[Bug]: Nextcloud virtual files cannot sync some files

[Gloryandel]

⚠️ 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

I hate to say it, but Nextcloud 3.15 is the worst release ever.

• 3.15.0 When Windows 10 virtual files are synchronized to the local computer, some files cannot be synchronized to the local computer, resulting in a crash. After that, the system will automatically synchronize and crash again without any error message.
• 3.15.1 When Windows 10 + Windows 11 virtual files are synchronized to the local computer, the Nextcloud crashes, which is even more serious than the 3.15.0 version. It is directly unresponsive and stuck.
• No translation files were imported in the 3.15 version release.
• Error message: Multiple accounts are sharing the folder
  This configuration is known to lead to data loss and is no longer upported. Please consider removing this folder from the account and adding it again.
  Currently, Nextcloud is unavailable.

Steps to reproduce

1. Create a sync for a directory with many files, using virtual files
2. Use "Always keep local" for a virtual file directory
3. Sync starts, crashes at one file (happens 100%), no error message

Expected behavior

。。。

Which files are affected by this bug

all

Operating system

Windows

Which version of the operating system you are running.

Windows10、Windows11

Package

Official Windows MSI

Nextcloud Server version

30.04

Nextcloud Desktop Client version

3.15.0

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 Disabled

Are you using an external user-backend?

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

Nextcloud Server logs

none

Additional info

log.zip

[camilasan]

• Error message: Multiple accounts are sharing the folder
  This configuration is known to lead to data loss and is no longer upported. Please consider removing this folder from the account and adding it again.
  Currently, Nextcloud is unavailable.

There is a bug report for this: #7657
it means your sync folder contains multiple database files. There is a workaround in the comments.

[Gloryandel]

@camilasan The biggest problem right now is that Nextcloud crashes when certain files are converted from virtual files to real files. There is nothing special about the files that cause the crashes.The only thing that is certain is that when the number of files is large, the probability of crash is close to 100%

[Gloryandel]

The synchronization is terminated at a temporary file of a file in the synchronization process, with the name pattern of .<filename>.extention~<string>. It can be seen that the synchronization is not completed and Nextcloud crashes. The current solution is to cancel the “keep locally” option for the original file and delete the temporary file. Unfortunately, the file that caused the crash can no longer be synchronized to the local computer. As soon as its synchronization starts, Nextcloud crashes immediately.

[Gloryandel]

Finally, I would like to add that due to the unexpected crash of Nextcloud, temporary files were not processed. If they are not cleaned up manually, the temporary files will be synchronized to the Server when Nextcloud is started again. Maybe they need to be ignored by default.