Add more accurate way of counting background migrations

Sidekiq::Queue only counts currently enqueued jobs. For background
migrations, most jobs are instead scheduled and therefore in
Sidekiq::ScheduledSet. This is because if we enqueued them all, we'd
process them as fast as Sidekiq could pick them up, which could:

1. Block other jobs from processing.
2. Overwhelm the database.

changelogs/unreleased/improve-background-migration-check.yml

+---
+title: Add more accurate way of counting remaining background migrations before upgrading
+merge_request:
+author:
+type: fixed

doc/update/README.md

@@ -116,14 +116,14 @@ following command:
 **For Omnibus installations**
 
 ```shell
-sudo gitlab-rails runner -e production 'puts Sidekiq::Queue.new("background_migration").size'
+sudo gitlab-rails runner -e production 'puts Gitlab::BackgroundMigration.remaining'
 ```
 
 **For installations from source**
 
 ```
 cd /home/git/gitlab
-sudo -u git -H bundle exec rails runner -e production 'puts Sidekiq::Queue.new("background_migration").size'
+sudo -u git -H bundle exec rails runner -e production 'puts Gitlab::BackgroundMigration.remaining'
 ```
 
 ## Upgrading to a new major version

lib/gitlab/background_migration.rb

@@ -58,6 +58,14 @@ module Gitlab
       migration_class_for(class_name).new.perform(*arguments)
     end
 
+    def self.remaining
+      scheduled = Sidekiq::ScheduledSet.new.count do |job|
+        job.queue == self.queue
+      end
+
+      scheduled + Sidekiq::Queue.new(self.queue).size
+    end
+
     def self.exists?(migration_class, additional_queues = [])
       enqueued = Sidekiq::Queue.new(self.queue)
       scheduled = Sidekiq::ScheduledSet.new

spec/lib/gitlab/background_migration_spec.rb

@@ -165,6 +165,32 @@ describe Gitlab::BackgroundMigration do
     end
   end
 
+  describe '.remaining', :redis do
+    context 'when there are jobs remaining' do
+      let(:queue) { Array.new(12) }
+
+      before do
+        allow(Sidekiq::Queue).to receive(:new)
+          .with(described_class.queue)
+          .and_return(Array.new(12))
+
+        Sidekiq::Testing.disable! do
+          BackgroundMigrationWorker.perform_in(10.minutes, 'Foo')
+        end
+      end
+
+      it 'returns the enqueued jobs plus the scheduled jobs' do
+        expect(described_class.remaining).to eq(13)
+      end
+    end
+
+    context 'when there are no jobs remaining' do
+      it 'returns zero' do
+        expect(described_class.remaining).to be_zero
+      end
+    end
+  end
+
   describe '.exists?' do
     context 'when there are enqueued jobs present' do
       let(:queue) do