Add experimental support for Pumaan-multithreading

This allows us (and others) to test drive Puma without it affecting all
users. Puma can be enabled by setting the environment variable
"EXPERIMENTAL_PUMA" to a non empty value.

23 files changed, 966 insertions, 75 deletions

diff --git a/.gitignore b/.gitignore
index 82b3d08f7a8..aecaae95b8c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -40,6 +40,7 @@ eslint-report.html
 /config/redis.queues.yml
 /config/redis.shared_state.yml
 /config/unicorn.rb
+/config/puma.rb
 /config/secrets.yml
 /config/sidekiq.yml
 /config/registry.key
diff --git a/Gemfile b/Gemfile
index c442ed9065e..d8c996cf6c4 100644
--- a/Gemfile
+++ b/Gemfile
@@ -153,6 +153,11 @@ group :unicorn do
   gem 'unicorn-worker-killer', '~> 0.4.4'
 end
 
+group :puma do
+  gem 'puma', '~> 3.12', require: false
+  gem 'puma_worker_killer', require: false
+end
+
 # State machine
 gem 'state_machines-activerecord', '~> 0.5.1'
 
diff --git a/Gemfile.lock b/Gemfile.lock
index bf16bef4f32..da74cae3f5f 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -598,6 +598,10 @@ GEM
     pry-rails (0.3.6)
       pry (>= 0.10.4)
     public_suffix (3.0.3)
+    puma (3.12.0)
+    puma_worker_killer (0.1.0)
+      get_process_mem (~> 0.2)
+      puma (>= 2.7, < 4)
     pyu-ruby-sasl (0.0.3.3)
     rack (1.6.10)
     rack-accept (0.4.5)
@@ -1076,6 +1080,8 @@ DEPENDENCIES
   prometheus-client-mmap (~> 0.9.4)
   pry-byebug (~> 3.4.1)
   pry-rails (~> 0.3.4)
+  puma (~> 3.12)
+  puma_worker_killer
   rack-attack (~> 4.4.1)
   rack-cors (~> 1.0.0)
   rack-oauth2 (~> 1.2.1)
diff --git a/Gemfile.rails5.lock b/Gemfile.rails5.lock
index 81547303ed2..a1018753510 100644
--- a/Gemfile.rails5.lock
+++ b/Gemfile.rails5.lock
@@ -602,6 +602,10 @@ GEM
     pry-rails (0.3.6)
       pry (>= 0.10.4)
     public_suffix (3.0.3)
+    puma (3.12.0)
+    puma_worker_killer (0.1.0)
+      get_process_mem (~> 0.2)
+      puma (>= 2.7, < 4)
     pyu-ruby-sasl (0.0.3.3)
     rack (2.0.5)
     rack-accept (0.4.5)
@@ -1085,6 +1089,8 @@ DEPENDENCIES
   prometheus-client-mmap (~> 0.9.4)
   pry-byebug (~> 3.4.1)
   pry-rails (~> 0.3.4)
+  puma (~> 3.12)
+  puma_worker_killer
   rack-attack (~> 4.4.1)
   rack-cors (~> 1.0.0)
   rack-oauth2 (~> 1.2.1)
diff --git a/bin/web b/bin/web
index ecd0bbd10b0..06ff7c39296 100755
--- a/bin/web
+++ b/bin/web
@@ -3,6 +3,11 @@
 cd $(dirname $0)/..
 app_root=$(pwd)
 
+# Switch to experimental PUMA configuration
+if [ -n "${EXPERIMENTAL_PUMA}" ]; then
+  exec bin/web_puma "$@"
+fi
+
 unicorn_pidfile="$app_root/tmp/pids/unicorn.pid"
 unicorn_config="$app_root/config/unicorn.rb"
 unicorn_cmd="bundle exec unicorn_rails -c $unicorn_config -E $RAILS_ENV"
diff --git a/bin/web_puma b/bin/web_puma
new file mode 100755
index 00000000000..178fe84800d
--- /dev/null
+++ b/bin/web_puma
@@ -0,0 +1,63 @@
+#!/bin/sh
+
+set -e
+
+cd $(dirname $0)/..
+app_root=$(pwd)
+
+puma_pidfile="$app_root/tmp/pids/puma.pid"
+puma_config="$app_root/config/puma.rb"
+
+spawn_puma()
+{
+  exec bundle exec puma --config "${puma_config}" "$@"
+}
+
+get_puma_pid()
+{
+  pid=$(cat "${puma_pidfile}")
+  if [ -z "$pid" ] ; then
+    echo "Could not find a PID in $puma_pidfile"
+    exit 1
+  fi
+  echo "${pid}"
+}
+
+start()
+{
+  spawn_puma -d
+}
+
+start_foreground()
+{
+  spawn_puma
+}
+
+stop()
+{
+  get_puma_pid
+  kill -QUIT "$(get_puma_pid)"
+}
+
+reload()
+{
+  kill -USR2 "$(get_puma_pid)"
+}
+
+case "$1" in
+  start)
+    start
+    ;;
+  start_foreground)
+    start_foreground
+    ;;
+  stop)
+    stop
+    ;;
+  reload)
+    reload
+    ;;
+  *)
+    echo "Usage: RAILS_ENV=your_env $0 {start|stop|reload}"
+    ;;
+esac
diff --git a/changelogs/unreleased/an-multithreading.yml b/changelogs/unreleased/an-multithreading.yml
new file mode 100644
index 00000000000..fca847e6ea4
--- /dev/null
+++ b/changelogs/unreleased/an-multithreading.yml
@@ -0,0 +1,5 @@
+---
+title: Experimental support for running Puma multithreaded web-server
+merge_request: 22372
+author:
+type: performance
diff --git a/config/initializers/7_prometheus_metrics.rb b/config/initializers/7_prometheus_metrics.rb
index 146c4b1e024..8052880cc3d 100644
--- a/config/initializers/7_prometheus_metrics.rb
+++ b/config/initializers/7_prometheus_metrics.rb
@@ -26,9 +26,25 @@ Sidekiq.configure_server do |config|
 end
 
 if !Rails.env.test? && Gitlab::Metrics.prometheus_metrics_enabled?
-  unless Sidekiq.server?
-    Gitlab::Metrics::Samplers::UnicornSampler.initialize_instance(Settings.monitoring.unicorn_sampler_interval).start
+  Gitlab::Cluster::LifecycleEvents.on_worker_start do
+    defined?(::Prometheus::Client.reinitialize_on_pid_change) && Prometheus::Client.reinitialize_on_pid_change
+
+    unless Sidekiq.server?
+      Gitlab::Metrics::Samplers::UnicornSampler.initialize_instance(Settings.monitoring.unicorn_sampler_interval).start
+    end
+
+    Gitlab::Metrics::Samplers::RubySampler.initialize_instance(Settings.monitoring.ruby_sampler_interval).start
   end
+end
 
-  Gitlab::Metrics::Samplers::RubySampler.initialize_instance(Settings.monitoring.ruby_sampler_interval).start
+Gitlab::Cluster::LifecycleEvents.on_master_restart do
+  # The following is necessary to ensure stale Prometheus metrics don't
+  # accumulate over time. It needs to be done in this hook as opposed to
+  # inside an init script to ensure metrics files aren't deleted after new
+  # unicorn workers start after a SIGUSR2 is received.
+  prometheus_multiproc_dir = ENV['prometheus_multiproc_dir']
+  if prometheus_multiproc_dir
+    old_metrics = Dir[File.join(prometheus_multiproc_dir, '*.db')]
+    FileUtils.rm_rf(old_metrics)
+  end
 end
diff --git a/config/initializers/8_metrics.rb b/config/initializers/8_metrics.rb
index eccf82ab8dc..c8d261d415e 100644
--- a/config/initializers/8_metrics.rb
+++ b/config/initializers/8_metrics.rb
@@ -158,7 +158,9 @@ if Gitlab::Metrics.enabled? && !Rails.env.test?
 
   GC::Profiler.enable
 
-  Gitlab::Metrics::Samplers::InfluxSampler.initialize_instance.start
+  Gitlab::Cluster::LifecycleEvents.on_worker_start do
+    Gitlab::Metrics::Samplers::InfluxSampler.initialize_instance.start
+  end
 
   module TrackNewRedisConnections
     def connect(*args)
diff --git a/config/initializers/active_record_lifecycle.rb b/config/initializers/active_record_lifecycle.rb
new file mode 100644
index 00000000000..7fa37121efc
--- /dev/null
+++ b/config/initializers/active_record_lifecycle.rb
@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+# Don't handle sidekiq configuration as it
+# has its own special active record configuration here
+if defined?(ActiveRecord::Base) && !Sidekiq.server?
+  Gitlab::Cluster::LifecycleEvents.on_worker_start do
+    ActiveSupport.on_load(:active_record) do
+      ActiveRecord::Base.establish_connection
+
+      Rails.logger.debug("ActiveRecord connection established")
+    end
+  end
+end
+
+if defined?(ActiveRecord::Base)
+  Gitlab::Cluster::LifecycleEvents.on_before_fork do
+    # the following is highly recommended for Rails + "preload_app true"
+    # as there's no need for the master process to hold a connection
+    ActiveRecord::Base.connection.disconnect!
+
+    Rails.logger.debug("ActiveRecord connection disconnected")
+  end
+end
diff --git a/config/initializers/macos.rb b/config/initializers/macos.rb
new file mode 100644
index 00000000000..f410af6ed47
--- /dev/null
+++ b/config/initializers/macos.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+if /darwin/ =~ RUBY_PLATFORM
+  Gitlab::Cluster::LifecycleEvents.on_before_fork do
+    require 'fiddle'
+
+    # Dynamically load Foundation.framework, ~implicitly~ initialising
+    # the Objective-C runtime before any forking happens in Unicorn
+    #
+    # From https://bugs.ruby-lang.org/issues/14009
+    Fiddle.dlopen '/System/Library/Frameworks/Foundation.framework/Foundation'
+  end
+end
diff --git a/config/initializers/rbtrace.rb b/config/initializers/rbtrace.rb
new file mode 100644
index 00000000000..6a1b71bf4bd
--- /dev/null
+++ b/config/initializers/rbtrace.rb
@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+if ENV['ENABLE_RBTRACE']
+  Gitlab::Cluster::LifecycleEvents.on_worker_start do
+    # Unicorn clears out signals before it forks, so rbtrace won't work
+    # unless it is enabled after the fork.
+    require 'rbtrace'
+  end
+end
diff --git a/config/initializers/sidekiq.rb b/config/initializers/sidekiq.rb
index bc6b7aed6aa..565efc858d1 100644
--- a/config/initializers/sidekiq.rb
+++ b/config/initializers/sidekiq.rb
@@ -14,8 +14,6 @@ Sidekiq.default_worker_options = { retry: 3 }
 enable_json_logs = Gitlab.config.sidekiq.log_format == 'json'
 
 Sidekiq.configure_server do |config|
-  require 'rbtrace' if ENV['ENABLE_RBTRACE']
-
   config.redis = queues_config_hash
 
   config.server_middleware do |chain|
diff --git a/config/puma.example.development.rb b/config/puma.example.development.rb
new file mode 100644
index 00000000000..490c940077a
--- /dev/null
+++ b/config/puma.example.development.rb
@@ -0,0 +1,77 @@
+# frozen_string_literal: true
+
+# -----------------------------------------------------------------------
+# This file is used by the GDK to generate a default config/puma.rb file
+# Note that `/home/git` will be substituted for the actual GDK root
+# directory when this file is generated
+# -----------------------------------------------------------------------
+
+# Load "path" as a rackup file.
+#
+# The default is "config.ru".
+#
+rackup 'config.ru'
+pidfile '/home/git/gitlab/tmp/pids/puma.pid'
+state_path '/home/git/gitlab/tmp/pids/puma.state'
+
+stdout_redirect '/home/git/gitlab/log/puma.stdout.log',
+  '/home/git/gitlab/log/puma.stderr.log',
+  true
+
+# Configure "min" to be the minimum number of threads to use to answer
+# requests and "max" the maximum.
+#
+# The default is "0, 16".
+#
+threads 1, 4
+
+# By default, workers accept all requests and queue them to pass to handlers.
+# When false, workers accept the number of simultaneous requests configured.
+#
+# Queueing requests generally improves performance, but can cause deadlocks if
+# the app is waiting on a request to itself. See https://github.com/puma/puma/issues/612
+#
+# When set to false this may require a reverse proxy to handle slow clients and
+# queue requests before they reach puma. This is due to disabling HTTP keepalive
+queue_requests false
+
+# Bind the server to "url". "tcp://", "unix://" and "ssl://" are the only
+# accepted protocols.
+bind 'unix:///home/git/gitlab.socket'
+
+workers 2
+
+require_relative "/home/git/gitlab/lib/gitlab/cluster/lifecycle_events"
+require_relative "/home/git/gitlab/lib/gitlab/cluster/puma_worker_killer_initializer"
+
+on_restart do
+  # Signal application hooks that we're about to restart
+  Gitlab::Cluster::LifecycleEvents.do_master_restart
+end
+
+before_fork do
+  # Signal to the puma killer
+  Gitlab::Cluster::PumaWorkerKillerInitializer.start @config.options unless ENV['DISABLE_PUMA_WORKER_KILLER']
+
+  # Signal application hooks that we're about to fork
+  Gitlab::Cluster::LifecycleEvents.do_before_fork
+end
+
+Gitlab::Cluster::LifecycleEvents.set_puma_options @config.options
+on_worker_boot do
+  # Signal application hooks of worker start
+  Gitlab::Cluster::LifecycleEvents.do_worker_start
+end
+
+# Preload the application before starting the workers; this conflicts with
+# phased restart feature. (off by default)
+
+preload_app!
+
+tag 'gitlab-puma-worker'
+
+# Verifies that all workers have checked in to the master process within
+# the given timeout. If not the worker process will be restarted. Default
+# value is 60 seconds.
+#
+worker_timeout 60
diff --git a/config/unicorn.rb.example b/config/unicorn.rb.example
index e06cce3e97a..4637eb8bc6e 100644
--- a/config/unicorn.rb.example
+++ b/config/unicorn.rb.example
@@ -81,22 +81,16 @@ preload_app true
 # fast LAN.
 check_client_connection false
 
+require_relative "/home/git/gitlab/lib/gitlab/cluster/lifecycle_events"
+
 before_exec do |server|
-  # The following is necessary to ensure stale Prometheus metrics don't
-  # accumulate over time. It needs to be done in this hook as opposed to
-  # inside an init script to ensure metrics files aren't deleted after new
-  # unicorn workers start after a SIGUSR2 is received.
-  if ENV['prometheus_multiproc_dir']
-    old_metrics = Dir[File.join(ENV['prometheus_multiproc_dir'], '*.db')]
-    FileUtils.rm_rf(old_metrics)
-  end
+  # Signal application hooks that we're about to restart
+  Gitlab::Cluster::LifecycleEvents.do_master_restart
 end
 
 before_fork do |server, worker|
-  # the following is highly recommended for Rails + "preload_app true"
-  # as there's no need for the master process to hold a connection
-  defined?(ActiveRecord::Base) &&
-    ActiveRecord::Base.connection.disconnect!
+  # Signal application hooks that we're about to fork
+  Gitlab::Cluster::LifecycleEvents.do_before_fork
 
   # The following is only recommended for memory/DB-constrained
   # installations.  It is not needed if your system can house
@@ -124,25 +118,10 @@ before_fork do |server, worker|
 end
 
 after_fork do |server, worker|
-  # Unicorn clears out signals before it forks, so rbtrace won't work
-  # unless it is enabled after the fork.
-  require 'rbtrace' if ENV['ENABLE_RBTRACE']
+  # Signal application hooks of worker start
+  Gitlab::Cluster::LifecycleEvents.do_worker_start
 
   # 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)
-
-  # the following is *required* for Rails + "preload_app true",
-  defined?(ActiveRecord::Base) &&
-    ActiveRecord::Base.establish_connection
-
-  # reset prometheus client, this will cause any opened metrics files to be closed
-  defined?(::Prometheus::Client.reinitialize_on_pid_change) &&
-    Prometheus::Client.reinitialize_on_pid_change
-
-  # if preload_app is true, then you may also want to check and
-  # restart any other shared sockets/descriptors such as Memcached,
-  # and Redis.  TokyoCabinet file handles are safe to reuse
-  # between any number of forked children (assuming your kernel
-  # correctly implements pread()/pwrite() system calls)
 end
diff --git a/config/unicorn.rb.example.development b/config/unicorn.rb.example.development
index f31df66015a..f7541bb9d55 100644
--- a/config/unicorn.rb.example.development
+++ b/config/unicorn.rb.example.development
@@ -1,32 +1,61 @@
+# frozen_string_literal: true
+
+# -------------------------------------------------------------------------
+# This file is used by the GDK to generate a default config/unicorn.rb file
+# Note that `/home/git` will be substituted for the actual GDK root
+# directory when this file is generated
+# -------------------------------------------------------------------------
+
 worker_processes 2
 timeout 60
 
+listen '/home/git/gitlab.socket'
+
 preload_app true
 check_client_connection false
 
+require_relative "/home/git/gitlab/lib/gitlab/cluster/lifecycle_events"
+
+before_exec do |server|
+  # Signal application hooks that we're about to restart
+  Gitlab::Cluster::LifecycleEvents.do_master_restart
+end
+
 before_fork do |server, worker|
-  # the following is highly recommended for Rails + "preload_app true"
-  # as there's no need for the master process to hold a connection
-  defined?(ActiveRecord::Base) &&
-    ActiveRecord::Base.connection.disconnect!
-
-  if /darwin/ =~ RUBY_PLATFORM
-    require 'fiddle'
-
-    # Dynamically load Foundation.framework, ~implicitly~ initialising
-    # the Objective-C runtime before any forking happens in Unicorn
-    #
-    # From https://bugs.ruby-lang.org/issues/14009
-    Fiddle.dlopen '/System/Library/Frameworks/Foundation.framework/Foundation'
+  # Signal application hooks that we're about to fork
+  Gitlab::Cluster::LifecycleEvents.do_before_fork
+
+  # The following is only recommended for memory/DB-constrained
+  # installations.  It is not needed if your system can house
+  # twice as many worker_processes as you have configured.
+  #
+  # 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
+  #
+  # Throttle the master from forking too quickly by sleeping.  Due
+  # to the implementation of standard Unix signal handlers, this
+  # helps (but does not completely) prevent identical, repeated signals
+  # from being lost when the receiving process is busy.
+  # sleep 1
 end
 
 after_fork do |server, worker|
-  # Unicorn clears out signals before it forks, so rbtrace won't work
-  # unless it is enabled after the fork.
-  require 'rbtrace' if ENV['ENABLE_RBTRACE']
+  # Signal application hooks of worker start
+  Gitlab::Cluster::LifecycleEvents.do_worker_start
 
-  # the following is *required* for Rails + "preload_app true",
-  defined?(ActiveRecord::Base) &&
-    ActiveRecord::Base.establish_connection
+  # 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)
 end
+
diff --git a/doc/update/11.4-to-11.5.md b/doc/update/11.4-to-11.5.md
new file mode 100644
index 00000000000..e64ab2acae2
--- /dev/null
+++ b/doc/update/11.4-to-11.5.md
@@ -0,0 +1,404 @@
+---
+comments: false
+---
+
+# From 11.4 to 11.5
+
+Make sure you view this update guide from the branch (version) of GitLab you would
+like to install (e.g., `11-5-stable`. You can select the branch in the version
+dropdown at the top left corner of GitLab (below the menu bar).
+
+If the highest number stable branch is unclear please check the
+[GitLab Blog](https://about.gitlab.com/blog/archives.html) for installation
+guide links by version.
+
+### 1. Stop server
+
+```bash
+sudo service gitlab stop
+```
+
+### 2. Backup
+
+NOTE: If you installed GitLab from source, make sure `rsync` is installed.
+
+```bash
+cd /home/git/gitlab
+
+sudo -u git -H bundle exec rake gitlab:backup:create RAILS_ENV=production
+```
+
+### 3. Update Ruby
+
+NOTE: GitLab 11.0 and higher only support Ruby 2.4.x and dropped support for Ruby 2.3.x. Be
+sure to upgrade your interpreter if necessary.
+
+You can check which version you are running with `ruby -v`.
+
+Download Ruby and compile it:
+
+```bash
+mkdir /tmp/ruby && cd /tmp/ruby
+curl --remote-name --progress https://cache.ruby-lang.org/pub/ruby/2.4/ruby-2.4.5.tar.gz
+echo '4d650f302f1ec00256450b112bb023644b6ab6dd  ruby-2.4.5.tar.gz' | shasum -c - && tar xzf ruby-2.4.5.tar.gz
+cd ruby-2.4.5
+
+./configure --disable-install-rdoc
+make
+sudo make install
+```
+
+Install Bundler:
+
+```bash
+sudo gem install bundler --no-ri --no-rdoc
+```
+
+### 4. Update Node
+
+GitLab utilizes [webpack](http://webpack.js.org) to compile frontend assets.
+This requires a minimum version of node v6.0.0.
+
+You can check which version you are running with `node -v`. If you are running
+a version older than `v6.0.0` you will need to update to a newer version. You
+can find instructions to install from community maintained packages or compile
+from source at the nodejs.org website.
+
+<https://nodejs.org/en/download/>
+
+GitLab also requires the use of yarn `>= v1.2.0` to manage JavaScript
+dependencies.
+
+```bash
+curl --silent --show-error https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -
+echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
+sudo apt-get update
+sudo apt-get install yarn
+```
+
+More information can be found on the [yarn website](https://yarnpkg.com/en/docs/install).
+
+### 5. Update Go
+
+NOTE: GitLab 11.4 and higher only supports Go 1.10.x and newer, and dropped support for Go
+1.9.x. Be sure to upgrade your installation if necessary.
+
+You can check which version you are running with `go version`.
+
+Download and install Go:
+
+```bash
+# Remove former Go installation folder
+sudo rm -rf /usr/local/go
+
+curl --remote-name --progress https://dl.google.com/go/go1.10.3.linux-amd64.tar.gz
+echo 'fa1b0e45d3b647c252f51f5e1204aba049cde4af177ef9f2181f43004f901035  go1.10.3.linux-amd64.tar.gz' | shasum -a256 -c - && \
+  sudo tar -C /usr/local -xzf go1.10.3.linux-amd64.tar.gz
+sudo ln -sf /usr/local/go/bin/{go,godoc,gofmt} /usr/local/bin/
+rm go1.10.3.linux-amd64.tar.gz
+```
+
+### 6. Get latest code
+
+```bash
+cd /home/git/gitlab
+
+sudo -u git -H git fetch --all --prune
+sudo -u git -H git checkout -- db/schema.rb # local changes will be restored automatically
+sudo -u git -H git checkout -- locale
+```
+
+For GitLab Community Edition:
+
+```bash
+cd /home/git/gitlab
+
+sudo -u git -H git checkout 11-5-stable
+```
+
+OR
+
+For GitLab Enterprise Edition:
+
+```bash
+cd /home/git/gitlab
+
+sudo -u git -H git checkout 11-5-stable-ee
+```
+
+### 7. Update gitlab-shell
+
+```bash
+cd /home/git/gitlab-shell
+
+sudo -u git -H git fetch --all --tags --prune
+sudo -u git -H git checkout v$(</home/git/gitlab/GITLAB_SHELL_VERSION)
+sudo -u git -H bin/compile
+```
+
+### 8. Update gitlab-workhorse
+
+Install and compile gitlab-workhorse. GitLab-Workhorse uses
+[GNU Make](https://www.gnu.org/software/make/).
+If you are not using Linux you may have to run `gmake` instead of
+`make` below.
+
+```bash
+cd /home/git/gitlab-workhorse
+
+sudo -u git -H git fetch --all --tags --prune
+sudo -u git -H git checkout v$(</home/git/gitlab/GITLAB_WORKHORSE_VERSION)
+sudo -u git -H make
+```
+
+### 9. Update Gitaly
+
+#### New Gitaly configuration options required
+
+In order to function Gitaly needs some additional configuration information. Below we assume you installed Gitaly in `/home/git/gitaly` and GitLab Shell in `/home/git/gitlab-shell`.
+
+```shell
+echo '
+[gitaly-ruby]
+dir = "/home/git/gitaly/ruby"
+
+[gitlab-shell]
+dir = "/home/git/gitlab-shell"
+' | sudo -u git tee -a /home/git/gitaly/config.toml
+```
+
+#### Check Gitaly configuration
+
+Due to a bug in the `rake gitlab:gitaly:install` script your Gitaly
+configuration file may contain syntax errors. The block name
+`[[storages]]`, which may occur more than once in your `config.toml`
+file, should be `[[storage]]` instead.
+
+```shell
+sudo -u git -H sed -i.pre-10.1 's/\[\[storages\]\]/[[storage]]/' /home/git/gitaly/config.toml
+```
+
+#### Compile Gitaly
+
+```shell
+cd /home/git/gitaly
+sudo -u git -H git fetch --all --tags --prune
+sudo -u git -H git checkout v$(</home/git/gitlab/GITALY_SERVER_VERSION)
+sudo -u git -H make
+```
+
+### 10. Update gitlab-pages
+
+#### Only needed if you use GitLab Pages.
+
+Install and compile gitlab-pages. GitLab-Pages uses
+[GNU Make](https://www.gnu.org/software/make/).
+If you are not using Linux you may have to run `gmake` instead of
+`make` below.
+
+```bash
+cd /home/git/gitlab-pages
+
+sudo -u git -H git fetch --all --tags --prune
+sudo -u git -H git checkout v$(</home/git/gitlab/GITLAB_PAGES_VERSION)
+sudo -u git -H make
+```
+
+### 11. Update MySQL permissions
+
+If you are using MySQL you need to grant the GitLab user the necessary
+permissions on the database:
+
+```bash
+mysql -u root -p -e "GRANT TRIGGER ON \`gitlabhq_production\`.* TO 'git'@'localhost';"
+```
+
+If you use MySQL with replication, or just have MySQL configured with binary logging,
+you will need to also run the following on all of your MySQL servers:
+
+```bash
+mysql -u root -p -e "SET GLOBAL log_bin_trust_function_creators = 1;"
+```
+
+You can make this setting permanent by adding it to your `my.cnf`:
+
+```
+log_bin_trust_function_creators=1
+```
+
+### 12. Update configuration files
+
+#### New `unicorn.rb` configuration
+
+Note: we have made [changes](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22372) to `unicorn.rb` to allow GitLab run with both Unicorn and Puma in future.
+
+- Make `/home/git/gitlab/config/unicorn.rb` the same as https://gitlab.com/gitlab-org/gitlab-ce/blob/11-5-stable/config/unicorn.rb.example but with your settings.
+    - In particular, make sure that `require_relative "/home/git/gitlab/lib/gitlab/cluster/lifecycle_events"` line exists and the `before_exec`, `before_fork`, and `after_fork` handlers are configured as shown below:
+
+```ruby
+require_relative "/home/git/gitlab/lib/gitlab/cluster/lifecycle_events"
+
+before_exec do |server|
+  # Signal application hooks that we're about to restart
+  Gitlab::Cluster::LifecycleEvents.do_master_restart
+end
+
+before_fork do |server, worker|
+  # Signal application hooks that we're about to fork
+  Gitlab::Cluster::LifecycleEvents.do_before_fork
+end
+
+after_fork do |server, worker|
+  # Signal application hooks of worker start
+  Gitlab::Cluster::LifecycleEvents.do_worker_start
+end
+```
+
+#### New configuration options for `gitlab.yml`
+
+There might be configuration options available for [`gitlab.yml`][yaml]. View them with the command below and apply them manually to your current `gitlab.yml`:
+
+```sh
+cd /home/git/gitlab
+
+git diff origin/11-4-stable:config/gitlab.yml.example origin/11-5-stable:config/gitlab.yml.example
+```
+
+#### Nginx configuration
+
+Ensure you're still up-to-date with the latest NGINX configuration changes:
+
+```sh
+cd /home/git/gitlab
+
+# For HTTPS configurations
+git diff origin/11-1-stable:lib/support/nginx/gitlab-ssl origin/11-5-stable:lib/support/nginx/gitlab-ssl
+
+# For HTTP configurations
+git diff origin/11-1-stable:lib/support/nginx/gitlab origin/11-5-stable:lib/support/nginx/gitlab
+```
+
+If you are using Strict-Transport-Security in your installation to continue using it you must enable it in your Nginx
+configuration as GitLab application no longer handles setting it.
+
+If you are using Apache instead of NGINX please see the updated [Apache templates].
+Also note that because Apache does not support upstreams behind Unix sockets you
+will need to let gitlab-workhorse listen on a TCP port. You can do this
+via [/etc/default/gitlab].
+
+[Apache templates]: https://gitlab.com/gitlab-org/gitlab-recipes/tree/master/web-server/apache
+[/etc/default/gitlab]: https://gitlab.com/gitlab-org/gitlab-ce/blob/11-5-stable/lib/support/init.d/gitlab.default.example#L38
+
+#### SMTP configuration
+
+If you're installing from source and use SMTP to deliver mail, you will need to add the following line
+to config/initializers/smtp_settings.rb:
+
+```ruby
+ActionMailer::Base.delivery_method = :smtp
+```
+
+See [smtp_settings.rb.sample] as an example.
+
+[smtp_settings.rb.sample]: https://gitlab.com/gitlab-org/gitlab-ce/blob/11-5-stable/config/initializers/smtp_settings.rb.sample#L13
+
+#### Init script
+
+There might be new configuration options available for [`gitlab.default.example`][gl-example]. View them with the command below and apply them manually to your current `/etc/default/gitlab`:
+
+```sh
+cd /home/git/gitlab
+
+git diff origin/11-1-stable:lib/support/init.d/gitlab.default.example origin/11-5-stable:lib/support/init.d/gitlab.default.example
+```
+
+Ensure you're still up-to-date with the latest init script changes:
+
+```bash
+cd /home/git/gitlab
+
+sudo cp lib/support/init.d/gitlab /etc/init.d/gitlab
+```
+
+For Ubuntu 16.04.1 LTS:
+
+```bash
+sudo systemctl daemon-reload
+```
+
+### 13. Install libs, migrations, etc.
+
+```bash
+cd /home/git/gitlab
+
+# MySQL installations (note: the line below states '--without postgres')
+sudo -u git -H bundle install --without postgres development test --deployment
+
+# PostgreSQL installations (note: the line below states '--without mysql')
+sudo -u git -H bundle install --without mysql development test --deployment
+
+# Optional: clean up old gems
+sudo -u git -H bundle clean
+
+# Run database migrations
+sudo -u git -H bundle exec rake db:migrate RAILS_ENV=production
+
+# Compile GetText PO files
+
+sudo -u git -H bundle exec rake gettext:compile RAILS_ENV=production
+
+# Update node dependencies and recompile assets
+sudo -u git -H bundle exec rake yarn:install gitlab:assets:clean gitlab:assets:compile RAILS_ENV=production NODE_ENV=production
+
+# Clean up cache
+sudo -u git -H bundle exec rake cache:clear RAILS_ENV=production
+```
+
+**MySQL installations**: Run through the `MySQL strings limits` and `Tables and data conversion to utf8mb4` [tasks](../install/database_mysql.md).
+
+### 14. Start application
+
+```bash
+sudo service gitlab start
+sudo service nginx restart
+```
+
+### 15. Check application status
+
+Check if GitLab and its environment are configured correctly:
+
+```bash
+cd /home/git/gitlab
+
+sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production
+```
+
+To make sure you didn't miss anything run a more thorough check:
+
+```bash
+cd /home/git/gitlab
+
+sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production
+```
+
+If all items are green, then congratulations, the upgrade is complete!
+
+## Things went south? Revert to previous version (11.4)
+
+### 1. Revert the code to the previous version
+
+Follow the [upgrade guide from 11.3 to 11.4](11.3-to-11.4.md), except for the
+database migration (the backup is already migrated to the previous version).
+
+### 2. Restore from the backup
+
+```bash
+cd /home/git/gitlab
+
+sudo -u git -H bundle exec rake gitlab:backup:restore RAILS_ENV=production
+```
+
+If you have more than one backup `*.tar` file(s) please add `BACKUP=timestamp_of_backup` to the command above.
+
+[yaml]: https://gitlab.com/gitlab-org/gitlab-ce/blob/11-5-stable/config/gitlab.yml.example
+[gl-example]: https://gitlab.com/gitlab-org/gitlab-ce/blob/11-5-stable/lib/support/init.d/gitlab.default.example
diff --git a/lib/gitlab/cluster/lifecycle_events.rb b/lib/gitlab/cluster/lifecycle_events.rb
new file mode 100644
index 00000000000..b05dca409d1
--- /dev/null
+++ b/lib/gitlab/cluster/lifecycle_events.rb
@@ -0,0 +1,99 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Cluster
+    #
+    # LifecycleEvents lets Rails initializers register application startup hooks
+    # that are sensitive to forking. For example, to defer the creation of
+    # watchdog threads. This lets us abstract away the Unix process
+    # lifecycles of Unicorn, Sidekiq, Puma, Puma Cluster, etc.
+    #
+    # We have three lifecycle events.
+    #
+    # - before_fork (only in forking processes)
+    # - worker_start
+    # - before_master_restart (only in forking processes)
+    #
+    # Blocks will be executed in the order in which they are registered.
+    #
+    class LifecycleEvents
+      class << self
+        #
+        # Hook registration methods (called from initializers)
+        #
+        def on_worker_start(&block)
+          if in_clustered_environment?
+            # Defer block execution
+            (@worker_start_hooks ||= []) << block
+          else
+            yield
+          end
+        end
+
+        def on_before_fork(&block)
+          return unless in_clustered_environment?
+
+          # Defer block execution
+          (@before_fork_hooks ||= []) << block
+        end
+
+        def on_master_restart(&block)
+          return unless in_clustered_environment?
+
+          # Defer block execution
+          (@master_restart_hooks ||= []) << block
+        end
+
+        #
+        # Lifecycle integration methods (called from unicorn.rb, puma.rb, etc.)
+        #
+        def do_worker_start
+          @worker_start_hooks&.each do |block|
+            block.call
+          end
+        end
+
+        def do_before_fork
+          @before_fork_hooks&.each do |block|
+            block.call
+          end
+        end
+
+        def do_master_restart
+          @master_restart_hooks && @master_restart_hooks.each do |block|
+            block.call
+          end
+        end
+
+        # Puma doesn't use singletons (which is good) but
+        # this means we need to pass through whether the
+        # puma server is running in single mode or cluster mode
+        def set_puma_options(options)
+          @puma_options = options
+        end
+
+        private
+
+        def in_clustered_environment?
+          # Sidekiq doesn't fork
+          return false if Sidekiq.server?
+
+          # Unicorn always forks
+          return true if defined?(::Unicorn)
+
+          # Puma sometimes forks
+          return true if in_clustered_puma?
+
+          # Default assumption is that we don't fork
+          false
+        end
+
+        def in_clustered_puma?
+          return false unless defined?(::Puma)
+
+          @puma_options && @puma_options[:workers] && @puma_options[:workers] > 0
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/cluster/puma_worker_killer_initializer.rb b/lib/gitlab/cluster/puma_worker_killer_initializer.rb
new file mode 100644
index 00000000000..331c39f7d6b
--- /dev/null
+++ b/lib/gitlab/cluster/puma_worker_killer_initializer.rb
@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Cluster
+    class PumaWorkerKillerInitializer
+      def self.start(puma_options, puma_per_worker_max_memory_mb: 650)
+        require 'puma_worker_killer'
+
+        PumaWorkerKiller.config do |config|
+          # Note! ram is expressed in megabytes (whereas GITLAB_UNICORN_MEMORY_MAX is in bytes)
+          # Importantly RAM is for _all_workers (ie, the cluster),
+          # not each worker as is the case with GITLAB_UNICORN_MEMORY_MAX
+          worker_count = puma_options[:workers] || 1
+          config.ram = worker_count * puma_per_worker_max_memory_mb
+
+          config.frequency = 20 # seconds
+
+          # We just want to limit to a fixed maximum, unrelated to the total amount
+          # of available RAM.
+          config.percent_usage = 0.98
+
+          # Ideally we'll never hit the maximum amount of memory. If so the worker
+          # is restarted already, thus periodically restarting workers shouldn't be
+          # needed.
+          config.rolling_restart_frequency = false
+        end
+
+        PumaWorkerKiller.start
+      end
+    end
+  end
+end
diff --git a/spec/rack_servers/configs/config.ru b/spec/rack_servers/configs/config.ru
new file mode 100644
index 00000000000..63daeb9eec5
--- /dev/null
+++ b/spec/rack_servers/configs/config.ru
@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+app = proc do |env|
+  if env['REQUEST_METHOD'] == 'GET'
+    [200, {}, ["#{Process.pid}"]]
+  else
+    Process.kill(env['QUERY_STRING'], Process.pid)
+    [200, {}, ['Bye!']]
+  end
+end
+
+run app
diff --git a/spec/rack_servers/configs/puma.rb b/spec/rack_servers/configs/puma.rb
new file mode 100644
index 00000000000..d6b6d83d648
--- /dev/null
+++ b/spec/rack_servers/configs/puma.rb
@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+# Note: this file is used for testing puma in `spec/rack_servers/puma_spec.rb` only
+# Note: as per the convention in `config/puma.example.development.rb`,
+# this file will replace `/home/git` with the actual working directory
+
+directory '/home/git'
+threads 1, 10
+queue_requests false
+pidfile '/home/git/gitlab/tmp/pids/puma.pid'
+bind 'unix:///home/git/gitlab/tmp/tests/puma.socket'
+workers 1
+preload_app!
+worker_timeout 60
+
+require_relative "/home/git/gitlab/lib/gitlab/cluster/lifecycle_events"
+require_relative "/home/git/gitlab/lib/gitlab/cluster/puma_worker_killer_initializer"
+
+before_fork do
+  Gitlab::Cluster::PumaWorkerKillerInitializer.start @config.options
+  Gitlab::Cluster::LifecycleEvents.do_before_fork
+end
+
+Gitlab::Cluster::LifecycleEvents.set_puma_options @config.options
+on_worker_boot do
+  Gitlab::Cluster::LifecycleEvents.do_worker_start
+  File.write('/home/git/gitlab/tmp/tests/puma-worker-ready', Process.pid)
+end
+
+on_restart do
+  Gitlab::Cluster::LifecycleEvents.do_master_restart
+end
diff --git a/spec/rack_servers/puma_spec.rb b/spec/rack_servers/puma_spec.rb
new file mode 100644
index 00000000000..431fab87857
--- /dev/null
+++ b/spec/rack_servers/puma_spec.rb
@@ -0,0 +1,84 @@
+# frozen_string_literal: true
+
+require 'fileutils'
+
+require 'excon'
+
+require 'spec_helper'
+
+describe 'Puma' do
+  before(:all) do
+    project_root = File.expand_path('../..', __dir__)
+
+    config_lines = File.read('spec/rack_servers/configs/puma.rb')
+      .gsub('/home/git/gitlab', project_root)
+      .gsub('/home/git', project_root)
+
+    config_path = File.join(project_root, "tmp/tests/puma.rb")
+    @socket_path = File.join(project_root, 'tmp/tests/puma.socket')
+
+    File.write(config_path, config_lines)
+
+    cmd = %W[puma -e test -C #{config_path} #{File.join(__dir__, 'configs/config.ru')}]
+    @puma_master_pid = spawn(*cmd)
+    wait_puma_boot!(@puma_master_pid, File.join(project_root, 'tmp/tests/puma-worker-ready'))
+    WebMock.allow_net_connect!
+  end
+
+  %w[SIGQUIT SIGTERM SIGKILL].each do |signal|
+    it "has a worker that self-terminates on signal #{signal}" do
+      response = Excon.get('unix://', socket: @socket_path)
+      expect(response.status).to eq(200)
+
+      worker_pid = response.body.to_i
+      expect(worker_pid).to be > 0
+
+      begin
+        Excon.post("unix://?#{signal}", socket: @socket_path)
+      rescue Excon::Error::Socket
+        # The connection may be closed abruptly
+      end
+
+      expect(pid_gone?(worker_pid)).to eq(true)
+    end
+  end
+
+  after(:all) do
+    begin
+      WebMock.disable_net_connect!(allow_localhost: true)
+      Process.kill('TERM', @puma_master_pid)
+    rescue Errno::ESRCH
+    end
+  end
+
+  def wait_puma_boot!(master_pid, ready_file)
+    # We have seen the boot timeout after 2 minutes in CI so let's set it to 5 minutes.
+    timeout = 5 * 60
+    timeout.times do
+      return if File.exist?(ready_file)
+
+      pid = Process.waitpid(master_pid, Process::WNOHANG)
+      raise "puma failed to boot: #{$?}" unless pid.nil?
+
+      sleep 1
+    end
+
+    raise "puma boot timed out after #{timeout} seconds"
+  end
+
+  def pid_gone?(pid)
+    # Worker termination should take less than a second. That makes 10
+    # seconds a generous timeout.
+    10.times do
+      begin
+        Process.kill(0, pid)
+      rescue Errno::ESRCH
+        return true
+      end
+
+      sleep 1
+    end
+
+    false
+  end
+end
diff --git a/spec/unicorn/unicorn_spec.rb b/spec/rack_servers/unicorn_spec.rb
index a4cf479a339..6a02ebcd048 100644
--- a/spec/unicorn/unicorn_spec.rb
+++ b/spec/rack_servers/unicorn_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'fileutils'
 
 require 'excon'
@@ -6,12 +8,16 @@ require 'spec_helper'
 
 describe 'Unicorn' do
   before(:all) do
-    config_lines = File.read('config/unicorn.rb.example').split("\n")
+    project_root = File.expand_path('../..', __dir__)
+
+    config_lines = File.read('config/unicorn.rb.example')
+      .gsub('/home/git/gitlab', project_root)
+      .gsub('/home/git', project_root)
+      .split("\n")
 
     # Remove these because they make setup harder.
     config_lines = config_lines.reject do |line|
       %w[
-        working_directory
         worker_processes
         listen
         pid
@@ -26,33 +32,18 @@ describe 'Unicorn' do
     # predictable which process will handle our requests.
     config_lines << 'worker_processes 1'
 
-    @socket_path = File.join(Dir.pwd, 'tmp/tests/unicorn.socket')
+    @socket_path = File.join(project_root, 'tmp/tests/unicorn.socket')
     config_lines << "listen '#{@socket_path}'"
 
-    ready_file = 'tmp/tests/unicorn-worker-ready'
+    ready_file = File.join(project_root, 'tmp/tests/unicorn-worker-ready')
     FileUtils.rm_f(ready_file)
     after_fork_index = config_lines.index { |l| l.start_with?('after_fork') }
     config_lines.insert(after_fork_index + 1, "File.write('#{ready_file}', Process.pid)")
 
-    config_path = 'tmp/tests/unicorn.rb'
+    config_path = File.join(project_root, 'tmp/tests/unicorn.rb')
     File.write(config_path, config_lines.join("\n") + "\n")
 
-    rackup_path = 'tmp/tests/config.ru'
-    File.write(rackup_path, <<~EOS)
-      app =
-        proc do |env|
-          if env['REQUEST_METHOD'] == 'GET'
-            [200, {}, [Process.pid]]
-          else
-            Process.kill(env['QUERY_STRING'], Process.pid)
-            [200, {}, ['Bye!']]
-          end
-        end
-
-      run app
-    EOS
-
-    cmd = %W[unicorn -E test -c #{config_path} #{rackup_path}]
+    cmd = %W[unicorn -E test -c #{config_path} spec/rack_servers/configs/config.ru]
     @unicorn_master_pid = spawn(*cmd)
     wait_unicorn_boot!(@unicorn_master_pid, ready_file)
     WebMock.allow_net_connect!