Store reports with Active Storage

This will enable us to offer download links and clean them up
automatically.

app/controllers/admin/reports_controller.rb

@@ -57,8 +57,9 @@ module Admin
 
     def render_report_as(format)
       if OpenFoodNetwork::FeatureToggle.enabled?(:background_reports, spree_current_user)
+        blob = ReportJob.create_blob!
         job = ReportJob.perform_later(
-          report_class, spree_current_user, params, format
+          report_class, spree_current_user, params, format, blob
         )
         Timeout.timeout(max_wait_time) do
           sleep 1 until job.done?

app/jobs/report_job.rb

@@ -1,15 +1,29 @@
 # frozen_string_literal: true
 
-# Renders a report and saves it to a temporary file.
+# Renders a report and stores it in a given blob.
 class ReportJob < ApplicationJob
-  def perform(report_class, user, params, format)
+  def self.create_blob!
+    # ActiveStorage discourages modifying a blob later but we need a blob
+    # before we know anything about the report file. It enables us to use the
+    # same blob in the controller to read the result.
+    ActiveStorage::Blob.create_before_direct_upload!(
+      filename: "tbd",
+      byte_size: 0,
+      checksum: "0",
+      content_type: "application/octet-stream",
+    ).tap do |blob|
+      ActiveStorage::PurgeJob.set(wait: 1.month).perform_later(blob)
+    end
+  end
+
+  def perform(report_class, user, params, format, blob)
     report = report_class.new(user, params, render: true)
     result = report.render_as(format)
-    write(result)
+    write(result, blob)
   end
 
   def done?
-    @done ||= File.file?(filename)
+    @done ||= blob.reload.checksum != "0"
   end
 
   def result
@@ -18,17 +32,17 @@ class ReportJob < ApplicationJob
 
   private
 
-  def write(result)
-    File.write(filename, result, mode: "wb")
+  def write(result, blob)
+    io = StringIO.new(result)
+    blob.upload(io, identify: false)
+    blob.save!
   end
 
   def read_result
-    File.read(filename)
-  ensure
-    File.unlink(filename)
+    blob.download
   end
 
-  def filename
-    Rails.root.join("tmp/report-#{job_id}")
+  def blob
+    arguments[4]
   end
 end

spec/jobs/report_job_spec.rb

@@ -3,12 +3,13 @@
 require 'spec_helper'
 
 describe ReportJob do
-  let(:report_args) { [report_class, user, params, format] }
+  let(:report_args) { [report_class, user, params, format, blob] }
   let(:report_class) { Reporting::Reports::UsersAndEnterprises::Base }
   let(:user) { enterprise.owner }
   let(:enterprise) { create(:enterprise) }
   let(:params) { {} }
   let(:format) { :csv }
+  let(:blob) { ReportJob.create_blob! }
 
   it "generates a report" do
     job = perform_enqueued_jobs(only: ReportJob) do