capistrano: clean up unicorn:terminate task.

Do a SIGQUIT (graceful shutdown, allow workers to finish current
request) before doing a SIGTERM (immediate shutdown).

lib/capistrano/tasks/unicorn.rake

@@ -1,16 +1,25 @@
+# https://bogomips.org/unicorn/SIGNALS.html
 namespace :unicorn do
   desc "Terminate unicorn processes (blocks until complete)"
   task :terminate do
     on roles(:app) do
-      execute "[[ -n $(pgrep -f unicorn) ]] && pgrep -f unicorn | xargs kill -SIGTERM"
-      sleep(5)
+      within current_path do
+        kill_unicorn("SIGQUIT")
+        sleep(10)
+        kill_unicorn("SIGTERM")
+        sleep(2)
+        kill_unicorn("SIGKILL")
+      end
     end
   end
 
-  desc "Kills unicorn processes (blocks until complete)"
-  task :kill do
-    on roles(:app) do
-      execute "[[ -n $(pgrep -f unicorn) ]] && pgrep -f unicorn | xargs kill -SIGKILL"
+  def unicorn_running?
+    test("[ -f #{fetch(:unicorn_pid)} ] && pkill --count --pidfile #{fetch(:unicorn_pid)}")
+  end
+
+  def kill_unicorn(signal)
+    if unicorn_running?
+      execute :pkill, "--signal #{signal}", "--pidfile #{fetch(:unicorn_pid)}"
     end
   end
 end