[Bug]: Incorrect handling of read-only folders

[Tegenwind]

⚠️ 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

Placing a file in a local folder that's read-only on the server side used to throw an error, with a clear "path" to the offending files by simply following the yellow exclamation mark through the file structure, but this is now no longer handled and results in a re-starting sync every few seconds that simply hangs and restarts without ever doing anything.

Upon creating a new sync that contains a read-only folder, the local permissions are now also set to read-only. However, if you have a pre-existing file sync created by an earlier version that upon creating it still used read-write for all local folders, there's a high chance of a user placing files in a read-only folder and borking their sync, without giving any clues in the GUI as to what's wrong.

Downgrading to 3.12.7 fixes the issue and clearly shows the offending files.

nextcloud.zip

Steps to reproduce

1. Sync a folder with read-only permissions on the server side
2. Add "Test 1.pdf" through the web interface with r/w account. File is synced normally.
3. Change local folder permissions to read-write (or enter admin credentials when prompted to override)
4. Place "Test 2.pdf" in the local folder. Notice how sync restarts about every second
5. Place "Test 3.pdf" through the web interface with R/W account. Notice how the sync is practically borked and the new file is never synced to the client

Expected behavior

Upon sync, the file should be skipped and show an error in the sync client and finder extension (previous behaviour).
Or, the client should have a more waterproof way of handling these situations.

The first is probably easier to re-implement.

Which files are affected by this bug

Test 2.pdf

Operating system

macOS

Which version of the operating system you are running.

macOS 12 through 15

Package

Official macOS 12+ universal pkg

Nextcloud Server version

29.0.9 through 3.0.5

Nextcloud Desktop Client version

3.15.3

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

Updated to a major version (ex. 3.3.6 to 3.4.0)

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

Additional info

No response

[loulous24]

We have the same issue. It is also related to the management of permissions for local folders, see the issue here #7318.

[camilasan]

I believe we have a fix for it. I am creating a build with it so you can try the fix.