Verify background migration execution in specs

This extends the existing matchers for scheduled migrations to also
verify that executing the background migration with the given arguments
doesn't raise any errors.

This makes sure that the post-deployment migration passes the arguments
correctly to the background migration, and also catches incomplete test
setups.

spec/migrations/schedule_populate_personal_snippet_statistics_spec.rb

@@ -6,11 +6,15 @@ require Rails.root.join('db', 'post_migrate', '20200714075739_schedule_populate_
 
 RSpec.describe SchedulePopulatePersonalSnippetStatistics do
   let(:users) { table(:users) }
+  let(:namespaces) { table(:namespaces) }
   let(:snippets) { table(:snippets) }
   let(:projects) { table(:projects) }
-  let(:user1) { users.create!(id: 1, email: 'user1@example.com', projects_limit: 10, username: 'test1', name: 'Test1', state: 'active') }
-  let(:user2) { users.create!(id: 2, email: 'user2@example.com', projects_limit: 10, username: 'test2', name: 'Test2', state: 'active') }
-  let(:user3) { users.create!(id: 3, email: 'user3@example.com', projects_limit: 10, username: 'test3', name: 'Test3', state: 'active') }
+  let!(:user1) { users.create!(id: 1, email: 'user1@example.com', projects_limit: 10, username: 'test1', name: 'Test1', state: 'active') }
+  let!(:user2) { users.create!(id: 2, email: 'user2@example.com', projects_limit: 10, username: 'test2', name: 'Test2', state: 'active') }
+  let!(:user3) { users.create!(id: 3, email: 'user3@example.com', projects_limit: 10, username: 'test3', name: 'Test3', state: 'active') }
+  let!(:namespace1) { namespaces.create!(id: 1, owner_id: user1.id, name: 'test1', path: 'test1') }
+  let!(:namespace2) { namespaces.create!(id: 2, owner_id: user2.id, name: 'test2', path: 'test2') }
+  let!(:namespace3) { namespaces.create!(id: 3, owner_id: user3.id, name: 'test3', path: 'test3') }
 
   def create_snippet(id, user_id, type = 'PersonalSnippet')
     params = {

spec/support/matchers/background_migrations_matchers.rb

 # frozen_string_literal: true
 
+RSpec::Matchers.define :be_background_migration_with_arguments do |arguments|
+  define_method :matches? do |migration|
+    expect do
+      Gitlab::BackgroundMigration.perform(migration, arguments)
+    end.not_to raise_error
+  end
+end
+
 RSpec::Matchers.define :be_scheduled_delayed_migration do |delay, *expected|
-  match do |migration|
+  define_method :matches? do |migration|
+    expect(migration).to be_background_migration_with_arguments(expected)
+
     BackgroundMigrationWorker.jobs.any? do |job|
       job['args'] == [migration, expected] &&
         job['at'].to_i == (delay.to_i + Time.now.to_i)
@@ -16,7 +26,9 @@ RSpec::Matchers.define :be_scheduled_delayed_migration do |delay, *expected|
 end
 
 RSpec::Matchers.define :be_scheduled_migration do |*expected|
-  match do |migration|
+  define_method :matches? do |migration|
+    expect(migration).to be_background_migration_with_arguments(expected)
+
     BackgroundMigrationWorker.jobs.any? do |job|
       args = job['args'].size == 1 ? [BackgroundMigrationWorker.jobs[0]['args'][0], []] : job['args']
       args == [migration, expected]
@@ -29,7 +41,9 @@ RSpec::Matchers.define :be_scheduled_migration do |*expected|
 end
 
 RSpec::Matchers.define :be_scheduled_migration_with_multiple_args do |*expected|
-  match do |migration|
+  define_method :matches? do |migration|
+    expect(migration).to be_background_migration_with_arguments(expected)
+
     BackgroundMigrationWorker.jobs.any? do |job|
       args = job['args'].size == 1 ? [BackgroundMigrationWorker.jobs[0]['args'][0], []] : job['args']
       args[0] == migration && compare_args(args, expected)