adjust generate previews

lib/templates/process_document.rb

@@ -90,23 +90,28 @@ module Templates
 
       pool = Concurrent::FixedThreadPool.new(concurrency)
 
-      promises =
-        range.map do |page_number|
-          doc_page = doc.get_page(page_number)
+      blobs =
+        range.each_slice(concurrency).flat_map do |page_numbers|
+          promises =
+            page_numbers.map do |page_number|
+              doc_page = doc.get_page(page_number)
 
-          hide_placeholder_widgets(doc_page, hide_empty: !flatten_pages)
-          doc_page.flatten if flatten_pages
+              hide_placeholder_widgets(doc_page, hide_empty: !flatten_pages)
+              doc_page.flatten if flatten_pages
 
-          bytes, width, height = doc_page.render_to_bitmap(width: MAX_WIDTH)
+              bytes, width, height = doc_page.render_to_bitmap(width: MAX_WIDTH)
 
-          image = Vips::Image.new_from_memory_copy(bytes, width, height, 4, :uchar)
+              image = Vips::Image.new_from_memory_copy(bytes, width, height, 4, :uchar)
 
-          Concurrent::Promise.execute(executor: pool) { build_and_upload_blob(image, page_number) }
-        ensure
-          doc_page&.close
+              Concurrent::Promise.execute(executor: pool) { build_and_upload_blob(image, page_number) }
+            ensure
+              doc_page&.close
+            end
+
+          Concurrent::Promise.zip(*promises).value!
         end
 
-      Concurrent::Promise.zip(*promises).value!.each do |blob|
+      blobs.each do |blob|
         next unless blob
 
         ApplicationRecord.no_touching do