Isolate report rendering in separate child process

Sidekiq doesn't have any features to limit memory usage or execution
time. We need a separate process for this. Forking avoids the boot time
of a fresh process and copy-on-write ensures minimal memory overheads.

.rubocop_todo.yml

@@ -695,6 +695,7 @@ Rails/ApplicationJob:
     - 'app/jobs/report_job.rb'
     - 'app/jobs/subscription_confirm_job.rb'
     - 'app/jobs/subscription_placement_job.rb'
+    - 'spec/services/job_processor_spec.rb'
 
 # Offense count: 1
 # This cop supports unsafe autocorrection (--autocorrect-all).

app/controllers/admin/reports_controller.rb

@@ -55,10 +55,11 @@ module Admin
 
     def render_report_as(format)
       if OpenFoodNetwork::FeatureToggle.enabled?(:background_reports, spree_current_user)
-        job = ReportJob.perform_later(
+        job = ReportJob.new
+        JobProcessor.perform_forked(
+          job,
           report_class, spree_current_user, params, format
         )
-        sleep 1 until job.done?
 
         # This result has been rendered by Rails in safe mode already.
         job.result.html_safe # rubocop:disable Rails/OutputSafety

app/services/job_processor.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+# Forks into a separate process to contain memory usage and timeout errors.
+class JobProcessor
+  def self.perform_forked(job, *args)
+    fork do
+      Process.setproctitle("Job worker #{job.job_id}")
+      job.perform(*args)
+
+      # Exit is not a good idea within a Rails process but Rubocop doesn't know
+      # that we are in a forked process.
+      exit # rubocop:disable Rails/Exit
+    end
+
+    Process.waitall
+  end
+end

spec/services/job_processor_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+class TestJob < ActiveJob::Base
+  def initialize
+    @file = Tempfile.new("test-job-result")
+    super
+  end
+
+  def perform(message)
+    @file.write(message)
+  end
+
+  def result
+    @file.rewind
+    @file.read
+  end
+end
+
+describe JobProcessor do
+  describe ".perform_forked" do
+    let(:job) { TestJob.new }
+
+    it "executes a job" do
+      JobProcessor.perform_forked(job, "hello")
+
+      expect(job.result).to eq "hello"
+    end
+  end
+end