🐞 Eliminate spurious UnknownError from CC workers

- Previously, the old records cleanup generates UnknownErrors when there are no
  records & it has been directed to preserve the most recent.  Although
  these UnknownErrors are harmless, they pollute logs, and can cause consternation.
- These UnknownErrors are not visible to the user via API calls; they appear only
  in the worker logs.
- We now check to make sure that there is at least one record before
  attempting to preserve it.

fixes:

```json
{
  "error_code": "UnknownError",
  "description": "An unknown error occurred.",
  "code": 10001,
  "test_mode_info": {
    "description": "undefined method `id' for nil:NilClass",
    "error_code": "CF-id"
  }
}
```

[finishes #172773653]

Co-authored-by: Reid Mitchell <rmitchell@pivotal.io>
Co-authored-by: Brian Cunnie <bcunnie@pivotal.io>

Author: reidmit

lib/database/old_record_cleanup.rb

@@ -16,8 +16,10 @@ def delete
 
       old_records = model.dataset.where(Sequel.lit('created_at < ?', cutoff_date))
       if keep_at_least_one_record
-        last_id = model.order(:id).last.id
-        old_records = old_records.where(Sequel.lit('id < ?', last_id))
+        last_record = model.order(:id).last
+        if last_record
+          old_records = old_records.where(Sequel.lit('id < ?', last_record.id))
+        end
       end
       logger.info("Cleaning up #{old_records.count} #{model.table_name} table rows")
 

spec/unit/lib/database/old_record_cleanup_spec.rb

@@ -20,6 +20,15 @@
       expect { stale_event2.reload }.to raise_error(Sequel::NoExistingObject)
     end
 
+    context "when there are no records at all but you're trying to keep at least one" do
+      it "doesn't keep one because there aren't any to keep" do
+        record_cleanup = Database::OldRecordCleanup.new(VCAP::CloudController::ServiceUsageEvent, 1, keep_at_least_one_record: true)
+
+        expect { record_cleanup.delete }.not_to raise_error
+        expect(VCAP::CloudController::ServiceUsageEvent.count).to eq(0)
+      end
+    end
+
     it 'only retrieves the current timestamp from the database once' do
       expect(VCAP::CloudController::Event.db).to receive(:fetch).with('SELECT CURRENT_TIMESTAMP as now').once.and_call_original
       record_cleanup = Database::OldRecordCleanup.new(VCAP::CloudController::Event, 1)