Fix sharing with user-key encryption

[jvillafanez]

Description

Having user-key encryption enabled, shares being received by the user might not be properly decrypted.

The cause was a wrong path and a length-base check. Basically a //files/path/to/file was used (notice the double //) and the check was using a strlen('/files'); the resulting path was s/path/to/file instead of the expected /path/to/file.
When we checked for the shares of the target path (s/path/to/file), we didn't find it, so we returned an access list containing only the owner of the file instead of the owner and the sharees. In the end, the key file for the sharee wasn't present in the right place.

Related Issue

owncloud/encryption#329

Motivation and Context

How Has This Been Tested?

Manually tested

1. Setup up user-key encryption
2. user1 shares a file with user2
3. user2 tries to download the file.

With the patch, user2 can preview the file in the file list and he can also download the file without problems

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

[ownclouders]

💥 Acceptance tests pipeline apiShareManagementBasicToShares-mariadb10.2-php7.4 failed. The build has been cancelled.

https://drone.owncloud.com/owncloud/core/35034/66

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

100.0% Coverage
0.0% Duplication

[phil-davis]

Looks sensible anyway, to remove the accidental // that was being produced if mountPoint was the empty string. Let's merge this, and we will get a new result from nightly CI. If there are other issues, we can followup with another PR(s).