Rails 3 controller gets empty parameters, but only sometimes, after loading the ajax file

Rails 3.2.11

I need to upload a whole bunch of large files to a web service. I want to take files and slice them into smaller chunks by uploading them one at a time and then collecting them on the server.

Javascript (Coffee), work continues, but it sends chunks

class ChunkUploader
  constructor: (@file) ->
    console.debug('wooo')
    @chunkSize = 1024 * 512

  startUpload: ->
    console.debug('startUpload')
    this.postChunk(0)

  postChunk: (index) ->
    that = this
    console.debug('postChunk')
    offset = index * @chunkSize

    blob = @file.slice(offset, offset + @chunkSize)

    formData = new FormData()
    formData.append('utf8','✓')
    formData.append('authenticity_token', AUTH_TOKEN)
    formData.append('index', index)
    formData.append('offset', offset)
    formData.append('chunk_size', @chunkSize)
    formData.append('chunk', blob)

    $.ajax
      contentType: false
      processData: false
      cache: false
      url: $('#drop_zone').attr('data-action')
      type: 'POST'
      data: formData
      error: ->
        console.debug('error')
      success: ->
        if blob.size < that.chunkSize
          console.debug("I think we're done")
          return true
        else
          that.postChunk(index + 1)

# and then

form = document.getElementById 'drop_zone'
form.addEventListener 'drop', (e) ->
  e.stopPropagation()
  e.preventDefault()

  files = e.dataTransfer.files
  cu = new ChunkUploader(files[0])
  cu.startUpload()
  return false

      

        
        
        
      

    

The controller accepting the request is pretty simple so far:

def create
  head params[:chunk].nil? ? :internal_server_error : :ok
end

      

        
        
        
      

    

The part I'm stuck with is that sometimes it works and sometimes it just doesn't. The inspector in Chrome says the form has been submitted, but Rails refuses to provide me with the form data.

Then the log (local dev env) looks like this (notice how the parameters are missing on the second and 4th requests):

17:36:53 logger.1   | Started POST "/admin/products/testproduct/downloads" for 127.0.0.1 at 2013-02-15 17:36:53 +0800
17:36:53 logger.1   | Processing by Admin::DownloadsController#create as */*
17:36:53 logger.1   |   Parameters: {"utf8"=>"✓", "authenticity_token"=>"aQCyWZo3vADr5xUBNs1ECC8/bRPXtBxPCuMArXHbJVI=", "index"=>"3", "offset"=>"1572864", "chunk_size"=>"524288", "chunk"=>#<ActionDispatch::Http::UploadedFile:0x007fbedbfc2870 @original_filename="blob", @content_type="application/octet-stream", @headers="Content-Disposition: form-data; name=\"chunk\"; filename=\"blob\"\r\nContent-Type: application/octet-stream\r\n", @tempfile=#<File:/var/folders/t6/wbym5ghx3r3g3ch1wgl1fg640000gn/T/RackMultipart20130215-789-2ckjj9>>, "product_id"=>"testproduct"}
17:36:53 logger.1   | Completed 200 OK in 6ms
17:36:53 logger.1   | 
17:36:53 logger.1   | 
17:36:53 logger.1   | Started POST "/admin/products/testproduct/downloads" for 127.0.0.1 at 2013-02-15 17:36:53 +0800
17:36:53 logger.1   | Processing by Admin::DownloadsController#create as */*
17:36:53 logger.1   |   Parameters: {"product_id"=>"testproduct"}
17:36:53 logger.1   | Completed 500 Internal Server Error in 6ms
17:37:04 logger.1   | 
17:37:04 logger.1   | 
17:37:04 logger.1   | Started POST "/admin/products/testproduct/downloads" for 127.0.0.1 at 2013-02-15 17:37:04 +0800
17:37:04 logger.1   | Processing by Admin::DownloadsController#create as */*
17:37:04 logger.1   |   Parameters: {"utf8"=>"✓", "authenticity_token"=>"aQCyWZo3vADr5xUBNs1ECC8/bRPXtBxPCuMArXHbJVI=", "index"=>"0", "offset"=>"0", "chunk_size"=>"524288", "chunk"=>#<ActionDispatch::Http::UploadedFile:0x007fbedbfbe9a0 @original_filename="blob", @content_type="application/octet-stream", @headers="Content-Disposition: form-data; name=\"chunk\"; filename=\"blob\"\r\nContent-Type: application/octet-stream\r\n", @tempfile=#<File:/var/folders/t6/wbym5ghx3r3g3ch1wgl1fg640000gn/T/RackMultipart20130215-789-154zeln>>, "product_id"=>"testproduct"}
17:37:04 logger.1   | Completed 200 OK in 6ms
17:37:04 logger.1   | 
17:37:04 logger.1   | 
17:37:04 logger.1   | Started POST "/admin/products/testproduct/downloads" for 127.0.0.1 at 2013-02-15 17:37:04 +0800
17:37:04 logger.1   | Processing by Admin::DownloadsController#create as */*
17:37:04 logger.1   |   Parameters: {"product_id"=>"testproduct"}
17:37:04 logger.1   | Completed 500 Internal Server Error in 6ms

      

        
        
        
      

    

Sometimes the entire file will be downloaded. Sometimes he suffocates on the first segment. Sometimes he is torn between them. I have no idea why this is happening or where it breaks down. Why does my form sometimes appear to be empty even though Chrome insists that all the form data has been created?