
        "file": "/var/www/html/lib/private/App/AppStore/Fetcher/AppFetcher.php",
        "line": 187,
        "function": "get",
        "class": "OC\\App\\AppStore\\Fetcher\\Fetcher",
        "type": "->",
        "args": [
          false
        ]
      },
      {
        "file": "/var/www/html/lib/private/Installer.php",
        "line": 423,
        "function": "get",
        "class": "OC\\App\\AppStore\\Fetcher\\AppFetcher",
        "type": "->",
        "args": [
          false
        ]
      },
      {
        "file": "/var/www/html/apps/settings/lib/Controller/AppSettingsController.php",
        "line": 154,
        "function": "isUpdateAvailable",
        "class": "OC\\Installer",
        "type": "->",
        "args": [
          "files_versions"
        ]
      },
      {
        "file": "/var/www/html/apps/settings/lib/Controller/AppSettingsController.php",
        "line": 137,
        "function": "getAppsWithUpdates",
        "class": "OCA\\Settings\\Controller\\AppSettingsController",
        "type": "->",
        "args": []
      },
      {
        "file": "/var/www/html/lib/private/AppFramework/Http/Dispatcher.php",
        "line": 230,
        "function": "viewApps",
        "class": "OCA\\Settings\\Controller\\AppSettingsController",
        "type": "->",
        "args": []
      },
      {
        "file": "/var/www/html/lib/private/AppFramework/Http/Dispatcher.php",
        "line": 137,
        "function": "executeController",
        "class": "OC\\AppFramework\\Http\\Dispatcher",
        "type": "->",
        "args": [
          [
            "OCA\\Settings\\Controller\\AppSettingsController"
          ],
          "viewApps"
        ]
      },
      {
        "file": "/var/www/html/lib/private/AppFramework/App.php",
        "line": 184,
        "function": "dispatch",
        "class": "OC\\AppFramework\\Http\\Dispatcher",
        "type": "->",
        "args": [
          [
            "OCA\\Settings\\Controller\\AppSettingsController"
          ],
          "viewApps"
        ]
      },
      {
        "file": "/var/www/html/lib/private/Route/Router.php",
        "line": 315,
        "function": "main",
        "class": "OC\\AppFramework\\App",
        "type": "::",
        "args": [
          "OCA\\Settings\\Controller\\AppSettingsController",
          "viewApps",
          [
            "OC\\AppFramework\\DependencyInjection\\DIContainer"
          ],
          [
            "",
            "",
            "settings.AppSettings.viewApps"
          ]
        ]
      },
      {
        "file": "/var/www/html/lib/base.php",
        "line": 1069,
        "function": "match",
        "class": "OC\\Route\\Router",
        "type": "->",
        "args": [
          "/settings/apps"
        ]
      },
      {
        "file": "/var/www/html/index.php",
        "line": 37,
        "function": "handleRequest",
        "class": "OC",
        "type": "::",
        "args": []
      }
    ],
    "File": "/var/www/html/lib/private/Files/Node/File.php",
    "Line": 56,
    "message": "",
    "exception": {},
    "CustomMessage": "Exception thrown: OCP\\Files\\GenericFileException"
  }
}

Steps to reproduce

1. Clear appdata
2. Internal server error
3. 😞

Expected behavior

I assume it's okay that your instance is bricked if you deleted app data.

However, there are also some weird paths in our code.

https://github.com/nextcloud/server/blob/d14d881080e7c994afb5e8b9a03c10e37939f769/lib/private/App/AppStore/Fetcher/Fetcher.php#L183-L186

Above code should handle that situation, a local copy of apps.json does not exist, properly.
It does not because a OCP\Files\GenericFileException is thrown instead of the expected OCP\Files\NotFoundException.

https://github.com/nextcloud/server/blob/8b5997483c852bcee6b44188982073c8213de25f/lib/private/Files/SimpleFS/SimpleFile.php#L80-L95

ISimpleFile.getContent does a check for false.
After https://github.com/nextcloud/server/pull/37943 the method does not return false but throws a OCP\Files\GenericFileException.

Installation method

None

Nextcloud Server version

master

Operating system

Debian/Ubuntu

PHP engine version

Other

Web server

Apache (supported)

Database engine version

None

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 with lib/private/App/AppStore/Fetcher/Fetcher.php around the local apps.json handling, then inspect lib/private/Files/SimpleFS/SimpleFile.php and the linked change in ISimpleFile::getContent. Reproduce with cleared appdata and follow the AppFetcher path from the provided stack trace. Done means the missing local copy follows the expected NotFoundException path instead of receiving GenericFileException.