Detect external storage of type Local as unavailable when path is unmounted on system

[PVince81]

Steps

1. Create a local folder "/mnt/mountedstorage/nextcloud_external"
2. Give the web server permissions for that folder
3. Mount it as Local external storage under the local path "/nextcloud_external"
4. In the web UI, create a folder "/nextcloud_external/sub"
5. On the CLI, rename "nextcloud_external" to something else to simulate as if "/mnt/mountedstorage" was not mounted
6. PROPFIND "/nextcloud_external"

Expected result

PROPFIND returns a 503 with StorageNotAvailable since the storage/path is temporarily not there.
A configured desktop client would not touch that folder locally, no deletion.

Actual result

PROPFIND returns a 404 on the path.
A configured desktop client would proceed to delete the local "/nextcloud_external" folder until mounted again. (assuming this, I haven't tested)
The next time the mount point is back, the desktop client would redownload everything.

Versions

Observed on master 9a7e2b1 and it was like that since forever 😅

Notes

My actual use case is that I have an "archive" hard disk connected to my Rpi4 but I don't want to have it powered up every time.
My expectation is that whenever I power it up the mount point becomes available again to access the archive, but I don't want desktop clients to delete and redownload files again from that place.

From what I remember desktop clients are already able to deal with unavailable storages and will skip these seamlessly (503 StorageNotAvailable). @tobiasKaminsky

Thoughts @icewind1991 @artonge @come-nc ?

I have a PR incoming...

[PVince81]

PR here: #39707

[PVince81]

Follow up in the PR as I discovered more things related to desktop client and the sabre exception ^

[kesselb]

Thank you, good finding 👍

The comment nextcloud/desktop#260 (comment) sounds exactly like your case (disk gone, desktop deleted the files).

[joshtrichards]

This sounds similar to #37934. We return the 404 but then also sometimes change our mind and start doing the 503. I figured out at least one way to trigger the switch. My notes in case they prove helpful:

#37934 (comment)