[Bug]: Array to string conversion in CalDAV backend

[ChristophWurst]

⚠️ 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 (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• Nextcloud Server is running on 64bit capable CPU, PHP and OS.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

Array to string conversion at https://github.com/nextcloud/server/blob/v26.0.0rc1/apps/dav/lib/CalDAV/Activity/Backend.php#L561

server/apps/dav/lib/CalDAV/Activity/Backend.php

Line 561 in 6466c8e

$users = $this->getUsersForShares(array_intersect($sourceShares, $targetShares));

Steps to reproduce

1. Connect mac os to Nextcloud's CalDAV backend
2. ???

Expected behavior

No errors

Installation method

None

Operating system

None

PHP engine version

None

Web server

None

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

n/a

List of activated Apps

n/a

Nextcloud Signing status

No response

Nextcloud Logs

{
  "reqId": "9DlC6Z02DsTFC2tvNxU2",
  "level": 3,
  "time": "2023-03-03T06:56:13+00:00",
  "remoteAddr": "redacted",
  "user": "user123",
  "app": "PHP",
  "method": "MOVE",
  "url": "/remote.php/dav/calendars/user123/personal/sabredav-abc-def-ghi.ics",
  "message": "Array to string conversion at /var/www/cloud.nextcloud.com/nextcloud/apps/dav/lib/CalDAV/Activity/Backend.php#561",
  "userAgent": "macOS/13.1 (22C65) dataaccessd/1.0",
  "version": "26.0.0.8",
  "data": {
    "app": "PHP"
  },
  "id": "123def567"
}

Additional info

No response

[ChristophWurst]

array_intersect does not like nested array: https://3v4l.org/TYrqn

[tcitworld]

I just saw the same thing (in 25). I don't see how shares can contain an array itself. 🤔

[kartoffelheinz]

This bug prevents certain events from syncing (from Apple devices, mostly fantastical) and had been fixed in early 26 beta but apparently has now been re-introduced. Unfortunate. Any idea on when this is going to be fixed again?

[Pazu]

Still seeing this running 27.1.3:

{
    "reqId": "[removed]",
    "level": 3,
    "time": "2023-11-05T17:35:11+00:00",
    "remoteAddr": "[removed]",
    "user": "[removed]",
    "app": "PHP",
    "method": "MOVE",
    "url": "/remote.php/dav/calendars/[removed]/[removed]/[GUID].ics",
    "message": "Array to string conversion at /var/www/path_to_web_root/apps/dav/lib/CalDAV/Activity/Backend.php#561",
    "userAgent": "iOS/17.0.3 (21A360) dataaccessd/1.0",
    "version": "27.1.3.2",
    "data": {
        "app": "PHP"
    },
    "id": "[removed]"
}

[solracsf]

What if values are flatten before using the arrays, could this fix it?

$flatten = function ($array) use (&$flatten) {
	$result = [];
	foreach ($array as $value) {
		if (is_array($value)) {
			$result = array_merge($result, $flatten($value));
		} else {
			$result[] = $value;
		}
	}
	return $result;
};

$sourceShares = is_array($sourceShares) ? $flatten($sourceShares) : [];
$targetShares = is_array($targetShares) ? $flatten($targetShares) : [];

[jameskimmel]

Same error on 28.0.2

{
  "reqId": "removed",
  "level": 3,
  "time": "2024-02-21T11:07:11+00:00",
  "remoteAddr": "1.1.1.1",
  "user": "UserName1",
  "app": "PHP",
  "method": "MOVE",
  "url": "/remote.php/dav/calendars/UserName1/sitzungen_shared_by_UserName2/58B95D5D-32EA-490B-82E1-ADC31B854C3C.ics",
  "message": "Array to string conversion at /var/www/nextcloud/apps/dav/lib/CalDAV/Activity/Backend.php#561",
  "userAgent": "Mozilla/5.0 (iPad; CPU OS 15_6 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) CriOS/121.0.6167.171 Mobile/15E148 Safari/604.1",
  "version": "28.0.2.5",
  "data": {
    "app": "PHP"
  },
  "id": "65defd55416b7"
}

[joshtrichards]

Same solution as used in #43794 should work here (using array_map serialize/unserialize). I don't have a test setup for this, so I'll leave trying it to someone that feels like giving it a shot + can also reproduce this situation on-demand.