Re: [libvirt] [PATCH 0/2] Fix interface state transitions logic

On 12/11/2013 11:16 AM, Michal Privoznik wrote: > Right now it's possible to start an interface that is already running, or > destroy an interface multiple times. Such state transitions are not allowed and > we check for such cases explicitly in other areas like qemu driver. I recall that someone brought up this issue before, and we didn't change the code. If I remember correctly, the reason was that the virInterfaceCreate() API is basically just a wrapper around /sbin/ifup, and /sbin/ifup can be called (and returns success after doing something useful - see below) if the interface is already up. Why would you want to do that? Well, calling ifup on an interface that is already up causes it to be "re-started", renewing its IP (and other) configuration from any changes that have been made, *without* needing to ifdown the interface in the process (and completely losing network connectivity, possibly making it impossible for the program calling the API to re-start the interface). (It's not directly applicable in this case, but when a bridge device is added, subsuming an existing ethernet, we rely on allowing virInterfaceCreate() to be called for the new bridge device even though the subordinate ethernet is already up; this permits us to add a bridge to the host's config without losing network connectivity.) I'm not convinced that it's a bad thing that virInterfaceCreate/Destroy can, by definition, be called when the device is already in the desired state, and wouldn't want to end up with other unforeseen problems just because we disabled it.