Guard against a missing area page in result attachment generation

The issue here is result-attachment generation 500s with `undefined method
'<' for nil` whenever a template field area has no `page`. It's a
permitted-but-not-required area param, so an imported or API-built area can
omit it, and `pdf.pages[nil]` walks straight into HexaPDF's `nil < 0` at
page_tree_node.rb:96. The existing `next if page.nil?` guard runs one line too
late: the exception fires inside the `pages[...]` call itself.

This adds a nil-guard before the index and skips the area, mirroring the
surrounding `next if pdf.nil?` style. It logs a warning on the way out so a
genuinely malformed template doesn't silently drop a field from a signed
document without a trace. Adds a spec for the missing / nil / out-of-range
page cases.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: spaulsandhu

lib/submissions/generate_result_attachments.rb

@@ -194,6 +194,16 @@ def fill_submitter_fields(submitter, account, pdfs_index, with_signature_id:, is
 
           next if pdf.nil?
 
+          # HexaPDF's pages[nil] does `nil < 0` and raises. `page` is a
+          # permitted-but-not-required area param, so a malformed/imported area
+          # can omit it. Skip it, but log so silently-incomplete signed docs stay visible.
+          if area['page'].nil?
+            Rails.logger.warn(
+              "Skipping field area with no page (submitter=#{submitter.id}, attachment=#{area['attachment_uuid']})"
+            )
+            next
+          end
+
           page = pdf.pages[area['page']]
 
           next if page.nil?

spec/lib/submissions/generate_result_attachments_spec.rb

@@ -0,0 +1,81 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe Submissions::GenerateResultAttachments do
+  let(:account) { create(:account) }
+  let(:user) { create(:user, account:) }
+  let(:template) { create(:template, account:, author: user) }
+  let(:submission) { create(:submission, template:, created_by_user: user) }
+  let(:submitter) { create(:submitter, submission:, uuid: SecureRandom.uuid) }
+  let(:attachment_uuid) { SecureRandom.uuid }
+
+  # A single-page in-memory PDF keyed by the area's attachment_uuid, so the
+  # `pdf.nil?` guard passes and we reach the `pdf.pages[area['page']]` index.
+  let(:pdfs_index) do
+    doc = HexaPDF::Document.new
+    doc.pages.add
+    { attachment_uuid => doc }
+  end
+
+  # Point the submitter's submission at a single text field whose one area is
+  # `area`, so `fill_submitter_fields` reaches the page lookup for that area.
+  def set_field_area(area)
+    submitter.submission.update!(
+      template_fields: [
+        { 'uuid' => SecureRandom.uuid, 'submitter_uuid' => submitter.uuid,
+          'type' => 'text', 'areas' => [area] }
+      ]
+    )
+  end
+
+  def fill
+    described_class.send(
+      :fill_submitter_fields, submitter, account, pdfs_index,
+      with_signature_id: false, is_flatten: false, with_headings: false
+    )
+  end
+
+  before { allow(Rails.logger).to receive(:warn) }
+
+  describe '.fill_submitter_fields with a missing area page' do
+    context 'when the area omits the page key' do
+      before { set_field_area('attachment_uuid' => attachment_uuid) }
+
+      it 'skips the area instead of raising (regression: HexaPDF pages[nil])' do
+        expect { fill }.not_to raise_error
+      end
+
+      it 'logs that the area was skipped' do
+        fill
+        expect(Rails.logger).to have_received(:warn).with(/Skipping field area with no page/)
+      end
+    end
+
+    context 'when the area page is explicitly nil' do
+      before { set_field_area('attachment_uuid' => attachment_uuid, 'page' => nil) }
+
+      it 'skips the area instead of raising' do
+        expect { fill }.not_to raise_error
+      end
+
+      it 'logs that the area was skipped' do
+        fill
+        expect(Rails.logger).to have_received(:warn).with(/Skipping field area with no page/)
+      end
+    end
+
+    context 'when the area page is out of range' do
+      before { set_field_area('attachment_uuid' => attachment_uuid, 'page' => 5) }
+
+      it 'does not raise (existing next-if-page-nil handling still applies)' do
+        expect { fill }.not_to raise_error
+      end
+
+      it 'does not log a skipped-no-page warning (the guard only catches nil)' do
+        fill
+        expect(Rails.logger).not_to have_received(:warn).with(/Skipping field area with no page/)
+      end
+    end
+  end
+end