complete config reset with filled disk

### Steps to reproduce
1. Fill entire available diskspace
2. Visit instance landingpage (might need to repeat this step a couple of times, not sure if instantly reproducible)

### Expected behaviour
Config should never be reset, even in edgecases.
Config should never ever be opened with write-permission from a public / non-admin endpoint. 

### Actual behaviour
Find your `config/config.php` truncated to a minimal config file with only the `instanceid` key filled in. This is serious.

Landing page says something like  "Looks like you're trying to reinstall nextcloud but file CAN_INSTALL was not found"

### Server configuration

**Operating system:** debian 9

**Web server:** nginx

**Database:** mysql

**PHP version:** 7.3

**Nextcloud version:** (see Nextcloud admin page) 17.0.1

**Updated from an older Nextcloud/ownCloud or fresh install:** updated since NC 15

**Where did you install Nextcloud from:** source tarball

**Signing status:**
<details>
<summary>Signing status</summary>

```
No errors have been found.
```
</details>

**List of activated apps:**
<details>
<summary>App list</summary>

```
Enabled:
  - accessibility: 1.3.0
  - apporder: 0.8.0
  - audioplayer: 2.8.4
  - calendar: 1.7.1
  - cloud_federation_api: 1.0.0
  - contacts: 3.1.6
  - dav: 1.13.0
  - deck: 0.7.0
  - encryption: 2.5.0
  - federatedfilesharing: 1.7.0
  - files: 1.12.0
  - files_linkeditor: 1.0.11
  - files_markdown: 2.1.0
  - files_pdfviewer: 1.6.0
  - files_readmemd: 1.1.3
  - files_rightclick: 0.15.1
  - files_sharing: 1.9.0
  - files_videoplayer: 1.6.0
  - gallery: 18.4.0
  - logreader: 2.2.0
  - lookup_server_connector: 1.5.0
  - nextcloud_announcements: 1.6.0
  - notes: 3.0.3
  - notifications: 2.5.0
  - oauth2: 1.5.0
  - password_policy: 1.7.0
  - polls: 0.10.4
  - privacy: 1.1.0
  - provisioning_api: 1.7.0
  - recommendations: 0.5.0
  - serverinfo: 1.7.0
  - sharebymail: 1.7.0
  - sharerenamer: 2.7.2
  - survey_client: 1.5.0
  - tasks: 0.11.3
  - text: 1.1.1
  - theming: 1.8.0
  - twofactor_backupcodes: 1.6.0
  - updatenotification: 1.7.0
  - viewer: 1.2.0
  - workflowengine: 1.7.0

```
</details>

**Nextcloud configuration:**
<details>
<summary>Config report</summary>

```
{
    "system": {
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "***REMOVED SENSITIVE VALUE***"",
            "***REMOVED SENSITIVE VALUE***"",
            "***REMOVED SENSITIVE VALUE***""
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "17.0.1.1",
        "overwrite.cli.url": "***REMOVED SENSITIVE VALUE***",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "logtimezone": "UTC",
        "installed": true,
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpmode": "smtp",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "appstore.experimental.enabled": true,
        "loglevel": 2,
        "maintenance": false,
        "default_language": "de",
        "theme": "",
        "updater.release.channel": "stable",
        "mail_smtpauthtype": "LOGIN",
        "mail_smtpauth": 1,
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpport": "587",
        "mail_smtpname": "***REMOVED SENSITIVE VALUE***",
        "mail_smtppassword": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpsecure": "tls",
        "memcache.distributed": "\\OC\\Memcache\\Redis",
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "memcache.local": "\\OC\\Memcache\\APCu",
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "port": 6379,
            "timeout": 0,
            "dbindex": 0
        },
        "mysql.utf8mb4": true
    }
}

```
</details>

**Are you using external storage, if yes which one:** no
**Are you using encryption:** yes
**Are you using an external user-backend, if yes which one:** no

### Client configuration
**Browser:** latest firefox

**Operating system:** linux

### Logs
#### Web server error log
<details>
<summary>Web server error log</summary>

```
Insert your webserver log here
```
</details>

#### Nextcloud log (data/nextcloud.log)
<details>
<summary>Nextcloud log</summary>
At the point of config-reset, nothing is logged. Beforehand, redis complains about unavailable storage
</details>

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Uh oh!

complete config reset with filled disk #18620

Steps to reproduce

Expected behaviour

Actual behaviour

Server configuration

Client configuration

Logs

Web server error log

Nextcloud log (data/nextcloud.log)

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Uh oh!

complete config reset with filled disk #18620

Description

Steps to reproduce

Expected behaviour

Actual behaviour

Server configuration

Client configuration

Logs

Web server error log

Nextcloud log (data/nextcloud.log)

Metadata

Metadata

Assignees

Labels

Type

Fields

Projects

Milestone

Relationships

Development

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