Disable replication lag check for Aurora PostgreSQL databases

Replication slots are not supported in Aurora. Attempting to check
the lag results in the message:

```
ActiveRecord::StatementInvalid: PG::FeatureNotSupported: ERROR:
Replication slots are currently not supported in Aurora : SELECT
pg_xlog_location_diff(pg_current_xlog_insert_location(),
restart_lsn)::...
```

To avoid breaking support for background migrations in Aurora, we just
disable the check if we encounter this error.

This change also now checks whether there are any replication slots
present in the primary before checking the replication lag.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52176

1 files changed, 20 insertions, 0 deletions

diff --git a/spec/models/postgresql/replication_slot_spec.rb b/spec/models/postgresql/replication_slot_spec.rb
index 919a7526803..e100af7ddc7 100644
--- a/spec/models/postgresql/replication_slot_spec.rb
+++ b/spec/models/postgresql/replication_slot_spec.rb
@@ -3,7 +3,27 @@
 require 'spec_helper'
 
 describe Postgresql::ReplicationSlot, :postgresql do
+  describe '.in_use?' do
+    it 'returns true when replication slots are present' do
+      expect(described_class).to receive(:exists?).and_return(true)
+      expect(described_class.in_use?).to be_truthy
+    end
+
+    it 'returns false when replication slots are not present' do
+      expect(described_class.in_use?).to be_falsey
+    end
+
+    it 'returns false if the existence check is invalid' do
+      expect(described_class).to receive(:exists?).and_raise(ActiveRecord::StatementInvalid.new('PG::FeatureNotSupported'))
+      expect(described_class.in_use?).to be_falsey
+    end
+  end
+
   describe '.lag_too_great?' do
+    before do
+      expect(described_class).to receive(:in_use?).and_return(true)
+    end
+
     it 'returns true when replication lag is too great' do
       expect(described_class)
         .to receive(:pluck)