Don't assume a process with > 0 instances exists (#4251)

* also lock all web_processes for an app during a scaling action

Author: sethboyles

lib/cloud_controller/deployment_updater/actions/scale.rb

@@ -28,8 +28,7 @@ def call
 
             app.lock!
 
-            oldest_web_process_with_instances.lock!
-            deploying_web_process.lock!
+            app.web_processes.each(&:lock!)
 
             deployment.update(
               status_value: DeploymentModel::ACTIVE_STATUS_VALUE,
@@ -52,11 +51,6 @@ def call
 
         private
 
-        def oldest_web_process_with_instances
-          # TODO: lock all web processes?  We might alter all of them, depending on max-in-flight size
-          @oldest_web_process_with_instances ||= app.web_processes.select { |process| process.instances > 0 }.min_by { |p| [p.created_at, p.id] }
-        end
-
         def instance_count_summary
           @instance_count_summary ||= instance_reporters.instance_count_summary(deploying_web_process)
         end

spec/unit/lib/cloud_controller/deployment_updater/actions/scale_spec.rb

@@ -89,6 +89,15 @@ module VCAP::CloudController
       }.by(1)
     end
 
+    context 'when the app has been scaled to 0 instances' do
+      let(:current_web_instances) { 0 }
+      let(:target_total_instance_count) { 0 }
+
+      it 'finishes scaling' do
+        expect(subject.call).to be true
+      end
+    end
+
     context 'when the max_in_flight is set to 2' do
       let(:deployment) do
         DeploymentModel.make(