[lxc-devel] [PATCH] Add remove_snapshots_entry() (rebased - v2)

[Serge Hallyn]

Quoting Christian Brauner (christianvanbrauner at gmail.com):
> On Mon, Sep 14, 2015 at 02:50:39PM +0000, Serge Hallyn wrote:
> > Quoting Christian Brauner (christianvanbrauner at gmail.com):
> > > When creating ephemeral containers that have the option lxc.ephemeral = 1 set
> > > in their config, they will be destroyed on shutdown. As they are simple overlay
> > > clones of an existing container they should be registered in the lxc_snapshots
> > > file of the original container to stay consistent and adhere to the
> > > expectancies of the users. Most of all, it ensure that we cannot remove a
> > > container that has clones, even if they are just ephemeral snapshot-clones. The
> > > function adds further consistency because remove_snapshots_entry() ensures that
> > > ephemeral clone-snapshots deregister themselves from the lxc_snapshots file
> > > when they are destroyed.
> > > 
> > > POSSIBLE GLITCH:
> > > I was thinking hard about racing conditions and concurrent acces on the
> > > lxc_snapshots file when lxc-destroy is called on the container while we
> > > shutdown then container from inside. Here is what my thoughts are so far:
> > > 
> > > There should be no racing condition when lxc-destroy including all snapshots is
> > 
> > Note that lxcapi_destroy_with_snapshots() deletes the *snapshots*, not the
> > snapshot clones.  This is an unfortunate naming clash (which we could try
> > to correct henceforth but we need good names :), but they are different.
> > So anything under /var/lib/lxc/$container/snaps will be deleted.  But if
> > you've created an overlayfs clone, then containers listed in
> > /var/lib/lxc/$container/lxc_snapshots will not be deleted. There is no
> > API call or program to automatically deleted those right now.
> 
> I think you are partially wrong here. I was not thinking about problems created
> by an API-call but by the lxc-destroy exectuable. A quick walkthrough: With the

D'oh.  Yeah, you'll need to mutex that somehow.

> recent patches by me to lxc-destroy when I do:
> 
>         lxc-clone -s -B overlayfs -o w1 -n w2
> 
> the snapshot-clone will be registered in the lxc_snapshots file and when I call
> 
>         lxc-destroy -n w1 -s
> 
> the original container including all it's snapshot-clones will be destroyed and
> deleted from the lxc_snapshots file. This is done by having lxc-destroy open the
> lxc_snapshots file of the original container, reading in all snapshot-clone
> entries in and deleting them.
> 
> > think we want to write one, but a program to show which snapshots exist
> > would be good).
> > 
> > (Actually, there seems to be a bug right now - The sequence:
> > 
> > 	lxc-create -t download -n w1 -- -d ubuntu -r wily -a amd64
> > 	lxc-clone -s -B overlayfs -o w1 -n w2
> > 	lxc-snapshot -n w2
> > 	lxc-snapshot -n w2 -r snap0
> > 
> > does not result in /var/lib/lxc/w2/snap0 being deleted, so a subsequent
> > 
> > 	lxc-destroy -n w2
> > 
> > is refused.
> > 
> > Do you have time to either look into that, or raise an issue on github
> > about it?
> 
> I'll take a look tonight. Don't know how fast I can fix this.
> 
> > 
> > > called and the container in question is running, lxc-destroy will simply fail
> > > with the warning that the clone is still running but lxc-destroy won't touch
> > > the lxc_snapshots file. The offending container will then exit and delete the
> > > container entry. lxc-destroy can then be called again and will delete the
> > > remaining containers.
> > > 
> > > The strange case seems to be when we create another clone-snapshot while
> > > another one shuts down. Does someone have any arguments against this way of
> > 
> > This should be fine, because mod_rdep does a container_disk_lock(c0).  So
> > the inc and dec will be mutually exclusive.
> > 
> > > implementing it? Do we expect trouble? Do we need flocks in start.c and
> > > lxccontainer.c?
> > > 
> > > Christian Brauner (1):
> > >   Add remove_snapshots_entry()
> > > 
> > >  src/lxc/start.c | 123 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 123 insertions(+)
> > > 
> > > -- 
> > > 2.5.1
> > >