Hack to reduce memory overhead when sending a file via post

postfile.nim

proc lightformat(p: var MultipartData): string =
  if p == nil or p.content.len == 0:
    return

    # Create boundary that is not in the data to be formatted
  var bound: string
  while true:
    bound = $random(int.high)
    var found = false
    for s in p.content:
      if bound in s:
        found = true
    if not found:
      break

  result = "multipart/form-data; boundary=" & bound
  for i in 0 ..< p.content.len:
    p.content[i] = "--" & bound & "\c\L" & p.content[i] & "\c\L"
  p.content.add("--" & bound & "--\c\L")

proc postFile*(client: HttpClient, url: string, filePath: string, name: string,
               multipart: var MultipartData): Response =
  var m = newMimetypes()
  var contentType: string
  let (_, fName, ext) = splitFile(filePath)
  if ext.len > 0:
    contentType = m.getMimetype(ext[1..ext.high], "")

  ## similar behaviour to the addFiles proc
  var str = "Content-Disposition: form-data; name=\"" & name &
    "\";filename=\"" & fName & ext &
    "\"\c\LContent-Type: " & contentType & "\c\L\c\L"

  multipart.content.add(str)
  multipart.content[^1] &= readFile(filePath)
  ## `str & readFile(..)` allocates the contents twice

  let mpContentType = lightformat(multipart)
  let xb = multipart.content.join("")

  var headers = newHttpHeaders()
  if multipart != nil:
    headers["Content-Type"] = mpContentType
  headers["Content-Length"] = $len(xb)

  result = client.requestAux(url, $HttpPOST, xb, headers)