[Bug]: Slow query getMountsForStorageId when user has many shared folders

[Ivansss]

⚠️ This issue respects the following points: ⚠️

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

Bug description

Detected the issue in the following scenario:

User A has 40K+ shared folders
User A share one of those folders with User B
User B experiences slowness because it has to cope with the 40K+ mount points

server/apps/files_sharing/lib/SharedStorage.php

Lines 443 to 444 in 8995272

	$mounts = $userMountCache->getMountsForStorageId($node->getStorageId());
	foreach ($mounts as $mount) {

The query was triggered by "Text" app's rich workspaces:

Expected behavior

User B should not experience that slowness when loading its files

[oleksandr-nc]

Detected the issue in the following scenario:

User A has 40K+ shared folders User A share one of those folders with User B User B experiences slowness because it has to cope with the 40K+ mount points

We have the same scenario, only in it disabling "rich workspaces" does not help in that case.

[icewind1991]

Having it such that loading the share for userB doesn't require setting up all the shares for userA would require a significant rework in how the filesystem is setup.

[icewind1991]

Looking at things again, this specific code path (via getWatcher) can maybe be improved. But I don't know how much that fixes the overall performance in a setup like this.

[icewind1991]

#49569