Inject default qemu_agent allowing for explicit disable (#1408)

Facilitate the injection of a default channel when qemu_agent is set to
true to make enabling it's use a simple `qemu_agent = true` for most
cases.

Where custom values are needed, they can be provided and the code will
skip adding a corresponding entry, or if necessary the user can add an
entry and mark it disabled similar to how synced folders work.

Closes: #1341

README.md

@@ -999,6 +999,19 @@ if it detects an attached channel during boot.
 * `qemu_use_agent` - false by default, if set to true, attempt to extract configured
   ip address via qemu agent.
 
+By default if `qemu_use_agent` is set to `true` the code will automatically
+inject a suitable channel unless there already exists an entry with a
+`:target_name` matching `'org.qemu.guest_agent.'`.
+Alternatively if setting `qemu_use_agent` but, needing to disable the addition
+of the channel, simply use a disabled flag as follows:
+```ruby
+Vagrant.configure(2) do |config|
+  config.vm.provider :libvirt do |libvirt|
+    libvirt.channel :type => 'unix', :target_name => 'org.qemu.guest_agent.0', :disabled => true
+  end
+end
+```
+
 To use the management network interface with an external dhcp service you need
 to setup a bridged host network manually and define it via
 `management_network_name` in your Vagrantfile.

lib/vagrant-libvirt/config.rb

@@ -498,7 +498,9 @@ module VagrantPlugins
                        target_address: options[:target_address],
                        target_name: options[:target_name],
                        target_port: options[:target_port],
-                       target_type: options[:target_type])
+                       target_type: options[:target_type],
+                       disabled: options[:disabled],
+                      )
       end
 
       def random(options = {})
@@ -906,7 +908,18 @@ module VagrantPlugins
         @inputs = [{ type: 'mouse', bus: 'ps2' }] if @inputs == UNSET_VALUE
 
         # Channels
-        @channels = [] if @channels == UNSET_VALUE
+        if @channels == UNSET_VALUE
+          @channels = []
+          if @qemu_use_agent == true
+            if @channels.all? { |channel| !channel.fetch(:target_name, '').start_with?('org.qemu.guest_agent.') }
+              channel(:type => 'unix', :target_name => 'org.qemu.guest_agent.0', :target_type => 'virtio')
+            end
+          end
+        end
+
+        # filter channels of anything explicitly disabled so it's possible to inject an entry to
+        # avoid the automatic addition of the guest_agent above, and disable it from subsequent use.
+        @channels = @channels.reject { |channel| channel[:disabled] }.tap {|channel| channel.delete(:disabled) }
 
         # PCI device passthrough
         @pcis = [] if @pcis == UNSET_VALUE

spec/unit/config_spec.rb

@@ -531,6 +531,47 @@ describe VagrantPlugins::ProviderLibvirt::Config do
         end
       end
     end
+
+    context '@channels' do
+      it 'should be empty by default' do
+        subject.finalize!
+
+        expect(subject.channels).to be_empty
+      end
+
+      context 'when qemu_use_agent is set' do
+        before do
+          subject.qemu_use_agent = true
+        end
+
+        it 'should inject a qemu agent channel' do
+          subject.finalize!
+
+          expect(subject.channels).to_not be_empty
+          expect(subject.channels).to match([a_hash_including({:target_name => 'org.qemu.guest_agent.0'})])
+        end
+
+        context 'when agent channel already added' do
+          it 'should not modify the channels' do
+            subject.channel :type => 'unix', :target_name => 'org.qemu.guest_agent.0', :target_type => 'virtio'
+
+            subject.finalize!
+
+            expect(subject.channels.length).to eq(1)
+          end
+
+          context 'when agent channel explicitly disbaled' do
+            it 'should not include an agent channel' do
+              subject.channel :type => 'unix', :target_name => 'org.qemu.guest_agent.0', :disabled => true
+
+              subject.finalize!
+
+              expect(subject.channels).to be_empty
+            end
+          end
+        end
+      end
+    end
   end
 
   def assert_invalid

tests/qemu_agent/Vagrantfile

@@ -7,7 +7,6 @@ Vagrant.configure("2") do |config|
   config.vm.box = "generic/debian10"
   config.vm.synced_folder ".", "/vagrant", disabled: true
   config.vm.provider :libvirt do |libvirt|
-    libvirt.channel :type => 'unix', :target_name => 'org.qemu.guest_agent.0', :target_type => 'virtio'
     libvirt.qemu_use_agent = true
   end
 end