The Flock issue

[kobergj]

While writing or reading nodes from the disk we want to avoid inconsistent states. That is why we lock (or readlock) files before writing/reading them. This is done by filelocks package.

This works fairly well, however there is a critical point:
When unlocking the file again, we want to delete the created lockfile. BUT we only want to do this when there are no more locks on the file. This leads to concurrency issues when two routines try to delete the file at the same time.

The critical part of the code is this: https://github.com/cs3org/reva/blob/edge/pkg/storage/utils/filelocks/filelocks.go#L144
We need to find a proper way to delete the lockfile without affecting other go routines.

This bug leads to an error message like:

{
  "error":"remove /var/lib/ocis/storage/users/spacetypes/personal/ddc2004c-0977-11eb-9d3f-a793888cd0f8.flock: no such file or directory",
}

It happens especially often on batch actions (restore, delete, upload, ...).

Whenever you see an error message like above it is most likely this issue that is causing it

[kobergj]

cc @dragotin @micbar

[kobergj]

Closing this since the supposed fix is merged