Greatly improve external_issue_tracker performance

This greatly improves the performance of Project#external_issue_tracker
by moving most of the fields queried in Ruby to the database and letting
the database handle all logic. Prior to this change the process of
finding an external issue tracker was along the lines of the following:

1. Load all project services into memory.
2. Reduce the list to only services where "issue_tracker?" returns true
3. Reduce the list from step 2 to service where "default?" returns false
4. Find the first service where "activated?" returns true

This has to two big problems:

1. Loading all services into memory only to reduce the list down to a
   single item later on is a waste of memory (and slow timing wise).
2. Calling Array#select followed by Array#reject followed by Array#find
   allocates extra objects when this really isn't needed.

To work around this the following service fields have been moved to the
database (instead of being hardcoded):

* category
* default

This in turn means we can get the external issue tracker using the
following query:

    SELECT *
    FROM services
    WHERE active IS TRUE
    AND default IS FALSE
    AND category = 'issue_tracker'
    AND project_id = XXX
    LIMIT 1

This coupled with memoizing the result (just as before this commit)
greatly reduces the time it takes for Project#external_issue_tracker to
complete. The exact reduction depends on one's environment, but locally
the execution time is reduced from roughly 230 ms to only 2 ms (= a
reduction of almost 180x).

Fixes gitlab-org/gitlab-ce#10771

app/models/project.rb

@@ -468,12 +468,9 @@ class Project < ActiveRecord::Base
     !external_issue_tracker
   end
 
-  def external_issues_trackers
-    services.select(&:issue_tracker?).reject(&:default?)
-  end
-
   def external_issue_tracker
-    @external_issues_tracker ||= external_issues_trackers.find(&:activated?)
+    @external_issue_tracker ||=
+      services.issue_trackers.active.without_defaults.first
   end
 
   def can_have_issues_tracker_id?

app/models/project_services/ci_service.rb

@@ -23,9 +23,7 @@
 # List methods you need to implement to get your CI service
 # working with GitLab Merge Requests
 class CiService < Service
-  def category
-    :ci
-  end
+  default_value_for :category, 'ci'
 
   def valid_token?(token)
     self.respond_to?(:token) && self.token.present? && self.token == token

app/models/project_services/gitlab_issue_tracker_service.rb

@@ -24,9 +24,7 @@ class GitlabIssueTrackerService < IssueTrackerService
 
   prop_accessor :title, :description, :project_url, :issues_url, :new_issue_url
 
-  def default?
-    true
-  end
+  default_value_for :default, true
 
   def to_param
     'gitlab'

app/models/project_services/issue_tracker_service.rb

@@ -23,12 +23,10 @@ class IssueTrackerService < Service
 
   validates :project_url, :issues_url, :new_issue_url, presence: true, if: :activated?
 
-  def category
-    :issue_tracker
-  end
+  default_value_for :category, 'issue_tracker'
 
   def default?
-    false
+    default
   end
 
   def issue_url(iid)

app/models/service.rb

@@ -43,6 +43,9 @@ class Service < ActiveRecord::Base
   validates :project_id, presence: true, unless: Proc.new { |service| service.template? }
 
   scope :visible, -> { where.not(type: ['GitlabIssueTrackerService', 'GitlabCiService']) }
+  scope :issue_trackers, -> { where(category: 'issue_tracker') }
+  scope :active, -> { where(active: true) }
+  scope :without_defaults, -> { where(default: false) }
 
   scope :push_hooks, -> { where(push_events: true, active: true) }
   scope :tag_push_hooks, -> { where(tag_push_events: true, active: true) }
@@ -51,6 +54,8 @@ class Service < ActiveRecord::Base
   scope :note_hooks, -> { where(note_events: true, active: true) }
   scope :build_hooks, -> { where(build_events: true, active: true) }
 
+  default_value_for :category, 'common'
+
   def activated?
     active
   end
@@ -60,7 +65,7 @@ class Service < ActiveRecord::Base
   end
 
   def category
-    :common
+    read_attribute(:category).to_sym
   end
 
   def initialize_properties

db/migrate/20160119111158_add_services_category.rb

+class AddServicesCategory < ActiveRecord::Migration
+  def up
+    add_column :services, :category, :string, default: 'common', null: false
+
+    category = quote_column_name('category')
+    type     = quote_column_name('type')
+
+    execute <<-EOF
+UPDATE services
+SET #{category} = 'issue_tracker'
+WHERE #{type} IN (
+  'CustomIssueTrackerService',
+  'GitlabIssueTrackerService',
+  'IssueTrackerService',
+  'JiraService',
+  'RedmineService'
+);
+EOF
+
+    execute <<-EOF
+UPDATE services
+SET #{category} = 'ci'
+WHERE #{type} IN (
+  'BambooService',
+  'BuildkiteService',
+  'CiService',
+  'DroneCiService',
+  'GitlabCiService',
+  'TeamcityService'
+);
+    EOF
+
+    add_index :services, :category
+  end
+
+  def down
+    remove_column :services, :category
+  end
+end

db/migrate/20160119112418_add_services_default.rb

+class AddServicesDefault < ActiveRecord::Migration
+  def up
+    add_column :services, :default, :boolean, default: false
+
+    default = quote_column_name('default')
+    type    = quote_column_name('type')
+
+    execute <<-EOF
+UPDATE services
+SET #{default} = true
+WHERE #{type} = 'GitlabIssueTrackerService'
+EOF
+
+    add_index :services, :default
+  end
+
+  def down
+    remove_column :services, :default
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160113111034) do
+ActiveRecord::Schema.define(version: 20160119112418) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -725,9 +725,9 @@ ActiveRecord::Schema.define(version: 20160113111034) do
     t.string   "type"
     t.string   "title"
     t.integer  "project_id"
-    t.datetime "created_at"
-    t.datetime "updated_at"
-    t.boolean  "active",                default: false, null: false
+    t.datetime "created_at",                                           null: false
+    t.datetime "updated_at",                                           null: false
+    t.boolean  "active",                                               null: false
     t.text     "properties"
     t.boolean  "template",                          default: false
     t.boolean  "push_events",                       default: true
@@ -736,9 +736,13 @@ ActiveRecord::Schema.define(version: 20160113111034) do
     t.boolean  "tag_push_events",                   default: true
     t.boolean  "note_events",                       default: true,     null: false
     t.boolean  "build_events",                      default: false,    null: false
+    t.string   "category",                          default: "common", null: false
+    t.boolean  "default",                           default: false
   end
 
+  add_index "services", ["category"], name: "index_services_on_category", using: :btree
   add_index "services", ["created_at", "id"], name: "index_services_on_created_at_and_id", using: :btree
+  add_index "services", ["default"], name: "index_services_on_default", using: :btree
   add_index "services", ["project_id"], name: "index_services_on_project_id", using: :btree
   add_index "services", ["template"], name: "index_services_on_template", using: :btree