FTP download fails using libcurl when TCP_FASTOPEN enabled

[KlausC]

I found, that the ftp example program, when modified by setting the TCP_FASTOPEN flag, fails.
My working environment is Fedora Linux release 33, but I think that is also for other Linux variants.

Linux klauspc 5.9.8-200.fc33.x86_64 #1 SMP Tue Nov 10 21:58:19 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

[crusius@klauspc src]$ curl-config --version
libcurl 7.71.1

[crusius@klauspc src]$ cc curlftp.c -lcurl -o curlftp

[crusius@klauspc src]$ curlftp --tcp-fastopen "ftp://xmlsoft.org/libxslt/libxslt-1.1.33.tar.gz"
*   Trying 91.121.203.120:21...
* Connected to xmlsoft.org () port 21 (#0)
< 220 (vsFTPd 2.2.2)
> USER anonymous
< 331 Please specify the password.
> PASS ftp@example.com
< 230 Login successful.
> PWD
< 257 "/"
* Entry path is '/'
> CWD libxslt
* ftp_perform ends with SECONDARY: 0
< 250 Directory successfully changed.
> EPSV
* Connect data stream passively
< 229 Entering Extended Passive Mode (|||13558|).
* Could not resolve host: 
* Can't resolve new host :13558
* Closing connection 0
curl told us 15

# For comparison with unset flag:
[crusius@klauspc src]$ curlftp "ftp://xmlsoft.org/libxslt/libxslt-1.1.33.tar.gz"
*   Trying 91.121.203.120:21...
* Connected to xmlsoft.org (91.121.203.120) port 21 (#0)
< 220 (vsFTPd 2.2.2)
> USER anonymous
< 331 Please specify the password.
> PASS ftp@example.com
< 230 Login successful.
> PWD
< 257 "/"
* Entry path is '/'
> CWD libxslt
* ftp_perform ends with SECONDARY: 0
< 250 Directory successfully changed.
> EPSV
* Connect data stream passively
< 229 Entering Extended Passive Mode (|||53060|).
*   Trying 91.121.203.120:53060...
* Connecting to 91.121.203.120 (91.121.203.120) port 53060
* Connected to xmlsoft.org (91.121.203.120) port 21 (#0)
> TYPE I
< 200 Switching to Binary mode.
> SIZE libxslt-1.1.33.tar.gz
< 213 3444093
> RETR libxslt-1.1.33.tar.gz
< 150 Opening BINARY mode data connection for libxslt-1.1.33.tar.gz (3444093 bytes).
* Maxdownload = -1
* Getting file with size: 3444093
* Remembering we are in dir "libxslt/"
< 226 Transfer complete.
* Connection #0 to host xmlsoft.org left intact


[crusius@klauspc src]$ cat curlftp.c

/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
 *
 * This software is licensed as described in the file COPYING, which
 * you should have received as part of this distribution. The terms
 * are also available at https://curl.se/docs/copyright.html.
 *
 * You may opt to use, copy, modify, merge, publish, distribute and/or sell
 * copies of the Software, and permit persons to whom the Software is
 * furnished to do so, under the terms of the COPYING file.
 *
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 * KIND, either express or implied.
 *
 ***************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <curl/curl.h>

/* <DESC>
 * Get a single file from an FTP server.
 * </DESC>
 */

struct FtpFile {
  const char *filename;
  FILE *stream;
};

static size_t my_fwrite(void *buffer, size_t size, size_t nmemb, void *stream)
{
  struct FtpFile *out = (struct FtpFile *)stream;
  if(!out->stream) {
    /* open file for writing */
    out->stream = fopen(out->filename, "wb");
    if(!out->stream)
      return -1; /* failure, can't open file to write */
  }
  return fwrite(buffer, size, nmemb, out->stream);
}


int main(int argc, char**argv)
{
  CURL *curl;
  CURLcode res;
  struct FtpFile ftpfile = {
    "curl.tar.gz", /* name to store the file as if successful */
    NULL
  };

  curl_global_init(CURL_GLOBAL_DEFAULT);

  long tcpfast = 0L;
  if( argc >= 3 && strcmp(argv[1], "--tcp-fastopen") == 0) {
    tcpfast = 1L;
    argc -= 1;
    argv += 1;
  }
  if (argc < 2) exit(1);

  curl = curl_easy_init();
  if(curl) {
    /*
     * You better replace the URL with one that works!
     */
    curl_easy_setopt(curl, CURLOPT_TCP_FASTOPEN, tcpfast);
    curl_easy_setopt(curl, CURLOPT_URL, argv[1]);

    /* Define our callback to get called when there's data to be written */
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, my_fwrite);
    /* Set a pointer to our struct to pass to the callback */
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &ftpfile);

    /* Switch on full protocol/debug output */
    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);

    res = curl_easy_perform(curl);

    /* always cleanup */
    curl_easy_cleanup(curl);

    if(CURLE_OK != res) {
      /* we failed */
      fprintf(stderr, "curl told us %d\n", res);
    }
  }

  if(ftpfile.stream)
    fclose(ftpfile.stream); /* close the local file */

  curl_global_cleanup();

  return 0;
}

[KlausC]

With the current version (7.74), compiled with -g -Og, the same run fails earlier:

 curlftp --tcp-fastopen "ftp://xmlsoft.org/libxslt/libxslt-1.1.33.tar.gz"
*   Trying 91.121.203.120:21...
* Connected to xmlsoft.org () port 21 (#0)
< 220 (vsFTPd 2.2.2)
* Send failure: Operation already in progress
* Closing connection 0
curl told us 55

[bagder]

I don't see how TCP Fast Open will help for an FTP transfer so I would recommend you don't enable that, as a rather safe work-around.

[KlausC]

I think we will disable that in our downstream library wrapper. See Julia: JuliaLang/Downloads.jl#76

[bagder]

That was wrong, the commit only fixed #6262, this should rather be fixed by #6265 ...