Fix GET Process endpoint for unstaged docker apps (#4415)

- The user field was added in #4407
- This failed for unstaged Docker apps since they do not have
  JSON-parseable execution metadata
- This fix updates the code to treat empty and unparseable execution
  metadata the same as an empty JSON object

Author: tcdowney

app/models/runtime/process_model.rb

@@ -579,8 +579,16 @@ def permitted_users
     def docker_run_action_user
       return DEFAULT_USER unless docker?
 
-      docker_exec_metadata = Oj.load(execution_metadata)
-      container_user = docker_exec_metadata['user']
+      container_user = ''
+      if execution_metadata.present?
+        begin
+          docker_exec_metadata = Oj.load(execution_metadata)
+          container_user = docker_exec_metadata['user']
+        rescue EncodingError
+          container_user = ''
+        end
+      end
+
       container_user.presence || 'root'
     end
 

spec/unit/models/runtime/process_model_spec.rb

@@ -708,6 +708,30 @@ def act_as_cf_admin
             expect(process.run_action_user).to eq('root')
           end
         end
+
+        context 'when the droplet execution metadata is an empty string' do
+          let(:droplet_execution_metadata) { '' }
+
+          it 'defaults the user to root' do
+            expect(process.run_action_user).to eq('root')
+          end
+        end
+
+        context 'when the droplet execution metadata is nil' do
+          let(:droplet_execution_metadata) { nil }
+
+          it 'defaults the user to root' do
+            expect(process.run_action_user).to eq('root')
+          end
+        end
+
+        context 'when the droplet execution metadata has invalid json' do
+          let(:droplet_execution_metadata) { '{' }
+
+          it 'defaults the user to root' do
+            expect(process.run_action_user).to eq('root')
+          end
+        end
       end
 
       context 'when the process DOES NOT belong to a Docker lifecycle app' do