[libvirt] [PATCH 2/2] qemu: use -incoming fd:n to avoid qemu holding fd indefinitely

[Eric Blake]

On 01/03/2011 06:39 AM, Paolo Bonzini wrote:
> On 12/23/2010 05:51 PM, Eric Blake wrote:
>> In the outgoing direction, we still have to use a unix socket (outgoing
>> migration is started via a monitor command, and I don't know how to pass
>> a new fd into qemu for using with an fd:n outgoing migration via the
>> monitor - it seems like fd is only useful on the command line).
> 
> See qemuMonitorCommandWithFd and qemuMonitorJSONCommandWithFd.

I'm looking at this problem again, now that we know that qemu exec:
migration is inherently racy (see
https://bugzilla.redhat.com/show_bug.cgi?id=678524).  I've managed to
work out the code to send an fd over to qemu, but how do I formulate the
fd:n migration command to use that fd?  That is, if I call
qemuMonitorCommandWithFd(,20), that copies fd 20 from libvirt's point of
view into qemu, but using the command 'migrate fd:20' is wrong because
qemu's copy is not necessarily fd 20.

Is there a monitor command that lets libvirt pass in an fd and get a
return value of what qemu sees for that newly copied fd, with no other
operation?  If so, then I can use that to do a two-step process - first
pass the fd, then pass the migrate fd:n command.  And what if the first
step succeeds but the second fails?  Is there a way to request that qemu
close an arbitrary fd that was passed to it earlier on?  Otherwise,
libvirt has just leaked an fd to qemu, and we have a repeat of the
earlier bug report where if the fd is a lvm partition, then the leaked
reference prevents one from closing out the logical volume.

I don't want to have to resort to brute-force searching through
/proc/<qemu-pid>/fd to find which fd in qemu matches the one that
libvirt sent over.

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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