capistrano: add deploy:rolling task (fix #4123).

config/deploy.rb

@@ -12,3 +12,6 @@ set :conditionally_migrate, true
 
 # run migrations on the primary app server
 set :migration_role, :app
+
+# how long unicorn:legacy_restart (used by deploy:rolling) waits until killing the old unicorn.
+set :unicorn_restart_sleep_time, 10

lib/capistrano/tasks/app.rake

@@ -0,0 +1,32 @@
+namespace :app do
+  set :rolling_deploy, false
+
+  before "deploy:migrate", "app:disable"
+  after "deploy:published", "app:enable"
+
+  task :disable do
+    if fetch(:rolling_deploy)
+      # do nothing
+    else
+      invoke "web:disable"
+      invoke "unicorn:terminate"
+    end
+  end
+
+  task :enable do
+    if fetch(:rolling_deploy)
+      invoke "unicorn:legacy_restart"
+    else
+      invoke "unicorn:start"
+      invoke "web:enable"
+    end
+  end
+end
+
+namespace :deploy do
+  desc "Deploy a rolling update without taking the site down for maintenance"
+  task :rolling do
+    set :rolling_deploy, true
+    invoke "deploy"
+  end
+end

lib/capistrano/tasks/unicorn.rake

@@ -14,7 +14,3 @@ namespace :unicorn do
     end
   end
 end
-
-after "deploy:published", "unicorn:terminate"
-after "deploy:published", "unicorn:kill"
-after "deploy:published", "unicorn:start"

lib/capistrano/tasks/web.rake

@@ -19,6 +19,3 @@ namespace :web do
     end
   end
 end
-
-before "deploy:started", "web:disable"
-after "deploy:published", "web:enable"