Migrate events into a new format

This commit migrates events data in such a way that push events are
stored much more efficiently. This is done by creating a shadow table
called "events_for_migration", and a table called "push_event_payloads"
which is used for storing push data of push events. The background
migration in this commit will copy events from the "events" table into
the "events_for_migration" table, push events in will also have a row
created in "push_event_payloads".

This approach allows us to reclaim space in the next release by simply
swapping the "events" and "events_for_migration" tables, then dropping
the old events (now "events_for_migration") table.

The new table structure is also optimised for storage space, and does
not include the unused "title" column nor the "data" column (since this
data is moved to "push_event_payloads").

== Newly Created Events

Newly created events are inserted into both "events" and
"events_for_migration", both using the exact same primary key value. The
table "push_event_payloads" in turn has a foreign key to the _shadow_
table. This removes the need for recreating and validating the foreign
key after swapping the tables. Since the shadow table also has a foreign
key to "projects.id" we also don't have to worry about orphaned rows.

This approach however does require some additional storage as we're
duplicating a portion of the events data for at least 1 release. The
exact amount is hard to estimate, but for GitLab.com this is expected to
be between 10 and 20 GB at most. The background migration in this commit
deliberately does _not_ update the "events" table as doing so would put
a lot of pressure on PostgreSQL's auto vacuuming system.

== Supporting Both Old And New Events

Application code has also been adjusted to support push events using
both the old and new data formats. This is done by creating a PushEvent
class which extends the regular Event class. Using Rails' Single Table
Inheritance system we can ensure the right class is used for the right
data, which in this case is based on the value of `events.action`. To
support displaying old and new data at the same time the PushEvent class
re-defines a few methods of the Event class, falling back to their
original implementations for push events in the old format.

Once all existing events have been migrated the various push event
related methods can be removed from the Event model, and the calls to
`super` can be removed from the methods in the PushEvent model.

The UI and event atom feed have also been slightly changed to better
handle this new setup, fortunately only a few changes were necessary to
make this work.

== API Changes

The API only displays push data of events in the new format. Supporting
both formats in the API is a bit more difficult compared to the UI.
Since the old push data was not really well documented (apart from one
example that used an incorrect "action" nmae) I decided that supporting
both was not worth the effort, especially since events will be migrated
in a few days _and_ new events are created in the correct format.

4 files changed, 216 insertions, 10 deletions

diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 3bb1910a441..18cd604a216 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -497,14 +497,24 @@ module API
       expose :author, using: Entities::UserBasic
     end
 
+    class PushEventPayload < Grape::Entity
+      expose :commit_count, :action, :ref_type, :commit_from, :commit_to
+      expose :ref, :commit_title
+    end
+
     class Event < Grape::Entity
       expose :title, :project_id, :action_name
       expose :target_id, :target_iid, :target_type, :author_id
-      expose :data, :target_title
+      expose :target_title
       expose :created_at
       expose :note, using: Entities::Note, if: ->(event, options) { event.note? }
       expose :author, using: Entities::UserBasic, if: ->(event, options) { event.author }
 
+      expose :push_event_payload,
+        as: :push_data,
+        using: PushEventPayload,
+        if: -> (event, _) { event.push? }
+
       expose :author_username do |event, options|
         event.author&.username
       end
diff --git a/lib/api/v3/entities.rb b/lib/api/v3/entities.rb
index 4a2e9c9cbb0..a9a35f2a4bd 100644
--- a/lib/api/v3/entities.rb
+++ b/lib/api/v3/entities.rb
@@ -25,14 +25,24 @@ module API
         expose(:downvote?)  { |note| false }
       end
 
+      class PushEventPayload < Grape::Entity
+        expose :commit_count, :action, :ref_type, :commit_from, :commit_to
+        expose :ref, :commit_title
+      end
+
       class Event < Grape::Entity
         expose :title, :project_id, :action_name
         expose :target_id, :target_type, :author_id
-        expose :data, :target_title
+        expose :target_title
         expose :created_at
         expose :note, using: Entities::Note, if: ->(event, options) { event.note? }
         expose :author, using: ::API::Entities::UserBasic, if: ->(event, options) { event.author }
 
+        expose :push_event_payload,
+          as: :push_data,
+          using: PushEventPayload,
+          if: -> (event, _) { event.push? }
+
         expose :author_username do |event, options|
           event.author&.username
         end
diff --git a/lib/gitlab/background_migration/migrate_events_to_push_event_payloads.rb b/lib/gitlab/background_migration/migrate_events_to_push_event_payloads.rb
new file mode 100644
index 00000000000..432f7c3e706
--- /dev/null
+++ b/lib/gitlab/background_migration/migrate_events_to_push_event_payloads.rb
@@ -0,0 +1,176 @@
+module Gitlab
+  module BackgroundMigration
+    # Class that migrates events for the new push event payloads setup. All
+    # events are copied to a shadow table, and push events will also have a row
+    # created in the push_event_payloads table.
+    class MigrateEventsToPushEventPayloads
+      class Event < ActiveRecord::Base
+        self.table_name = 'events'
+
+        serialize :data
+
+        BLANK_REF = ('0' * 40).freeze
+        TAG_REF_PREFIX = 'refs/tags/'.freeze
+        MAX_INDEX = 69
+        PUSHED = 5
+
+        def push_event?
+          action == PUSHED && data.present?
+        end
+
+        def commit_title
+          commit = commits.last
+
+          return nil unless commit && commit[:message]
+
+          index = commit[:message].index("\n")
+          message = index ? commit[:message][0..index] : commit[:message]
+
+          message.strip.truncate(70)
+        end
+
+        def commit_from_sha
+          if create?
+            nil
+          else
+            data[:before]
+          end
+        end
+
+        def commit_to_sha
+          if remove?
+            nil
+          else
+            data[:after]
+          end
+        end
+
+        def data
+          super || {}
+        end
+
+        def commits
+          data[:commits] || []
+        end
+
+        def commit_count
+          data[:total_commits_count] || 0
+        end
+
+        def ref
+          data[:ref]
+        end
+
+        def trimmed_ref_name
+          if ref_type == :tag
+            ref[10..-1]
+          else
+            ref[11..-1]
+          end
+        end
+
+        def create?
+          data[:before] == BLANK_REF
+        end
+
+        def remove?
+          data[:after] == BLANK_REF
+        end
+
+        def push_action
+          if create?
+            :created
+          elsif remove?
+            :removed
+          else
+            :pushed
+          end
+        end
+
+        def ref_type
+          if ref.start_with?(TAG_REF_PREFIX)
+            :tag
+          else
+            :branch
+          end
+        end
+      end
+
+      class EventForMigration < ActiveRecord::Base
+        self.table_name = 'events_for_migration'
+      end
+
+      class PushEventPayload < ActiveRecord::Base
+        self.table_name = 'push_event_payloads'
+
+        enum action: {
+          created: 0,
+          removed: 1,
+          pushed: 2
+        }
+
+        enum ref_type: {
+          branch: 0,
+          tag: 1
+        }
+      end
+
+      # start_id - The start ID of the range of events to process
+      # end_id - The end ID of the range to process.
+      def perform(start_id, end_id)
+        return unless migrate?
+
+        find_events(start_id, end_id).each { |event| process_event(event) }
+      end
+
+      def process_event(event)
+        replicate_event(event)
+        create_push_event_payload(event) if event.push_event?
+      end
+
+      def replicate_event(event)
+        new_attributes = event.attributes
+          .with_indifferent_access.except(:title, :data)
+
+        EventForMigration.create!(new_attributes)
+      rescue ActiveRecord::InvalidForeignKey
+        # A foreign key error means the associated event was removed. In this
+        # case we'll just skip migrating the event.
+      end
+
+      def create_push_event_payload(event)
+        commit_from = pack(event.commit_from_sha)
+        commit_to = pack(event.commit_to_sha)
+
+        PushEventPayload.create!(
+          event_id: event.id,
+          commit_count: event.commit_count,
+          ref_type: event.ref_type,
+          action: event.push_action,
+          commit_from: commit_from,
+          commit_to: commit_to,
+          ref: event.trimmed_ref_name,
+          commit_title: event.commit_title
+        )
+      rescue ActiveRecord::InvalidForeignKey
+        # A foreign key error means the associated event was removed. In this
+        # case we'll just skip migrating the event.
+      end
+
+      def find_events(start_id, end_id)
+        Event
+          .where('NOT EXISTS (SELECT true FROM events_for_migration WHERE events_for_migration.id = events.id)')
+          .where(id: start_id..end_id)
+      end
+
+      def migrate?
+        Event.table_exists? && PushEventPayload.table_exists? &&
+          EventForMigration.table_exists?
+      end
+
+      def pack(value)
+        value ? [value].pack('H*') : nil
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/import_export/import_export.yml b/lib/gitlab/import_export/import_export.yml
index c5c05bfe2fb..9d9ebcb389a 100644
--- a/lib/gitlab/import_export/import_export.yml
+++ b/lib/gitlab/import_export/import_export.yml
@@ -3,18 +3,22 @@ project_tree:
   - labels:
       :priorities
   - milestones:
-    - :events
+    - events:
+      - :push_event_payload
   - issues:
-    - :events
+    - events:
+      - :push_event_payload
     - :timelogs
     - notes:
       - :author
-      - :events
+      - events:
+        - :push_event_payload
     - label_links:
       - label:
           :priorities
     - milestone:
-      - :events
+      - events:
+        - :push_event_payload
   - snippets:
     - :award_emoji
     - notes:
@@ -25,21 +29,25 @@ project_tree:
   - merge_requests:
     - notes:
       - :author
-      - :events
+      - events:
+        - :push_event_payload
     - merge_request_diff:
       - :merge_request_diff_commits
       - :merge_request_diff_files
-    - :events
+    - events:
+      - :push_event_payload
     - :timelogs
     - label_links:
       - label:
           :priorities
     - milestone:
-      - :events
+      - events:
+        - :push_event_payload
   - pipelines:
     - notes:
       - :author
-      - :events
+      - events:
+        - :push_event_payload
     - :stages
     - :statuses
   - :triggers
@@ -107,6 +115,8 @@ excluded_attributes:
   statuses:
     - :trace
     - :token
+  push_event_payload:
+    - :event_id
 
 methods:
   labels: