Validate reshare permissions against actual path that the user tries to share

[juliusknorr]

Otherwise this could lead to taking the wrong user mount in case there
are multiple ones with different permissions that the user could use to
reshare

Possible reproduction:

• A creates a folder and shares it with group without reshare permissions
• A creates a subfolder inside of the share and shares it to B with reshare permisions
• B is member of group
• B Tries to create a share link of the subfolder

Before:
💥 Cannot increase permissions

After:
The proper source user mount is taken for permission comparison where the user has reshare permissions and the share link creation passes.

[juliusknorr]

Other bug found, to be filed separately:

• B is offered resharing in the UI for the subfolder (but not children) when accessing it through the original share path, but the backend doesn't allow resharing there

[juliusknorr]

Unit tests should be fine now and were actually using the wrong source node in case of a reshare as then the share would always be created from the resharing users filesystem instead of using the owners user folder as a base.

[nickvergessen]

We really need integrations tests for this.

[juliusknorr]

Will look into some tests for that and will double check the suggestion from @PVince81

[juliusknorr]

Pushed another attempt as the share node might be the one located in the owners filesystem

server/lib/private/Share20/Share.php

Lines 182 to 186 in 215aef3

	if ($this->userManager->userExists($this->shareOwner)) {
	$userFolder = $this->rootFolder->getUserFolder($this->shareOwner);
	} else {
	$userFolder = $this->rootFolder->getUserFolder($this->sharedBy);
	}

compared to the previously assumed shared by users filesystem https://github.com/nextcloud/server/blob/master/lib/private/Share20/Manager.php#L271-L275

[juliusknorr]

Closing as I don't have time to properly finish this