Run background migrations with a minimum intervaldelay-background-migrations

This adds a minimum interval to BackgroundMigrationWorker, ensuring
background migrations of the same class only run once every 5 minutes.
This prevents a thundering herd problem where scheduled migrations all
run at once due to their delays having been expired (e.g. as the result
of a queue being paused for a long time).

If a job was recently executed it's rescheduled with a delay that equals
the remaining time of the job's lease. This means that if the lease
expires in two minutes we only need to wait two minutes, instead of
five.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/41624

1 files changed, 44 insertions, 1 deletions

diff --git a/app/workers/background_migration_worker.rb b/app/workers/background_migration_worker.rb
index aeb3bc019b9..376703f6319 100644
--- a/app/workers/background_migration_worker.rb
+++ b/app/workers/background_migration_worker.rb
@@ -1,10 +1,53 @@
 class BackgroundMigrationWorker
   include ApplicationWorker
 
+  # The minimum amount of time between processing two jobs of the same migration
+  # class.
+  #
+  # This interval is set to 5 minutes so autovacuuming and other maintenance
+  # related tasks have plenty of time to clean up after a migration has been
+  # performed.
+  MIN_INTERVAL = 5.minutes.to_i
+
   # Performs the background migration.
   #
   # See Gitlab::BackgroundMigration.perform for more information.
+  #
+  # class_name - The class name of the background migration to run.
+  # arguments - The arguments to pass to the migration class.
   def perform(class_name, arguments = [])
-    Gitlab::BackgroundMigration.perform(class_name, arguments)
+    should_perform, ttl = perform_and_ttl(class_name)
+
+    if should_perform
+      Gitlab::BackgroundMigration.perform(class_name, arguments)
+    else
+      # If the lease could not be obtained this means either another process is
+      # running a migration of this class or we ran one recently. In this case
+      # we'll reschedule the job in such a way that it is picked up again around
+      # the time the lease expires.
+      self.class.perform_in(ttl || MIN_INTERVAL, class_name, arguments)
+    end
+  end
+
+  def perform_and_ttl(class_name)
+    if always_perform?
+      # In test environments `perform_in` will run right away. This can then
+      # lead to stack level errors in the above `#perform`. To work around this
+      # we'll just perform the migration right away in the test environment.
+      [true, nil]
+    else
+      lease = lease_for(class_name)
+
+      [lease.try_obtain, lease.ttl]
+    end
+  end
+
+  def lease_for(class_name)
+    Gitlab::ExclusiveLease
+      .new("#{self.class.name}:#{class_name}", timeout: MIN_INTERVAL)
+  end
+
+  def always_perform?
+    Rails.env.test?
   end
 end