active_storage: refactor concerns

Follow-up of #5953.

Refactor the concerns with two goals:

- Getting closer from the way ActiveStorage adds its own hooks.
  Usually ActiveStorage does this using an `Attachment#after_create`
  hook, which then delegates to the blob to enqueue the job.
- Enqueuing each job only once. By hooking on `Attachment#after_create`,
  we guarantee each job will be added only once.

We then let the jobs themselves check if they are relevant or not, and
retry or discard themselves if necessary.

We also need to update the tests a bit, because Rails'
`perform_enqueued_jobs(&block)` test helper doesn't honor the `retry_on`
clause of jobs. Instead it forwards the exception to the caller – which
makes the test fail.

Instead we use the inline version of `perform_enqueued_jobs()`, without
a block, which properly ignores errors catched by retry_on.

app/jobs/titre_identite_watermark_job.rb

@@ -1,9 +1,23 @@
 class TitreIdentiteWatermarkJob < ApplicationJob
+  class FileNotScannedYetError < StandardError
+  end
+
+  # If by the time the job runs the blob has been deleted, ignore the error
+  discard_on ActiveRecord::RecordNotFound
+  # If the file is deleted during the scan, ignore the error
+  discard_on ActiveStorage::FileNotFoundError
+  # If the file is not analyzed or scanned for viruses yet, retry later
+  # (to avoid modifying the file while it is being scanned).
+  retry_on FileNotScannedYetError, wait: :exponentially_longer, attempts: 10
+
   MAX_IMAGE_SIZE = 1500
   SCALE = 0.9
   WATERMARK = Rails.root.join("app/assets/images/#{WATERMARK_FILE}")
 
   def perform(blob)
+    if blob.virus_scanner.pending? then raise FileNotScannedYetError end
+    if blob.watermark_done? then return end
+
     blob.open do |file|
       watermark = resize_watermark(file)
 

app/jobs/virus_scanner_job.rb

@@ -1,15 +1,22 @@
 class VirusScannerJob < ApplicationJob
+  class FileNotAnalyzedYetError < StandardError
+  end
+
   queue_as :active_storage_analysis
 
   # If by the time the job runs the blob has been deleted, ignore the error
   discard_on ActiveRecord::RecordNotFound
   # If the file is deleted during the scan, ignore the error
   discard_on ActiveStorage::FileNotFoundError
-
+  # If the file is not analyzed yet, retry later (to avoid clobbering metadata)
+  retry_on FileNotAnalyzedYetError, wait: :exponentially_longer, attempts: 10
   # If for some reason the file appears invalid, retry for a while
   retry_on ActiveStorage::IntegrityError, attempts: 10, wait: 5.seconds
 
   def perform(blob)
+    if !blob.analyzed? then raise FileNotAnalyzedYetError end
+    if blob.virus_scanner.done? then return end
+
     metadata = extract_metadata_via_virus_scanner(blob)
     blob.update!(metadata: blob.metadata.merge(metadata))
   end

app/models/concerns/attachment_titre_identite_watermark_concern.rb

@@ -0,0 +1,17 @@
+# Request a watermark on files attached to a `Champs::TitreIdentiteChamp`.
+#
+# We're using a class extension here, but we could as well have a periodic
+# job that watermarks relevant attachments.
+module AttachmentTitreIdentiteWatermarkConcern
+  extend ActiveSupport::Concern
+
+  included do
+    after_create_commit :watermark_later
+  end
+
+  private
+
+  def watermark_later
+    blob&.watermark_later
+  end
+end

app/models/concerns/attachment_virus_scanner_concern.rb

@@ -0,0 +1,20 @@
+# Run a virus scan on all attachments after they are analyzed.
+#
+# We're using a class extension to ensure that all attachments get scanned,
+# regardless on how they were created. This could be an ActiveStorage::Analyzer,
+# but as of Rails 6.1 only the first matching analyzer is ever run on
+# a blob (and we may want to analyze the dimension of a picture as well
+# as scanning it).
+module AttachmentVirusScannerConcern
+  extend ActiveSupport::Concern
+
+  included do
+    after_create_commit :scan_for_virus_later
+  end
+
+  private
+
+  def scan_for_virus_later
+    blob&.scan_for_virus_later
+  end
+end

app/models/concerns/blob_titre_identite_watermark_concern.rb

@@ -1,38 +1,21 @@
-# Request a watermark on blobs attached to a `Champs::TitreIdentiteChamp`
-# after the virus scan has run.
-#
-# We're using a class extension here, but we could as well have a periodic
-# job that watermarks relevant attachments.
-#
-# The `after_commit` hook is triggered, among other cases, when
-# the analyzer or virus scan updates the blob metadata. When both the analyzer
-# and the virus scan have run, it is now safe to start the watermarking,
-# without  risking to replace the picture while it is being scanned in a
-# concurrent job.
 module BlobTitreIdentiteWatermarkConcern
-  extend ActiveSupport::Concern
-
-  included do
-    after_commit :enqueue_watermark_job
-  end
-
   def watermark_pending?
     watermark_required? && !watermark_done?
   end
 
+  def watermark_done?
+    metadata[:watermark]
+  end
+
+  def watermark_later
+    if watermark_required?
+      TitreIdentiteWatermarkJob.perform_later(self)
+    end
+  end
+
   private
 
   def watermark_required?
     attachments.any? { |attachment| attachment.record.class.name == 'Champs::TitreIdentiteChamp' }
   end
-
-  def watermark_done?
-    metadata[:watermark]
-  end
-
-  def enqueue_watermark_job
-    if analyzed? && virus_scanner.done? && watermark_pending?
-      TitreIdentiteWatermarkJob.perform_later(self)
-    end
-  end
 end

app/models/concerns/blob_virus_scanner_concern.rb

@@ -1,36 +1,21 @@
-# Run a virus scan on all blobs after they are analyzed.
-#
-# We're using a class extension to ensure that all blobs get scanned,
-# regardless on how they were created. This could be an ActiveStorage::Analyzer,
-# but as of Rails 6.1 only the first matching analyzer is ever run on
-# a blob (and we may want to analyze the dimension of a picture as well
-# as scanning it).
-#
-# The `after_commit` hook is triggered, among other cases, when
-# the analyzer updates the blob metadata. When the analyzer has run,
-# it is now safe to start our own scanning, without risking to have
-# two concurrent jobs overwriting the metadata of the blob.
 module BlobVirusScannerConcern
   extend ActiveSupport::Concern
 
   included do
     before_create :set_pending
-    after_commit :enqueue_virus_scan
   end
 
   def virus_scanner
     ActiveStorage::VirusScanner.new(self)
   end
 
+  def scan_for_virus_later
+    VirusScannerJob.perform_later(self)
+  end
+
   private
 
   def set_pending
-    self.metadata[:virus_scan_result] ||= ActiveStorage::VirusScanner::PENDING
-  end
-
-  def enqueue_virus_scan
-    if analyzed? && !virus_scanner.done?
-      VirusScannerJob.perform_later(self)
-    end
+    metadata[:virus_scan_result] ||= ActiveStorage::VirusScanner::PENDING
   end
 end