Handle ConnectionNotEstablished in the DB LB

The database load balancer would re-raise ConnectionNotEstablished
errors when encountering them. This can lead to user facing errors
whenever the application tries to connect to a new database that is not
(yet) online.

This was discovered while testing the changes for
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68042, after
adding a replica host that didn't exist.

Changelog: fixed

lib/gitlab/database/load_balancing.rb

@@ -4,8 +4,7 @@ module Gitlab
   module Database
     module LoadBalancing
       # The exceptions raised for connection errors.
-      CONNECTION_ERRORS = if defined?(PG)
-                            [
+      CONNECTION_ERRORS = [
         PG::ConnectionBad,
         PG::ConnectionDoesNotExist,
         PG::ConnectionException,
@@ -13,11 +12,11 @@ module Gitlab
         PG::UnableToSend,
         # During a failover this error may be raised when
         # writing to a primary.
-                              PG::ReadOnlySqlTransaction
+        PG::ReadOnlySqlTransaction,
+        # This error is raised when we can't connect to the database in the
+        # first place (e.g. it's offline or the hostname is incorrect).
+        ActiveRecord::ConnectionNotEstablished
       ].freeze
-                          else
-                            [].freeze
-                          end
 
       def self.proxy
         ActiveRecord::Base.load_balancing_proxy

lib/gitlab/database/load_balancing/host.rb

@@ -9,19 +9,12 @@ module Gitlab
 
         delegate :connection, :release_connection, :enable_query_cache!, :disable_query_cache!, :query_cache_enabled, to: :pool
 
-        CONNECTION_ERRORS =
-          if defined?(PG)
-            [
+        CONNECTION_ERRORS = [
           ActionView::Template::Error,
           ActiveRecord::StatementInvalid,
+          ActiveRecord::ConnectionNotEstablished,
           PG::Error
         ].freeze
-          else
-            [
-              ActionView::Template::Error,
-              ActiveRecord::StatementInvalid
-            ].freeze
-          end
 
         # host - The address of the database.
         # load_balancer - The LoadBalancer that manages this Host.

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

@@ -172,6 +172,14 @@ RSpec.describe Gitlab::Database::LoadBalancing::Host do
 
         expect(host).not_to be_online
       end
+
+      it 'returns false when ActiveRecord::ConnectionNotEstablished is raised' do
+        allow(host)
+          .to receive(:check_replica_status?)
+          .and_raise(ActiveRecord::ConnectionNotEstablished)
+
+        expect(host).not_to be_online
+      end
     end
   end
 

spec/lib/gitlab/database/load_balancing/load_balancer_spec.rb

@@ -277,31 +277,26 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer, :request_store do
   end
 
   describe '#connection_error?' do
-    before do
-      stub_const('Gitlab::Database::LoadBalancing::LoadBalancer::CONNECTION_ERRORS',
-                 [NotImplementedError])
-    end
-
     it 'returns true for a connection error' do
-      error = NotImplementedError.new
+      error = ActiveRecord::ConnectionNotEstablished.new
 
       expect(lb.connection_error?(error)).to eq(true)
     end
 
     it 'returns true for a wrapped connection error' do
-      wrapped = wrapped_exception(ActiveRecord::StatementInvalid, NotImplementedError)
+      wrapped = wrapped_exception(ActiveRecord::StatementInvalid, ActiveRecord::ConnectionNotEstablished)
 
       expect(lb.connection_error?(wrapped)).to eq(true)
     end
 
     it 'returns true for a wrapped connection error from a view' do
-      wrapped = wrapped_exception(ActionView::Template::Error, NotImplementedError)
+      wrapped = wrapped_exception(ActionView::Template::Error, ActiveRecord::ConnectionNotEstablished)
 
       expect(lb.connection_error?(wrapped)).to eq(true)
     end
 
     it 'returns true for deeply wrapped/nested errors' do
-      top = twice_wrapped_exception(ActionView::Template::Error, ActiveRecord::StatementInvalid, NotImplementedError)
+      top = twice_wrapped_exception(ActionView::Template::Error, ActiveRecord::StatementInvalid, ActiveRecord::ConnectionNotEstablished)
 
       expect(lb.connection_error?(top)).to eq(true)
     end