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

6 files changed, 139 insertions, 26 deletions

diff --git a/app/helpers/projects_helper.rb b/app/helpers/projects_helper.rb
index c0114dd0256..0c8cb9ba235 100644
--- a/app/helpers/projects_helper.rb
+++ b/app/helpers/projects_helper.rb
@@ -137,15 +137,7 @@ module ProjectsHelper
   end
 
   def last_push_event
-    return unless current_user
-    return current_user.recent_push unless @project
-
-    project_ids = [@project.id]
-    if fork = current_user.fork_of(@project)
-      project_ids << fork.id
-    end
-
-    current_user.recent_push(project_ids)
+    current_user&.recent_push(@project)
   end
 
   def project_feature_access_select(field)
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
 
diff --git a/app/services/event_create_service.rb b/app/services/event_create_service.rb
index 0b7e4f187f7..6328d567a07 100644
--- a/app/services/event_create_service.rb
+++ b/app/services/event_create_service.rb
@@ -74,12 +74,19 @@ class EventCreateService
     # We're using an explicit transaction here so that any errors that may occur
     # when creating push payload data will result in the event creation being
     # rolled back as well.
-    Event.transaction do
-      event = create_event(project, current_user, Event::PUSHED)
+    event = Event.transaction do
+      new_event = create_event(project, current_user, Event::PUSHED)
 
-      PushEventPayloadService.new(event, push_data).execute
+      PushEventPayloadService
+        .new(new_event, push_data)
+        .execute
+
+      new_event
     end
 
+    Users::LastPushEventService.new(current_user)
+      .cache_last_push_event(event)
+
     Users::ActivityService.new(current_user, 'push').execute
   end
 
diff --git a/app/services/users/last_push_event_service.rb b/app/services/users/last_push_event_service.rb
new file mode 100644
index 00000000000..f2bfb60604f
--- /dev/null
+++ b/app/services/users/last_push_event_service.rb
@@ -0,0 +1,83 @@
+module Users
+  # Service class for caching and retrieving the last push event of a user.
+  class LastPushEventService
+    EXPIRATION = 2.hours
+
+    def initialize(user)
+      @user = user
+    end
+
+    # Caches the given push event for the current user in the Rails cache.
+    #
+    # event - An instance of PushEvent to cache.
+    def cache_last_push_event(event)
+      keys = [
+        project_cache_key(event.project),
+        user_cache_key
+      ]
+
+      if event.project.forked?
+        keys << project_cache_key(event.project.forked_from_project)
+      end
+
+      keys.each { |key| set_key(key, event.id) }
+    end
+
+    # Returns the last PushEvent for the current user.
+    #
+    # This method will return nil if no event was found.
+    def last_event_for_user
+      find_cached_event(user_cache_key)
+    end
+
+    # Returns the last PushEvent for the current user and the given project.
+    #
+    # project - An instance of Project for which to retrieve the PushEvent.
+    #
+    # This method will return nil if no event was found.
+    def last_event_for_project(project)
+      find_cached_event(project_cache_key(project))
+    end
+
+    def find_cached_event(cache_key)
+      event_id = get_key(cache_key)
+
+      return unless event_id
+
+      unless (event = find_event_in_database(event_id))
+        # We don't want to keep querying the same data over and over when a
+        # merge request has been created, thus we remove the key if no event
+        # (meaning an MR was created) is returned.
+        Rails.cache.delete(cache_key)
+      end
+
+      event
+    end
+
+    private
+
+    def find_event_in_database(id)
+      PushEvent
+        .without_existing_merge_requests
+        .find_by(id: id)
+    end
+
+    def user_cache_key
+      "last-push-event/#{@user.id}"
+    end
+
+    def project_cache_key(project)
+      "last-push-event/#{@user.id}/#{project.id}"
+    end
+
+    def get_key(key)
+      Rails.cache.read(key, raw: true)
+    end
+
+    def set_key(key, value)
+      # We're using raw values here since this takes up less space and we don't
+      # store complex objects.
+      Rails.cache.write(key, value, raw: true, expires_in: EXPIRATION)
+    end
+  end
+end