Export Filter - processing in batches

Question

I'm writing an export filter that uses an external tool to process the photos. The tool can/should be multithreaded and I want to take advantage of this by processing the images in groups of 2 or 4 or 8 (or whatever). This means that I gather the paths and the renditions to satisfy in a table and once the table reaches the desired size I process the images in the batch together, empty the table and continue.
For some reason I can't make this work. To illustrate, I've reduced this problem to this scenario (Example A)
1. While iterating filterContext:renditions() I copy the paths and renditionToSatisify to a table
2. Iterate over the table of copies and process the images one by one.
When running this, I'm getting this error:
This plug-in’s post-processing task did not finish successfully.
?:0: attempt to index field 'exportSettings' (a nil value)
It seems to me that LR doesn't like me to call "renditionIsDone" out of sequence but I don't know what to make of this.
I'm also adding "Example B" to show that if I call processOne right after adding its entry to the table all is well.
Example A
function processOne(table)
     local commandResult
     commandResult = PluginManager.doit({table.a})
     if commandResult ~= 0 then
         table.b:renditionIsDone( false, "Image Processing Failed (" ..  commandResult .. ")")
     else
         table.b:renditionIsDone (true,"very good indeed")
     end
end
function JPEGminiFilterProvider.postProcessRenderedPhotos( functionContext, filterContext )
  local renditionOptions = {
  plugin = _PLUGIN,
  renditionsToSatisfy = filterContext.renditionsToSatisfy,
  filterSettings = function( renditionToSatisfy, exportSettings )
  end,
  local maxGroupSize = 1
  local groupSize = 0
  local group = {}
  for sourceRendition, renditionToSatisfy in filterContext:renditions( renditionOptions ) do
  local success, pathOrMessage = sourceRendition:waitForRender()
  if success then
  groupSize = groupSize + 1
  group[groupSize]={a=pathOrMessage, b=renditionToSatisfy}
  end
  end
  for i,v in ipairs(group) do
  processOne(v)
  end
end
Example B
function JPEGminiFilterProvider.postProcessRenderedPhotos( functionContext, filterContext )
     local renditionOptions = {
         plugin = _PLUGIN,
         renditionsToSatisfy = filterContext.renditionsToSatisfy,
         filterSettings = function( renditionToSatisfy, exportSettings )
         end,
     local groupSize = 0
     local group = {}
     for sourceRendition, renditionToSatisfy in filterContext:renditions( renditionOptions ) do
         local success, pathOrMessage = sourceRendition:waitForRender()
         if success then
               groupSize = groupSize + 1
               group[groupSize]={a=pathOrMessage, b=renditionToSatisfy}
               processOne(group[groupSize])
         end
     end
end

Answer 1

I dunno the problem, but consider naming parameters something besides 'table' since that masks the global lua table object.
Rob