[systemd-devel] Suggestion for a lowlevel fsnotify change daemon.

[Stef Bon]

Hi all,

for some time I have been looking at the issue why fsnotify does not work
with network filesystems and FUSE (with a shared backend).

I've found out that changes initiated on the localhost, on the filesystem
are supported by the fs change subsystems on Linux, and events initiated at
the backend (from another host with network fs)  are not detected. This is
because the filesystem are not "aware" a watch has been set on an inode,
and thus cannot act on it.
(if they act if they are aware is another question).

I've tried to tackle this in the kernel. I've made this working with a FUSE:
- when a watch is set on a FUSE fs, a message is forwarded to the userspace
daemon containing the inode and the mask. I had to add a opcode
FUSE_FSNOTIFY.
- the fuse fs has to react in it, by setting a watch on the backend. I
wrote a simple overlay fs, and setting a watch on the backend is simple
- I had to add some calls to the fuse library to "push" changes to the VFS
where there is no direct related call from the VFS. (files are added and/or
files are changed)
- the FUSE kernel module in VFS has to trigger fsnotify call when events
are pushed to the VFS by the userspace daemon.

This worked but is I think not the best way to deal with it.

My suggestion it to write a fs notify change service which does all the
watching for clients, like there are already services for desktops right
now.

This service should also work with a console app like mc, but also with
desktop environments like Gnome and KDE.

It should also be able to "forward" a watch to a filesystem like FUSE and
cifs and nfs, so that they "know" a watch has been set.
They can act then on it, by forwarding the watch to the backend. SMB does
upport this, NFS4 also, and you can make FUSE also support it(depending the
protocol).
When the fs receives an event, it can send it back to the fs notify change
service, which informs the client(s). This way the filesystem also stays up
to date.

To forward a watch and to read to incoming fsevents, a
socket/filedescriptor is required. A FUSE fs can easily connect to it at
startup, the in kernel filesystems need some extra. Via mountoptions parse
the fd to the kernel?

Is this something what can be added to systemd? Please let me know what you
think of it.

Stef
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