[Virtio-fs] Lack of inotify for virtio-fs backed files/directories

[Eric Ernst]

Hey ya'll,

As highlighted nicely at [1], a common pattern in Kubernetes is to watch
for changes to files/directories passed in as Configmaps or secrets.
Sidecars typically use inotify to watch for changes and then signal the
primary container or perform other changes based on the reconfiguration.

In the context of Kata, these files/dirs are passed in utilizing virtio-fs.
Since FUSE itself doesn't support inotify, this pattern will fail. I'm
looking at ways to work-around this gap, including:

1) documenting this as a compatibility limitation ("don't do this!").
2) suggesting end-users modify their sidecars and utilize poll
3) doing an inotify on the host, and when changes are identified, notify
the guest agent who can then modify the files on disk to trigger an inotify
4) Working on contributing to libfuse in order to add inotify support.

(4) is the right thing, but will take quite a bit of time before it can be
used. I'm curious if other folks have suggestions for workarounds (I don't
like any of the ones I suggested), and if anyone has taken a look at (4)?

Thanks,
Eric

[1] - https://github.com/kata-containers/runtime/issues/1505
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