[lwip-users] How to use LWIP_HTTPD_FS_ASYNC_READ

[Giuseppe Modugno]

When I receive the request GET /log.json I can't immediately build the response data. This because data are stored in another device and must be downloaded through a serial link.

In this scenario, I think I have to use LWIP_HTTPD_FS_ASYNC_READ and define four custom functions: fs_read_async_custom(), fs_canread_custom(), fs_wait_read_custom(). I tried in the following way:

int fs_open_custom(struct fs_file *file, const char *name) {
  if (!strcmp(name, "/log.json")) {
    file->data = "">    // So hs->left = 0 in http_init_file()
    file->len = 300;      // First assumption of data length (must be >0)
    file->index = 0;      // It is needed for fs_bytes_left()
    get_log_from_other_device();
    return 1;
  }
  return 0;
}

u8_t fs_canread_custom(struct fs_file *file) {
  return 1;
}

u8_t fs_wait_read_custom(struct fs_file *file, fs_wait_cb callback_fn, void *callback_arg) {
  return 0;
}

int fs_read_async_custom(struct fs_file *file, char *buffer, int count, fs_wait_cb callback_fn, void *callback_arg) {
  if (!log_complete()) return FS_READ_DELAYED;
  memcpy(buffer, log_data, count);
  file->index = file->len = count;   // So we are at the end of file
  return count;
}

I have to make some additional tests, but it seems it works. I have some doubts.

1. What is the exact purpose of fs_canread_custom() and fs_wait_read_custom()? In this configuration, they are called from fs_is_file_ready() for every file, custom and not, static and dynamic, asyncronous and not. For most of the files, fs_canread_custom() should returns 1 and fs_wait_read_custom() should return 0. When they should return a different value?
2. I don't know the content length in advance. So I have to start with a length that is surely greater than the final length, otherwise the browser stops receveing data before the end. However what happens if Content Length header says 300 and only 100 bytes were received? Chrome seems ok, but other browsers? Is there a standard behaviour? From RFC7230 it seems sending a Content Length value greater than real message body length isn't good:
   

      5.  If a valid Content-Length header field is present without
          Transfer-Encoding, its decimal value defines the expected message
          body length in octets.  If the sender closes the connection or
          the recipient times out before the indicated number of octets are
          received, the recipient MUST consider the message to be
          incomplete and close the connection.

   Is it possible to avoid sending Content Length header for log.json file? Consider I'm using dynamic headers.
3. I couldn't understand the purpose of callback_fn (and its callback_arg parameter). They are passed to fs_wait_read_custom() and fs_read_async_custom(). When this callback (that is http_continue function) should be called from those functions?