Update parsing of incoming emails

Ensure we don't ever replace attachments which have been hidden using
prominence with newly parsed attachments.

This could in theory happen if the hexdigest which is based on the
attachment body changes. There have been occasions this has happened to
due to changes in the underlying mail gem and other parsing changes.

See: #7875

app/models/incoming_message.rb

@@ -41,6 +41,8 @@ class IncomingMessage < ApplicationRecord
   include CacheAttributesFromRawEmail
   include Taggable
 
+  UnableToExtractAttachments = Class.new(StandardError)
+
   MAX_ATTACHMENT_TEXT_CLIPPED = 1_000_000 # 1Mb ish
 
   belongs_to :info_request,
@@ -470,8 +472,18 @@ def extract_attachments
       attachments += _uudecode_attachments(main_part.body, c)
     end
 
-    # Purge old attachments that have been rebuilt with a new hexdigest
-    (foi_attachments - attachments).each(&:mark_for_destruction)
+    # Purge old public attachments that will be rebuilt with a new hexdigest
+    old_attachments = (foi_attachments - attachments)
+    hidden_old_attachments = old_attachments.reject { _1.is_public? }
+
+    if hidden_old_attachments.any?
+      # if there are hidden attachments error as we don't want to re-build and
+      # lose the prominence as this will make them public
+      raise UnableToExtractAttachments, "due to prominence of attachments " \
+        "(ID=#{hidden_old_attachments.map(&:id).join(', ')})"
+    else
+      old_attachments.each(&:mark_for_destruction)
+    end
   end
 
   # Returns body text as HTML with quotes flattened, and emails removed.

spec/models/incoming_message_spec.rb

@@ -1087,6 +1087,23 @@ def populate_raw_email(fixture)
     expect(im._get_attachment_text_internal.valid_encoding?).to be true
   end
 
+  it 'does not raise error if existing hidden attachment will be retained' do
+    incoming_message = FactoryBot.create(:incoming_message)
+    foi_attachment = incoming_message.foi_attachments.first
+    foi_attachment.update(prominence: 'hidden')
+
+    expect { incoming_message.extract_attachments! }.to_not raise_error
+  end
+
+  it 'raises error if existing hidden attachment will be deleted' do
+    incoming_message = FactoryBot.create(:incoming_message)
+    foi_attachment = incoming_message.foi_attachments.first
+    foi_attachment.update(prominence: 'hidden', hexdigest: '123')
+
+    expect { incoming_message.extract_attachments! }.to raise_error(
+      IncomingMessage::UnableToExtractAttachments
+    )
+  end
 end
 
 RSpec.describe IncomingMessage, 'when getting the body of a message for html display' do