[libvirt] RFC: 'old' event for leaseshelper.c when lease renews

In the current version of dnsmasq, the leases helper script/program specified by --dhcp-script to dnsmasq is invoked on three events, 'add', 'old', 'del'. In short, add: -> a new lease has to be added to db del: -> a lease has to be deleted from db old:-> if lease has changed Now, the lease can be considered to be changed in 2 ways. [standard-change] The change could be to the associated hostname or MAC address [auxiliary-change] The change associated with expiry time or clientid When only --dhcp-script is set, 'old' events are sent only for standard-lease changes. But when --dhcp-script is set along with --leasefile-ro, 'old' events are sent for any change in the lease. So, right now, if a lease is renewed, the reflected change doesn't appear in our JSON formatted lease database <interface-name>.status. We have the following options with this: (i) We can simply add the --leasefile-ro option. But in that case, the leases file database which is generated by dnsmasq by the name <network-name>.leases will cease to exist. It won't contain any lease information. All lease database handling will be done by our leaseshelper. Note: this option given a lot of information which is not stored in <network-name>.leases file. (ii) We ask the dnsmasq developer(s) to add an extra command line option to enable auxiliary changes in lease to be propagated to leaseshelper via 'old' events. I had a small conversation with Simon Kelley, and he said: "Yes. For that application (libvirt), you clearly don't want a third-party patch. At very least I'd be willing to add a boolean option to dnsmasq which enables "old" events when the lease expiry time changes, independent of leasefile-ro." If we do this, then can retain <network-name>.leases and have our helper too. (iii) We do nothing. IMO, we should go for (i), since <network-name>.leases becomes obsolete, once we have our own leases helper. Also, enabling --leasefile-ro enables a lot of other options, like vendor-class, user-class, circuit-id and much more, which may be asked for in future and get added to the virNetworkDHCPLeases struct. But we will have to remove the option --dhcp-leasefile. Although it won't affect any existing APIs, if people did rely on the dnsmasq's generated leases file before, they will find it missing. There is one more point. The man page of dnsmasq for the leasefile-ro option, states, "In addition to the invocations given in --dhcp-script the lease-change script is called once, at dnsmasq startup, with the single argument "init". When called like this the script should write the saved state of the lease database, in dnsmasq leasefile format, to stdout and exit with zero exit code." This is something extra that we will have to add to leaseshelper if we go with (i). The easier option is to go with (ii), since Simon is already willing to add that functionality. -- Nehal J Wani