Fix background migrations failing with unused replication slot

When there is an unused replication slot, the replication lag function
will return a nil value, resulting in "NoMethodError: undefined method
`>=' for nil:NilClass" error. We now just ignore these nil values.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63666

app/models/postgresql/replication_slot.rb

@@ -28,7 +28,7 @@ module Postgresql
       # We force the use of a transaction here so the query always goes to the
       # primary, even when using the EE DB load balancer.
       sizes = transaction { pluck(lag_function) }
-      too_great = sizes.count { |size| size >= max }
+      too_great = sizes.compact.count { |size| size >= max }
 
       # If too many replicas are falling behind too much, the availability of a
       # GitLab instance might suffer. To prevent this from happening we require

changelogs/unreleased/sh-handle-nil-replication-lag.yml

@@ -0,0 +1,5 @@
+---
+title: Fix background migrations failing with unused replication slot
+merge_request: 30042
+author:
+type: fixed

spec/models/postgresql/replication_slot_spec.rb

@@ -47,5 +47,13 @@ describe Postgresql::ReplicationSlot, :postgresql do
 
       expect(described_class.lag_too_great?).to eq(false)
     end
+
+    it 'returns false when there is a nil replication lag' do
+      expect(described_class)
+        .to receive(:pluck)
+        .and_return([0.megabytes, nil])
+
+      expect(described_class.lag_too_great?).to eq(false)
+    end
   end
 end