[RFC] Another take at restarting FUSE servers

[Luis Henriques <luis@igalia.com>]

Hi!

I know this has been discussed several times in several places, and the
recent(ish) addition of NOTIFY_RESEND is an important step towards being
able to restart a user-space FUSE server.

While looking at how to restart a server that uses the libfuse lowlevel
API, I've created an RFC pull request [1] to understand whether adding
support for this operation would be something acceptable in the project.
The PR doesn't do anything sophisticated, it simply hacks into the opaque
libfuse data structures so that a server could set some of the sessions'
fields.

So, a FUSE server simply has to save the /dev/fuse file descriptor and
pass it to libfuse while recovering, after a restart or a crash.  The
mentioned NOTIFY_RESEND should be used so that no requests are lost, of
course.  And there are probably other data structures that user-space file
systems will have to keep track as well, so that everything can be
restored.  (The parameters set in the INIT phase, for example.)

But, from the discussion with Bernd in the PR, one of the things that
would be good to have is for the kernel to send back to user-space the
information about the inodes it already knows about.

I have been playing with this idea with a patch that simply sends out
LOOKUPs for each of these inodes.  This could be done through a new
NOTIFY_RESEND_INODES, or maybe it could be an extra operation added to the
already existing NOTIFY_RESEND.

Anyway, before spending any more time with this, I wanted to ask whether
this is something that could be acceptable in the kernel, if people think
a different approach should be followed, or if I'm simply trying to solve
the wrong problem.

Thanks in advance for any feedback on this.

[1] https://github.com/libfuse/libfuse/pull/1219

Cheers,
-- 
Luís