Fix DB LB errors when escaping input

The Host class uses the pattern `connection.quote(input)` to quote
certain forms of input before passing these to queries. Rails will
unfortunately try to acquire a database connection when running such
code, resulting in errors should the database not be available.

This commit fixes this by simply catching the connection errors that may
be produced.

ee/changelogs/unreleased/load-balancing-connection-quoting.yml

+---
+title: Fix DB LB errors when escaping input
+merge_request:
+author:
+type: fixed

ee/lib/gitlab/database/load_balancing/host.rb

@@ -115,6 +115,8 @@ module Gitlab
           SQL
 
           row['diff'].to_i if row.any?
+        rescue *CONNECTION_ERRORS
+          nil
         end
 
         def primary_write_location
@@ -142,6 +144,8 @@ module Gitlab
           row = query_and_release(query)
 
           row['result'] == 't'
+        rescue *CONNECTION_ERRORS
+          false
         end
 
         def query_and_release(sql)

ee/spec/lib/gitlab/database/load_balancing/host_spec.rb

@@ -239,6 +239,14 @@ describe Gitlab::Database::LoadBalancing::Host, :postgresql do
 
       expect(host.replication_lag_size).to be_nil
     end
+
+    it 'returns nil when the database connection fails' do
+      allow(host)
+        .to receive(:connection)
+        .and_raise(ActionView::Template::Error.new('boom', StandardError.new))
+
+      expect(host.replication_lag_size).to be_nil
+    end
   end
 
   describe '#primary_write_location' do
@@ -268,6 +276,14 @@ describe Gitlab::Database::LoadBalancing::Host, :postgresql do
 
       expect(host.caught_up?('foo')).to eq(false)
     end
+
+    it 'returns false when the connection fails' do
+      allow(host)
+        .to receive(:connection)
+        .and_raise(ActionView::Template::Error.new('boom', StandardError.new))
+
+      expect(host.caught_up?('foo')).to eq(false)
+    end
   end
 
   describe '#query_and_release' do