Allow network updates (vs. recreate) where possible

[wking]

Version Reports:

Terraform Version Report

$ terraform -v
Terraform v0.11.8

Your version of Terraform is out of date! The latest version
is 0.11.10. You can update by downloading from www.terraform.io/downloads.html

Provider and libvirt versions

$ ~/.terraform.d/plugins/terraform-provider-libvirt -version
/home/trking/.terraform.d/plugins/terraform-provider-libvirt 07d7ab44a1e6056c063a70e94db9352c3f54aa70
Compiled against library: libvirt 3.9.0
Using library: libvirt 3.9.0
Running hypervisor: QEMU 2.9.0
Running against daemon: 3.9.0

---

Description of Issue/Question

Setup

provider "libvirt" {
  uri = "qemu+tcp:///system"
}

variable "bootstrap_dns" {
  default     = true
}

resource "libvirt_network" "tectonic_net" {
  name      = "wking"
  mode      = "nat"
  bridge    = "tt0"
  domain    = "installer.testing"
  addresses = ["192.168.126.0/24"]

  dns = [{
    local_only = true

    hosts = ["${flatten(list(
      data.libvirt_network_dns_host_template.bootstrap.*.rendered,
    ))}"]
  }]

  autostart = true
}

data "libvirt_network_dns_host_template" "bootstrap" {
  count    = "${var.bootstrap_dns ? 1 : 0}"
  ip       = "192.168.126.10"
  hostname = "wking-api"
}

Steps to Reproduce Issue

$ terraform init
$ terraform apply -input=false
$ terraform apply -var bootstrap_dns=0
libvirt_network.tectonic_net: Refreshing state... (ID: 08a83d14-9bca-466e-9a1f-d5f54f8a94a2)

An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
-/+ destroy and then create replacement

Terraform will perform the following actions:

-/+ libvirt_network.tectonic_net (new resource required)
      id:                     "08a83d14-9bca-466e-9a1f-d5f54f8a94a2" => <computed> (forces new resource)
      addresses.#:            "1" => "1"
      addresses.0:            "192.168.126.0/24" => "192.168.126.0/24"
      autostart:              "true" => "true"
      bridge:                 "tt0" => "tt0"
      dns.#:                  "1" => "1"
      dns.0.hosts.#:          "1" => "0" (forces new resource)
      dns.0.hosts.0.hostname: "wking-api" => "" (forces new resource)
      dns.0.hosts.0.ip:       "192.168.126.10" => "" (forces new resource)
      dns.0.local_only:       "true" => "true"
      domain:                 "installer.testing" => "installer.testing"
      mode:                   "nat" => "nat"
      name:                   "wking" => "wking"


Plan: 1 to add, 0 to change, 1 to destroy.

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: no


Error: Apply cancelled.

---

Additional Infos:

Recreating the nework detaches associated interfaces:

Sometimes, one needs to edit the network definition and apply the changes on the fly. The most common scenario for this is adding new static MAC+IP mappings for the network's DHCP server. If you edit the network with "virsh net-edit", any changes you make won't take effect until the network is destroyed and re-started, which unfortunately will cause a all guests to lose network connectivity with the host until their network interfaces are explicitly re-attached.

Ideally we'd be updating the network in-place to remove the hosts entry (like virsh net-update), instead of deleting and recreating the network:

$ virsh -c qemu+tcp:///system net-dumpxml wking
<network>
  <name>wking</name>
  <uuid>08a83d14-9bca-466e-9a1f-d5f54f8a94a2</uuid>
  <forward mode='nat'>
    <nat>
      <port start='1024' end='65535'/>
    </nat>
  </forward>
  <bridge name='tt0' stp='on' delay='0'/>
  <mac address='52:54:00:c9:32:9f'/>
  <domain name='installer.testing' localOnly='yes'/>
  <dns>
    <host ip='192.168.126.10'>
      <hostname>wking-api</hostname>
    </host>
  </dns>
  <ip family='ipv4' address='192.168.126.1' prefix='24'>
  </ip>
</network>

$ virsh -c qemu+tcp:///system net-update wking delete dns-host '<host ip="192.168.126.10"><hostname>wking-api</hostname></host>'
Updated network wking live state
$ virsh -c qemu+tcp:///system net-dumpxml wking
<network>
  <name>wking</name>
  <uuid>08a83d14-9bca-466e-9a1f-d5f54f8a94a2</uuid>
  <forward mode='nat'>
    <nat>
      <port start='1024' end='65535'/>
    </nat>
  </forward>
  <bridge name='tt0' stp='on' delay='0'/>
  <mac address='52:54:00:c9:32:9f'/>
  <domain name='installer.testing' localOnly='yes'/>
  <ip family='ipv4' address='192.168.126.1' prefix='24'>
  </ip>
</network>

[MalloZup]

#389

[wking]

Dup of #389.