Greatly improve external_issue_tracker performanceimprove-project-external-issue-trackers

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

3 files changed, 6 insertions, 12 deletions

diff --git a/app/models/project_services/ci_service.rb b/app/models/project_services/ci_service.rb
index c3f70d1f972..e10b5529b42 100644
--- a/app/models/project_services/ci_service.rb
+++ b/app/models/project_services/ci_service.rb
@@ -23,14 +23,12 @@
 # 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
   end
-  
+
   def supported_events
     %w(push)
   end
diff --git a/app/models/project_services/gitlab_issue_tracker_service.rb b/app/models/project_services/gitlab_issue_tracker_service.rb
index 7aa04309f54..05436cd0f79 100644
--- a/app/models/project_services/gitlab_issue_tracker_service.rb
+++ b/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'
diff --git a/app/models/project_services/issue_tracker_service.rb b/app/models/project_services/issue_tracker_service.rb
index ed201979d39..25045224ce5 100644
--- a/app/models/project_services/issue_tracker_service.rb
+++ b/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)