Add rbtrace to Gemfile

This will be enabled if ENABLE_RBTRACE environment variable is defined.
This will allow us to debug all sorts of issues in production:

https://github.com/tmm1/rbtrace

Gemfile

@@ -400,6 +400,7 @@ gem 'email_reply_trimmer', '~> 0.1'
 gem 'html2text'
 
 gem 'ruby-prof', '~> 0.17.0'
+gem 'rbtrace', '~> 0.4', require: false
 
 # OAuth
 gem 'oauth2', '~> 1.4'

Gemfile.lock

@@ -700,6 +700,10 @@ GEM
       ffi (>= 0.5.0, < 2)
     rblineprof (0.3.6)
       debugger-ruby_core_source (~> 1.3)
+    rbtrace (0.4.10)
+      ffi (>= 1.0.6)
+      msgpack (>= 0.4.3)
+      trollop (>= 1.16.2)
     rdoc (6.0.4)
     re2 (1.1.1)
     recaptcha (3.0.0)
@@ -911,6 +915,7 @@ GEM
       parslet (~> 1.5.0)
     toml-rb (1.0.0)
       citrus (~> 3.0, > 3.0)
+    trollop (2.1.3)
     truncato (0.7.10)
       htmlentities (~> 4.3.1)
       nokogiri (~> 1.8.0, >= 1.7.0)
@@ -1133,6 +1138,7 @@ DEPENDENCIES
   rainbow (~> 2.2)
   raindrops (~> 0.18)
   rblineprof (~> 0.3.6)
+  rbtrace (~> 0.4)
   rdoc (~> 6.0)
   re2 (~> 1.1.1)
   recaptcha (~> 3.0)

Gemfile.rails5.lock

@@ -709,6 +709,10 @@ GEM
       ffi (>= 0.5.0, < 2)
     rblineprof (0.3.6)
       debugger-ruby_core_source (~> 1.3)
+    rbtrace (0.4.10)
+      ffi (>= 1.0.6)
+      msgpack (>= 0.4.3)
+      trollop (>= 1.16.2)
     rdoc (6.0.4)
     re2 (1.1.1)
     recaptcha (3.0.0)
@@ -918,6 +922,7 @@ GEM
       parslet (~> 1.5.0)
     toml-rb (1.0.0)
       citrus (~> 3.0, > 3.0)
+    trollop (2.1.3)
     truncato (0.7.10)
       htmlentities (~> 4.3.1)
       nokogiri (~> 1.8.0, >= 1.7.0)
@@ -1144,6 +1149,7 @@ DEPENDENCIES
   rainbow (~> 2.2)
   raindrops (~> 0.18)
   rblineprof (~> 0.3.6)
+  rbtrace (~> 0.4)
   rdoc (~> 6.0)
   re2 (~> 1.1.1)
   recaptcha (~> 3.0)

changelogs/unreleased/sh-include-rbtrace.yml

+---
+title: Add rbtrace to Gemfile
+merge_request: 20831
+author:
+type: other

config/initializers/rbtrace.rb

+# frozen_string_literal: true
+
+require 'rbtrace' if ENV['ENABLE_RBTRACE']

doc/administration/troubleshooting/debug.md

@@ -66,6 +66,19 @@ On CentOS:
 sudo yum install gdb
 ```
 
+### rbtrace
+
+GitLab 11.2 ships with [rbtrace](https://github.com/tmm1/rbtrace), which
+allows you to trace Ruby code, view all running threads, take memory dumps,
+and more. However, this is not enabled by default. To enable it, define the
+`ENABLE_RBTRACE` variable to the environment. For example, in Omnibus:
+
+```ruby
+gitlab_rails['env'] = {"ENABLE_RBTRACE" => "1"}
+```
+
+Then reconfigure the system and restart Unicorn and Sidekiq.
+
 ## Common Problems
 
 Many of the tips to diagnose issues below apply to many different situations. We'll use one