Empty <default></default> for an ExApp environment variable is deployed as the literal string `Array`

[oleksandr-nc]

Describe the bug

If an ExApp declares an environment variable with an empty <default> element, the variable is deployed to the container with the literal string Array as its value, and PHP logs an Array to string conversion warning.

An empty XML element is parsed into an empty array, not an empty string, so it slips past the filter that is meant to drop empty values.

Steps/Code to Reproduce

appinfo/info.xml:

<external-app>
    <docker-install>
        <registry>example.local</registry>
        <image>minimal_exapp</image>
        <image-tag>1.0.0</image-tag>
    </docker-install>
    <environment-variables>
        <variable>
            <name>EMPTY_ELEM</name>
            <display-name>Empty</display-name>
            <description>d</description>
            <default></default>
        </variable>
    </environment-variables>
</external-app>

occ app_api:app:register <appid> <harp-daemon> --info-xml /tmp/info.xml --wait-finish
docker inspect nc_app_<appid> --format '{{range .Config.Env}}{{println .}}{{end}}' | grep EMPTY_ELEM

Expected Results

The variable is not passed to the container at all, which is what happens for the two neighbouring cases (both verified):

• a <variable> with no <default> element at all,
• a declared variable overridden with an empty value, --env EMPTY_ELEM=.

Actual Results

EMPTY_ELEM=Array

and in nextcloud.log (level 2):

Array to string conversion at /var/www/html/apps-extra/app_api/lib/DeployActions/DockerActions.php#1279

<default/> behaves identically to <default></default>.

Cause

info.xml is parsed with json_decode(json_encode((array)simplexml_load_string(...)), true), so an empty element becomes [] rather than ''.

• lib/Service/ExAppService.php#L305-L306 assigns that array to default and value.
• the filter at lib/Service/ExAppService.php#L318 ($envVar['value'] !== '') therefore keeps the entry, since [] !== ''.
• lib/DeployActions/DockerActions.php#L1189 then does sprintf('%s=%s', $envKey, ...['value']), producing Array. lib/DeployActions/KubernetesActions.php#L709 is the same code, so Kubernetes deployments are affected too.

App Store installs go through the same XML path, so a published ExApp with this in its manifest would ship Array to every installation.

The same code is present in main, stable34 and stable33.

Suggested fix

Normalise the parsed value to a string where it is read, which keeps the existing empty-value filter working for every shape:

$default = $envVar['default'] ?? '';
if (!is_scalar($default)) {
    $default = '';
}

Setup configuration

• Nextcloud: 35.0.0 dev
• AppAPI: 35.0.0-dev.1 (main)
• HaRP: 0.4.3, docker-install daemon
• Reproduced with a minimal framework-free ExApp; line numbers above are from main, the log line number is from the checkout used for the test.