CHOUETTE-2358 Change query syntax to avoid multiple rows in the Delay…

…ed Job reserve subquery

Approach described into collectiveidea/delayed_job_active_record#169

app/lib/delayed/uniq_reservation.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+module Delayed
+  # Used to customize Delayed::Backend::ActiveRecord::Job
+  #
+  # Fixes PostgreSQL SQL to ensure reservation uniqueness
+  # See https://github.com/collectiveidea/delayed_job_active_record/pull/169
+  module UniqReservation
+    def self.prepended(base)
+      class << base
+        prepend ClassMethods
+      end
+    end
+
+    module ClassMethods # rubocop:disable Style/Documentation
+      def reserve_with_scope_using_optimized_postgres(ready_scope, worker, now)
+        # Custom SQL required for PostgreSQL because postgres does not support UPDATE...LIMIT
+        # This locks the single record 'FOR UPDATE' in the subquery
+        # http://www.postgresql.org/docs/9.0/static/sql-select.html#SQL-FOR-UPDATE-SHARE
+        # Note: active_record would attempt to generate UPDATE...LIMIT like
+        # SQL for Postgres if we use a .limit() filter, but it would not
+        # use 'FOR UPDATE' and we would have many locking conflicts
+        quoted_name = connection.quote_table_name(table_name)
+        subquery    = ready_scope.limit(1).lock(true).select('id').to_sql
+        sql         = "UPDATE #{quoted_name} SET locked_at = ?, locked_by = ? WHERE id = (#{subquery}) RETURNING *"
+        reserved    = find_by_sql([sql, now, worker.name])
+        reserved[0]
+      end
+    end
+  end
+end

config/initializers/delayed_job.rb

@@ -5,6 +5,7 @@
 # Add organisation management for Job
 class Delayed::Job # rubocop:disable Style/ClassAndModuleChildren(RuboCop)
   prepend Delayed::WithOrganisation
+  prepend Delayed::UniqReservation
 end
 
 # Add :dead_worker hook