feat: adds some threads

[niquerio]

Adds a basic threadpool to speed up all of the calls to worldcat

@billdueber Mostly wanting you to look over the thread pool I put in for doing a bazillion calls to the worldcat api. Do I need to do anything to keep it from getting out of control? is 5 threads too many?

[billdueber]

Your thread code isn't safe -- the gap between barcode_queue.empty? and barcode_queue.shift is asking for a deadlock, because between this thread calling barcode_queue.empty? another thread might have emptied the queue, and it'll wait there forever.

You don't have any shared state, so it seems like it might be a good use for the in_threads gem. I think you can just do something like:

results = @barcodes.in_threads.map do |barcode|
  umich_item = @umich_catalog_items.item_for_barcode(barcode)
  if umich_item.nil?
    nil 
  else
    worldcat_summary = WorldCatSummary.for(umich_item.oclc || [])
    format_line(umich_item: umich_item, worldcat_summary: worldcat_summary)
  end
end
results.compact! # get rid of the nils

If you really just need to print them, skip the map and just call in_threads.each , replacing the if statement with the next if umich_item.nil? you had before.