nginx-lua: upload request-body to other service with LuaSocket

error.log

2015/12/21 20:31:38 [error] 7922#0: *70 lua entry thread aborted: runtime error: attempt to yield across C-call boundary
stack traceback:
coroutine 0:
	[C]: in function 'request'
	/opt/nginx/scripts/upload4.lua:45: in function </opt/nginx/scripts/upload4.lua:1>, client: 127.0.0.1, server: localhost, request: "POST /upload4 HTTP/1.0", host: "localhost"

see:
http://stackoverflow.com/questions/30111808/luasocket-nginx-error-lua-entry-thread-aborted-runtime-error-attempt-to-yi

upload_body.lua

-- using https://github.com/diegonehab/luasocket
local HTTP  = require("socket.http")
local LTN12 = require("ltn12")

-- source for ngx.req.socket()
function source_tcpsock(handle, bytes)
  bytes = tonumber(bytes)
  if handle then
      return function()
          if bytes <= 0 then
            return nil
          end
          local rsize = 2048
          if bytes < rsize then
            rsize = bytes
          end
          local chunk, err, partial = handle:receive(rsize)
          if err then
            ngx.log(ngx.ERR, 'failed to receive request body - ' .. err)
            ngx.exit(500)
          end
          if not chunk then
            chunk = partial
          end
          if chunk then
            bytes = bytes - #chunk
          end
          return chunk
      end
  else
    return nil
  end
end

-- upload uri
local post_uri = 'http://localhost:9292/teapot'

-- copy headers
local headers = ngx.req.get_headers()
local reqtype = headers['content-type'] or 'application/json'
local reqsize = headers['content-length'] or '0'

-- upload
local resbody = {}
local result, rescode, resheaders, resstatus = HTTP.request {
  method = 'POST',
  url = post_uri,
  source = source_tcpsock(ngx.req.socket(), reqsize),
  headers = {
    ['Content-Type'] = reqtype,
    ['Content-Length'] = reqsize
  },
  sink = LTN12.sink.table(resbody)
}

-- make response
if result then
  ngx.status = rescode
  ngx.header.content_type = resheaders['content-type']
  ngx.say(table.concat(resbody))
else
  ngx.exit(500)
end