Fix indirect Fields on Interactive Form

lib/rubrik/document.rb

@@ -91,7 +91,15 @@ def add_signature_field
         modified_objects << {id: first_page_reference, value: new_first_page}
       end
 
-      (interactive_form[:Fields] ||= []) << signature_field_id
+      fields_entry = interactive_form[:Fields]
+      if fields_entry.is_a?(PDF::Reader::Reference)
+        new_fields_array = objects.fetch(fields_entry).dup
+        new_fields_array << signature_field_id
+
+        modified_objects << {id: fields_entry, value: new_fields_array}
+      else
+        (interactive_form[:Fields] ||= []) << signature_field_id
+      end
 
       signature_value_id
     end

test/rubrik/document_test.rb

@@ -189,5 +189,27 @@ def test_add_signature_field_with_indirect_annots
     ensure
       input&.close
     end
+
+    def test_add_signature_field_with_indirect_fields
+      # Arrange
+      input = File.open(SupportPDF["indirect_fields"], "rb")
+      document = Document.new(input)
+      initial_number_of_objects = document.modified_objects.size
+
+      # Act
+      result = document.add_signature_field
+
+      # Assert
+      number_of_added_objects = document.modified_objects.size - initial_number_of_objects
+      assert_equal(4, number_of_added_objects)
+
+      assert_pattern do
+        document.modified_objects => [{id: PDF::Reader::Reference, value: {Fields: fields_ref}}, *]
+        document.modified_objects => [*, {id: ^fields_ref, value: [signature_field_ref]}]
+        document.modified_objects => [*, {id: ^signature_field_ref, value: {FT: :Sig}}, *]
+      end
+    ensure
+      input&.close
+    end
   end
 end

test/rubrik/sign_test.rb

@@ -151,5 +151,41 @@ def test_document_with_inline_interactive_form
       input_pdf&.close
       expected_output&.close
     end
+
+    def test_document_with_indirect_fields
+      # Arrange
+      input_pdf = File.open(SupportPDF["indirect_fields"], "rb")
+      output_pdf = StringIO.new
+      certificate_file = File.open("test/support/demo_cert.pem", "rb")
+
+      private_key = OpenSSL::PKey::RSA.new(certificate_file, "")
+      certificate_file.rewind
+      certificate = OpenSSL::X509::Certificate.new(certificate_file)
+
+      # Act
+      Sign.call(input_pdf, output_pdf, private_key:, certificate:)
+
+      # Assert
+      expected_output = File.open(SupportPDF["indirect_fields.expected"], "rb")
+
+      expected_output.readlines.zip(output_pdf.readlines).each do |(expected_line, actual_line)|
+        # We must erase the signature because it is timestampped
+        if actual_line&.match?("/Type /Sig")
+          actual_line.sub!(/<[a-f0-9]+>/, "")
+          expected_line.sub!(/<[a-f0-9]+>/, "")
+        # The signature field name is also random
+        elsif actual_line&.match?(/Signature-[a-f0-9]{4}/)
+          actual_line.sub!(/Signature-[a-f0-9]{4}/, "")
+          expected_line.sub!(/Signature-[a-f0-9]{4}/, "")
+        end
+
+        assert_equal(expected_line, actual_line)
+        end
+    ensure
+      certificate_file&.close
+      output_pdf&.close
+      input_pdf&.close
+      expected_output&.close
+    end
   end
 end