Run background migrations with a minimum interval

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, 15 insertions, 0 deletions

diff --git a/spec/lib/gitlab/exclusive_lease_spec.rb b/spec/lib/gitlab/exclusive_lease_spec.rb
index 7322a326b01..6193e177668 100644
--- a/spec/lib/gitlab/exclusive_lease_spec.rb
+++ b/spec/lib/gitlab/exclusive_lease_spec.rb
@@ -73,4 +73,19 @@ describe Gitlab::ExclusiveLease, :clean_gitlab_redis_shared_state do
       described_class.new(key, timeout: 3600).try_obtain
     end
   end
+
+  describe '#ttl' do
+    it 'returns the TTL of the Redis key' do
+      lease = described_class.new('kittens', timeout: 100)
+      lease.try_obtain
+
+      expect(lease.ttl <= 100).to eq(true)
+    end
+
+    it 'returns nil when the lease does not exist' do
+      lease = described_class.new('kittens', timeout: 10)
+
+      expect(lease.ttl).to be_nil
+    end
+  end
 end