force unicorn to terminate before restarting

Capfile

@@ -13,4 +13,5 @@ require 'capistrano3/unicorn'
 # Load custom tasks from `lib/capistrano/tasks` if you have any defined
 Dir.glob('lib/capistrano/tasks/*.cap').each { |r| import r }
 
-after "deploy:published", "unicorn:reload"
+after "deploy:published", "unicorn:terminate"
+after "deploy:published", "unicorn:start"

lib/capistrano/tasks/unicorn.cap

@@ -0,0 +1,16 @@
+namespace :unicorn do
+  desc "Terminate unicorn processes (blocks until complete)"
+  task :terminate do
+    on roles(:app) do
+      n = 0
+      while n < 10 && !capture("pgrep -f unicorn").blank?
+        execute "pgrep -f unicorn | xargs -SIGTERM"
+        n += 1
+      end
+
+      if n == 10
+        execute "pgrep -f unicorn | xargs -SIGKILL"
+      end
+    end
+  end
+end