Reschedule merge request diff background migration

The first attempt didn't migrate all rows on GitLab.com, due to a couple of
issues:

1. Some rows in merge_request_diffs had truly huge numbers of commits and diffs
   serialised - one in particular had 26,000 commits!
2. The jobs were sometimes on Sidekiq hosts with frequent OOM errors, leading to
   the job being lost.

The previous commit adds more logging, and a more robust insertion method. This
commit reschedules the jobs, with a generous pause between each.

changelogs/unreleased/36631-activerecord-statementinvalid-pg-querycanceled-error-canceling-statement-due-to-statement-timeout.yml

@@ -0,0 +1,6 @@
+---
+title: Reschedule merge request diff background migrations to catch failures from
+  9.5 run
+merge_request:
+author:
+type: fixed

db/post_migrate/20170926150348_schedule_merge_request_diff_migrations_take_two.rb

@@ -0,0 +1,32 @@
+class ScheduleMergeRequestDiffMigrationsTakeTwo < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  BATCH_SIZE = 500
+  MIGRATION = 'DeserializeMergeRequestDiffsAndCommits'
+  DELAY_INTERVAL = 10.minutes
+
+  disable_ddl_transaction!
+
+  class MergeRequestDiff < ActiveRecord::Base
+    self.table_name = 'merge_request_diffs'
+
+    include ::EachBatch
+
+    default_scope { where('st_commits IS NOT NULL OR st_diffs IS NOT NULL') }
+  end
+
+  # By this point, we assume ScheduleMergeRequestDiffMigrations - the first
+  # version of this - has already run. On GitLab.com, we have ~220k un-migrated
+  # rows, but these rows will, in general, take a long time.
+  #
+  # With a gap of 10 minutes per batch, and 500 rows per batch, these migrations
+  # are scheduled over 220_000 / 500 / 6 ~= 74 hours, which is a little over
+  # three days.
+  def up
+    queue_background_migration_jobs_by_range_at_intervals(MergeRequestDiff, MIGRATION, DELAY_INTERVAL, batch_size: BATCH_SIZE)
+  end
+
+  def down
+  end
+end

spec/migrations/schedule_merge_request_diff_migrations_take_two_spec.rb

@@ -0,0 +1,59 @@
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20170926150348_schedule_merge_request_diff_migrations_take_two')
+
+describe ScheduleMergeRequestDiffMigrationsTakeTwo, :migration, :sidekiq do
+  matcher :be_scheduled_migration do |time, *expected|
+    match do |migration|
+      BackgroundMigrationWorker.jobs.any? do |job|
+        job['args'] == [migration, expected] &&
+          job['at'].to_i == time.to_i
+      end
+    end
+
+    failure_message do |migration|
+      "Migration `#{migration}` with args `#{expected.inspect}` not scheduled!"
+    end
+  end
+
+  let(:merge_request_diffs) { table(:merge_request_diffs) }
+  let(:merge_requests) { table(:merge_requests) }
+  let(:projects) { table(:projects) }
+
+  before do
+    stub_const("#{described_class.name}::BATCH_SIZE", 1)
+
+    projects.create!(id: 1, name: 'gitlab', path: 'gitlab')
+
+    merge_requests.create!(id: 1, target_project_id: 1, source_project_id: 1, target_branch: 'feature', source_branch: 'master')
+
+    merge_request_diffs.create!(id: 1, merge_request_id: 1, st_commits: YAML.dump([]), st_diffs: nil)
+    merge_request_diffs.create!(id: 2, merge_request_id: 1, st_commits: nil, st_diffs: YAML.dump([]))
+    merge_request_diffs.create!(id: 3, merge_request_id: 1, st_commits: nil, st_diffs: nil)
+    merge_request_diffs.create!(id: 4, merge_request_id: 1, st_commits: YAML.dump([]), st_diffs: YAML.dump([]))
+  end
+
+  it 'correctly schedules background migrations' do
+    Sidekiq::Testing.fake! do
+      Timecop.freeze do
+        migrate!
+
+        expect(described_class::MIGRATION).to be_scheduled_migration(10.minutes.from_now, 1, 1)
+        expect(described_class::MIGRATION).to be_scheduled_migration(20.minutes.from_now, 2, 2)
+        expect(described_class::MIGRATION).to be_scheduled_migration(30.minutes.from_now, 4, 4)
+        expect(BackgroundMigrationWorker.jobs.size).to eq 3
+      end
+    end
+  end
+
+  it 'migrates the data' do
+    Sidekiq::Testing.inline! do
+      non_empty = 'st_commits IS NOT NULL OR st_diffs IS NOT NULL'
+
+      expect(merge_request_diffs.where(non_empty).count).to eq 3
+
+      migrate!
+
+      expect(merge_request_diffs.where(non_empty).count).to eq 0
+    end
+  end
+end