100% CPU consumption with in multi interface by using CURL_WRITEFUNC_PAUSE

[piru]

I did this

Return CURL_WRITEFUNC_PAUSE from CURLOPT_WRITEFUNCTION to pause a transfer. Specifically:

/*
 * Demonstrate the application busyloop issue with multi interface when
 * CURLOPT_WRITEFUNCTION returns CURL_WRITEFUNC_PAUSE.
 *
 * While the application does request pausing of the transfer by itself
 * it should still not lead to 100% CPU consumption. For example Webkit
 * cURL backend does use this functionality to pause transfers, and does
 * trigger this issue.
 *
 * Written by Harry Sintonen <sintonen@iki.fi>. Heavily based on multi.c
 * example by  Daniel Stenberg, Copyright (C) 1998 - 2020, Daniel Stenberg,
 * <daniel@haxx.se>, et al.
 */

#include <stdio.h>
#include <string.h>

/* somewhat unix-specific */
#include <sys/time.h>
#include <unistd.h>

/* curl stuff */
#include <curl/curl.h>

static size_t writefunc(void *data, size_t size, size_t nmemb, void *userp)
{
  printf("data %p size %lu nmemb %lu userp %p\n", data, size, nmemb, userp);
  return CURL_WRITEFUNC_PAUSE;
}

int main(void)
{
  CURL *handle;
  CURLM *multi_handle;

  int still_running = 0; /* keep number of running handles */

  CURLMsg *msg; /* for picking up messages with the transfer status */
  int msgs_left; /* how many messages are left */

  /* Allocate one CURL handle */
  handle = curl_easy_init();

  /* set the options */
  curl_easy_setopt(handle, CURLOPT_URL, "https://curl.se/");
  curl_easy_setopt(handle, CURLOPT_VERBOSE, 1L);
  curl_easy_setopt(handle, CURLOPT_WRITEFUNCTION, writefunc);

  /* init a multi stack */
  multi_handle = curl_multi_init();

  /* add the transfer */
  curl_multi_add_handle(multi_handle, handle);

  /* we start some action by calling perform right away */
  curl_multi_perform(multi_handle, &still_running);

  while(still_running) {
    struct timeval timeout;
    int rc; /* select() return code */
    CURLMcode mc; /* curl_multi_fdset() return code */

    fd_set fdread;
    fd_set fdwrite;
    fd_set fdexcep;
    int maxfd = -1;

    long curl_timeo = -1;

    FD_ZERO(&fdread);
    FD_ZERO(&fdwrite);
    FD_ZERO(&fdexcep);

    /* set a suitable timeout to play around with */
    timeout.tv_sec = 1;
    timeout.tv_usec = 0;

    curl_multi_timeout(multi_handle, &curl_timeo);
    if(curl_timeo >= 0) {
      timeout.tv_sec = curl_timeo / 1000;
      if(timeout.tv_sec > 1)
        timeout.tv_sec = 1;
      else
        timeout.tv_usec = (curl_timeo % 1000) * 1000;
    }

    /* get file descriptors from the transfers */
    mc = curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd);

    if(mc != CURLM_OK) {
      fprintf(stderr, "curl_multi_fdset() failed, code %d.\n", mc);
      break;
    }

    /* On success the value of maxfd is guaranteed to be >= -1. We call
       select(maxfd + 1, ...); specially in case of (maxfd == -1) there are
       no fds ready yet so we call select(0, ...) --or Sleep() on Windows--
       to sleep 100ms, which is the minimum suggested value in the
       curl_multi_fdset() doc. */

    if(maxfd == -1) {
#ifdef _WIN32
      Sleep(100);
      rc = 0;
#else
      /* Portable sleep for platforms other than Windows. */
      struct timeval wait = { 0, 100 * 1000 }; /* 100ms */
      rc = select(0, NULL, NULL, NULL, &wait);
#endif
    }
    else {
      /* Note that on some platforms 'timeout' may be modified by select().
         If you need access to the original value save a copy beforehand. */
      rc = select(maxfd + 1, &fdread, &fdwrite, &fdexcep, &timeout);
    }

    switch(rc) {
    case -1:
      /* select error */
      break;
    case 0: /* timeout */
    default: /* action */
      curl_multi_perform(multi_handle, &still_running);
      break;
    }
  }

  /* See how the transfers went */
  while((msg = curl_multi_info_read(multi_handle, &msgs_left))) {
    if(msg->msg == CURLMSG_DONE) {
      /* Find out which handle this message is about */
      if(msg->easy_handle == handle) {
        printf("HTTP transfer completed with status %d\n", msg->data.result);
      }
    }
  }

  curl_multi_cleanup(multi_handle);

  /* Free the CURL handle */
  curl_easy_cleanup(handle);

  return 0;
}

I expected the following

Transfer being paused without the process bysylooping.

curl/libcurl version

curl 7.72.0 (x86_64-pc-linux-gnu) libcurl/7.72.0 OpenSSL/1.1.1i zlib/1.2.11 brotli/1.0.9 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) libssh2/1.8.0 nghttp2/1.42.0 librtmp/2.3
Release-Date: 2020-08-19
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtmp rtsp scp sftp smb smbs smtp smtps telnet tftp
Features: AsynchDNS brotli GSS-API HTTP2 HTTPS-proxy IDN IPv6 Kerberos Largefile libz NTLM NTLM_WB PSL SPNEGO SSL TLS-SRP UnixSockets

operating system

Linux hostname 5.9.0-5-amd64 #1 SMP Debian 5.9.15-1 (2020-12-17) x86_64 GNU/Linux

[bagder]

Reproduces for me.