Create observable reports blob early

This will allow us to check for completion of the report later in case
websockets fail.

app/controllers/admin/reports_controller.rb

@@ -70,10 +70,12 @@ module Admin
           block: "start"
         ).broadcast
 
+      blob = ReportBlob.create_for_upload_later!(report_filename)
+
       ReportJob.perform_later(
         report_class:, user: spree_current_user, params:,
         format: report_format,
-        filename: report_filename,
+        blob:,
         channel: ScopedChannel.for_id(params[:uuid]),
       )
 

app/jobs/report_job.rb

@@ -9,12 +9,12 @@ class ReportJob < ApplicationJob
 
   NOTIFICATION_TIME = 5.seconds
 
-  def perform(report_class:, user:, params:, format:, filename:, channel: nil)
+  def perform(report_class:, user:, params:, format:, blob:, channel: nil)
     start_time = Time.zone.now
 
     report = report_class.new(user, params, render: true)
     result = report.render_as(format)
-    blob = ReportBlob.create!(filename, result)
+    blob.store(result)
 
     execution_time = Time.zone.now - start_time
 

app/models/report_blob.rb

@@ -5,7 +5,7 @@ class ReportBlob < ActiveStorage::Blob
   # AWS S3 limits URL expiry to one week.
   LIFETIME = 1.week
 
-  def self.create!(filename, content)
+  def self.create_locally!(filename, content)
     create_and_upload!(
       io: StringIO.new(content),
       filename:,
@@ -15,11 +15,34 @@ class ReportBlob < ActiveStorage::Blob
     )
   end
 
+  def self.create_for_upload_later!(filename)
+    # 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.
+    create_before_direct_upload!(
+      filename:,
+      byte_size: 0,
+      checksum: "0",
+      content_type: content_type(filename),
+      service_name: :local,
+    ).tap do |blob|
+      ActiveStorage::PurgeJob.set(wait: LIFETIME).perform_later(blob)
+    end
+  end
+
   def self.content_type(filename)
     MIME::Types.of(filename).first&.to_s || "application/octet-stream"
   end
 
+  def store(content)
+    io = StringIO.new(content)
+    upload(io, identify: false)
+    save!
+  end
+
   def result
+    return if checksum == "0"
+
     @result ||= download.force_encoding(Encoding::UTF_8)
   end
 

spec/jobs/report_job_spec.rb

@@ -6,13 +6,14 @@ RSpec.describe ReportJob do
   include CableReady::Broadcaster
 
   let(:report_args) {
-    { report_class:, user:, params:, format:, filename: }
+    { 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) { ReportBlob.create_for_upload_later!(filename) }
   let(:filename) { "report.csv" }
 
   it "generates a report" do
@@ -25,11 +26,12 @@ RSpec.describe ReportJob do
   it "enqueues a job for async processing" do
     expect {
       ReportJob.perform_later(**report_args)
-    }.not_to change { ActiveStorage::Blob.count }
+    }.not_to change { blob.checksum }
 
     expect {
       perform_enqueued_jobs(only: ReportJob)
-    }.to change { ActiveStorage::Blob.count }
+      blob.reload
+    }.to change { blob.checksum }
 
     expect_csv_report
   end
@@ -44,7 +46,8 @@ RSpec.describe ReportJob do
 
     expect {
       perform_enqueued_jobs(only: ReportJob)
-    }.to change { ActiveStorage::Blob.count }
+      blob.reload
+    }.to change { blob.checksum }
   end
 
   it "triggers an email when the report is done" do

spec/mailers/report_mailer_spec.rb

@@ -10,7 +10,7 @@ RSpec.describe ReportMailer do
         blob:,
       ).report_ready
     }
-    let(:blob) { ReportBlob.create!("customers.csv", "report content") }
+    let(:blob) { ReportBlob.create_locally!("customers.csv", "report content") }
 
     it "notifies about a report" do
       expect(email.subject).to eq "Report ready"

spec/models/report_blob_spec.rb

@@ -7,8 +7,16 @@ RSpec.describe ReportBlob, type: :model do
     content = "This works. ✓"
 
     expect do
-      blob = ReportBlob.create!("customers.html", content)
+      blob = ReportBlob.create_locally!("customers.html", content)
       content = blob.result
     end.not_to change { content.encoding }.from(Encoding::UTF_8)
   end
+
+  it "can be created first and filled later" do
+    blob = ReportBlob.create_for_upload_later!("customers.html")
+
+    expect { blob.store("Hello") }
+      .to change { blob.checksum }.from("0")
+      .and change { blob.result }.from(nil).to("Hello")
+  end
 end