Rework how recent push events are retrieveduser-recent-push

Whenever you push to a branch GitLab will show a button to create a
merge request (should one not exist already). The underlying code to
display this data was quite inefficient. For example, it involved
multiple slow queries just to figure out what the most recent push event
was.

This commit changes the way this data is retrieved so it's much faster.
This is achieved by caching the ID of the last push event on every push,
which is then retrieved when loading certain pages. Database queries are
only executed if necessary and the cached data is removed automatically
once a merge request has been created, or 2 hours after being stored.

A trade-off of this approach is that we _only_ track the last event.
Previously if you were to push to branch A and B then create a merge
request for branch B we'd still show the widget for branch A. As of this
commit this is no longer the case, instead we will only show the widget
for the branch you pushed to most recently. Once a merge request exists
the widget is no longer displayed. Alternative solutions are either too
complex and/or too slow, hence the decision was made to settle for this
trade-off.

Performance Impact
------------------

In the best case scenario (= a user didn't push anything for more than 2
hours) we perform a single Redis GET per page. Should there be cached
data we will run a single (and lightweight) SQL query to get the
event data from the database. If a merge request already exists we will
run an additional DEL to remove the cache key.

The difference in response timings can vary a bit per project. On
GitLab.com the 99th percentile of time spent in User#recent_push hovers
between 100 milliseconds and 1 second, while the mean hovers around 50
milliseconds. With the changes in this MR the expected time spent in
User#recent_push is expected to be reduced down to just a few
milliseconds.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/35990

3 files changed, 45 insertions, 14 deletions

diff --git a/app/models/event.rb b/app/models/event.rb
index c313bbb66f8..8e9490b66f4 100644
--- a/app/models/event.rb
+++ b/app/models/event.rb
@@ -49,7 +49,7 @@ class Event < ActiveRecord::Base
   belongs_to :author, class_name: "User"
   belongs_to :project
   belongs_to :target, polymorphic: true # rubocop:disable Cop/PolymorphicAssociations
-  has_one :push_event_payload, foreign_key: :event_id
+  has_one :push_event_payload
 
   # Callbacks
   after_create :reset_project_activity
diff --git a/app/models/push_event.rb b/app/models/push_event.rb
index 23ffb0d4ea8..708513c7861 100644
--- a/app/models/push_event.rb
+++ b/app/models/push_event.rb
@@ -30,6 +30,44 @@ class PushEvent < Event
   delegate :commit_count, to: :push_event_payload
   alias_method :commits_count, :commit_count
 
+  # Returns events of pushes that either pushed to an existing ref or created a
+  # new one.
+  def self.created_or_pushed
+    actions = [
+      PushEventPayload.actions[:pushed],
+      PushEventPayload.actions[:created]
+    ]
+
+    joins(:push_event_payload)
+      .where(push_event_payloads: { action: actions })
+  end
+
+  # Returns events of pushes to a branch.
+  def self.branch_events
+    ref_type = PushEventPayload.ref_types[:branch]
+
+    joins(:push_event_payload)
+      .where(push_event_payloads: { ref_type: ref_type })
+  end
+
+  # Returns PushEvent instances for which no merge requests have been created.
+  def self.without_existing_merge_requests
+    existing_mrs = MergeRequest.except(:order)
+      .select(1)
+      .where('merge_requests.source_project_id = events.project_id')
+      .where('merge_requests.source_branch = push_event_payloads.ref')
+
+    # For reasons unknown the use of #eager_load will result in the
+    # "push_event_payload" association not being set. Because of this we're
+    # using "joins" here, which does mean an additional query needs to be
+    # executed in order to retrieve the "push_event_association" when the
+    # returned PushEvent is used.
+    joins(:push_event_payload)
+      .where('NOT EXISTS (?)', existing_mrs)
+      .created_or_pushed
+      .branch_events
+  end
+
   def self.sti_name
     PUSHED
   end
diff --git a/app/models/user.rb b/app/models/user.rb
index d7549409b15..09c9b3250eb 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -651,20 +651,13 @@ class User < ActiveRecord::Base
     @personal_projects_count ||= personal_projects.count
   end
 
-  def recent_push(project_ids = nil)
-    # Get push events not earlier than 2 hours ago
-    events = recent_events.code_push.where("created_at > ?", Time.now - 2.hours)
-    events = events.where(project_id: project_ids) if project_ids
+  def recent_push(project = nil)
+    service = Users::LastPushEventService.new(self)
 
-    # Use the latest event that has not been pushed or merged recently
-    events.includes(:project).recent.find do |event|
-      next unless event.project.repository.branch_exists?(event.branch_name)
-
-      merge_requests = MergeRequest.where("created_at >= ?", event.created_at)
-        .where(source_project_id: event.project.id,
-               source_branch: event.branch_name)
-
-      merge_requests.empty?
+    if project
+      service.last_event_for_project(project)
+    else
+      service.last_event_for_user
     end
   end