[Bug]: nexcloud client freezes with circular symbolic links

[olivier-ploton]

⚠️ Before submitting, please verify the following: ⚠️

• This is a bug, not a question or a configuration issue.
• This issue is not already reported on Github (I've searched it).
• Nextcloud Server and Desktop Client are up to date. See Server Maintenance and Release Schedule and Desktop Releases for supported versions.
• I agree to follow Nextcloud's Code of Conduct

Bug description

Since recently, my nextcloud client freezes at startup, consuming 100% CPU for few minutes, then crashes. The problem seems to be related to circular symlinks (I mean, symlinks pointing to a parent directory of themselves). In fact, removing these symlinks makes nextcloud work fine again.

Steps to reproduce

1. In a directory do be sync-ed (for me ~/Nextcloud), untar the attached file BUG.TGZ. This creates a folder BUG with a minimal example. It contains two symlinks, ROOT and directory/ROOT, which both point to the directory itself: BUG.
2. launch nextcloud
3. nextcloud freezes and finally crashes after consuming all memory (16Gb, see log below)

$ command time -v nextcloud
nextcloud.gui.application: Migrating old config from "/home/plo/.local/share/Nextcloud" to "/home/plo/.config/Nextcloud"
nextcloud.gui.application: Failed to move the old config directory to its new location ( "/home/plo/.local/share/Nextcloud" to "/home/plo/.config/Nextcloud" )
nextcloud.gui.application: Will move the individual files ("Nextcloud_sync.log")
nextcloud.gui.application: Fallback move of  "Nextcloud_sync.log" also failed
Command terminated by signal 9
	Command being timed: "nextcloud"
	User time (seconds): 116.63
	System time (seconds): 199.70
	Percent of CPU this job got: 97%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 5:23.48
	Average shared text size (kbytes): 0
	Average unshared data size (kbytes): 0
	Average stack size (kbytes): 0
	Average total size (kbytes): 0
	Maximum resident set size (kbytes): 14325548
	Average resident set size (kbytes): 0
	Major (requiring I/O) page faults: 569
	Minor (reclaiming a frame) page faults: 3585062
	Voluntary context switches: 8733
	Involuntary context switches: 14269
	Swaps: 0
	File system inputs: 122528
	File system outputs: 84592
	Socket messages sent: 0
	Socket messages received: 0
	Signals delivered: 0
	Page size (bytes): 4096
	Exit status: 0

Expected behavior

The behaviour of the previous version was to ignore all symlinks, so they were harmless.

Nexcloud behaves correctly after removal of these symlinks.

Which files are affected by this bug

directory/ROOT

Operating system

Linux

Which version of the operating system you are running.

Ubuntu 22.04.4

Package

Distro package manager

Nextcloud Server version

24.0.9.2

Nextcloud Desktop Client version

3.13.0

Is this bug present after an update or on a fresh install?

Updated from a minor version (ex. 3.4.2 to 3.4.4)

Are you using the Nextcloud Server Encryption module?

Encryption is Enabled

Are you using an external user-backend?

• Default internal user-backend
• LDAP/ Active Directory
• SSO - SAML
• Other

Nextcloud Server logs

nexcloud crashes before producing any log

Additional info

Since 2019 and until now I synchronized about 10Gb in 60,000 files between 3 Ubuntu machines without any problem. The problem is quite recent for me (couple of weeks ?). Seems to be since a package update. Here are the packages I use:

$ apt list --installed nextcloud*
En train de lister... Fait
nextcloud-desktop-common/jammy,jammy,now 3.13.0-20240425.171743.1e5523e50-1.0~jammy1 all  [installé, automatique]
nextcloud-desktop-doc/jammy,jammy,now 3.13.0-20240425.171743.1e5523e50-1.0~jammy1 all  [installé]
nextcloud-desktop-l10n/jammy,jammy,now 3.13.0-20240425.171743.1e5523e50-1.0~jammy1 all  [installé]
nextcloud-desktop/jammy,now 3.13.0-20240425.171743.1e5523e50-1.0~jammy1 amd64  [installé]

[PlekszyGladz]

Something similar just happended on my system:

I upgraded the desktop client to 3.15.0 on my Win10 machine. The client started syncing all folders I symlinked into my Nextcloud folder, thus creating lots of duplicated files. Before that these links were ignored by the desktop client (as expected).

Currently I only encounter this problem on Win10. My Linux and OSX setups continue to work fine.