gitlab/unicorn.rb: Configure preload_app and pre-/post- forking actions

Unicorn is a forking server with the idea that master process preloads
heavy Ruby-on-Rails application, and then to handle new request a worker
process is forked with application already loaded in its memory (and
modification being tracked by OS via copy-on-write).

From this point of view the only reasonable value for preload_app is
always "true" and omnibus-gitlab does this:

    https://gitlab.com/gitlab-org/omnibus-gitlab/blob/8eda0933/files/gitlab-cookbooks/gitlab/definitions/unicorn_service.rb#L65

Then unicorn documentation shows what code has to be there in pre-/post-
forking event:

    http://bogomips.org/unicorn.git/tree/examples/unicorn.conf.rb?id=3312aca8#n57

GitLab uses only part of it that "allows a new master process to
incrementally phase out the old master process with SIGTTOU to avoid a
thundering herd":

    https://gitlab.com/gitlab-org/omnibus-gitlab/blob/8eda0933/files/gitlab-cookbooks/gitlab/definitions/unicorn_service.rb#L69
    http://bogomips.org/unicorn.git/tree/examples/unicorn.conf.rb?id=3312aca8#n75

but strangely does not use code parts that are "highly recommended" or
"require" for "Rails + "preload_app true"" case.

For the reference I've added such codes, but kept them being commented
out.

/cc @kazuhiko, @jerome

software/gitlab/template/unicorn.rb.in

@@ -18,25 +18,50 @@ working_directory '{{ gitlab_work.location }}'
 timeout {{ cfg('unicorn_worker_timeout') }}
 
 # Whether the app should be pre-loaded
-preload_app <%= @preload_app %>
+preload_app true
 
 # How many worker processes
 worker_processes {{ cfg('unicorn_worker_processes') }}
 
-<%- if @before_fork %>
+# about before_fork / after_fork - see:
+#   https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/files/gitlab-cookbooks/gitlab/definitions/unicorn_service.rb
+#   http://bogomips.org/unicorn.git/tree/examples/unicorn.conf.rb?id=3312aca8#n75
+
 # What to do before we fork a worker
 before_fork do |server, worker|
-  <%= @before_fork %>
+  # XXX why gitlab does not enable this?
+  # # the following is highly recomended for Rails + "preload_app true"
+  # # as there's no need for the master process to hold a connection
+  # defined?(ActiveRecord::Base) and
+  #   ActiveRecord::Base.connection.disconnect!
+
+  # This allows a new master process to incrementally
+  # phase out the old master process with SIGTTOU to avoid a
+  # thundering herd (especially in the "preload_app false" case)
+  # when doing a transparent upgrade.  The last worker spawned
+  # will then kill off the old master process with a SIGQUIT.
+  old_pid = "#{server.config[:pid]}.oldbin"
+  if old_pid != server.pid
+    begin
+      sig = (worker.nr + 1) >= server.worker_processes ? :QUIT : :TTOU
+      Process.kill(sig, File.read(old_pid).to_i)
+    rescue Errno::ENOENT, Errno::ESRCH
+    end
+  end
 end
 
-<%- end %>
-<%- if @after_fork %>
 # What to do after we fork a worker
 after_fork do |server, worker|
-  <%= @after_fork %>
+  # per-process listener ports for debugging/admin/migrations
+  # addr = "127.0.0.1:#{9293 + worker.nr}"
+  # server.listen(addr, :tries => -1, :delay => 5, :tcp_nopush => true)
+
+  # XXX why gitlab does not enable this?
+  # # the following is *required* for Rails + "preload_app true",
+  # defined?(ActiveRecord::Base) and
+  #   ActiveRecord::Base.establish_connection
 end
 
-<%- end %>
 
 # Where to drop a pidfile
 pid '{{ directory.run }}/unicorn.pid'