libcurl fd bug

example.c

/* Test program to verify that CURLMOPT_MAX_TOTAL_CONNECTIONS works as expected */

#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include <unistd.h>
#include <math.h>
#include <curl/curl.h>

/* Configure max concurrent connections */
#define MY_MAXCON 4

/* Count open connections from the FD's */
int print_multi_fd(CURLM *multi_handle){
    int max_fd, i, num_read = 0, num_write = 0, num_exc = 0;
    fd_set read_fd_set, write_fd_set, exc_fd_set;
    FD_ZERO(&read_fd_set);
    FD_ZERO(&write_fd_set);
    FD_ZERO(&exc_fd_set);
    curl_multi_fdset(multi_handle, &read_fd_set, &write_fd_set, &exc_fd_set, &max_fd);
    for (i = 0; i <= max_fd; i++){
      if (FD_ISSET(i, &read_fd_set)) num_read++;
      if (FD_ISSET(i, &write_fd_set)) num_write++;
      if (FD_ISSET(i, &exc_fd_set)) num_exc++;
    }
    fprintf(stderr, "Open Descriptors: READ %d, WRITE %d, EXC %d\n", num_read, num_write, num_exc);
    return num_read;
}

int main(void){
  int ret = 0;
  CURLM *multi_handle;
  FILE *output = fopen("/dev/null", "w");

  curl_global_init(CURL_GLOBAL_DEFAULT);

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

  /* disable multiplexing so request count matches connection count */
  curl_multi_setopt(multi_handle, CURLMOPT_PIPELINING, CURLPIPE_NOTHING);
  curl_multi_setopt(multi_handle, CURLMOPT_MAX_TOTAL_CONNECTIONS, (long) MY_MAXCON);
  curl_multi_setopt(multi_handle, CURLMOPT_MAX_HOST_CONNECTIONS, (long) MY_MAXCON);

  /* Add some requests */
  for(int i = 7; i >= 0; i--){
    char url[100] = "";
    sprintf(url, "https://eu.httpbin.org/delay/%d", i);
    CURL *http_handle = curl_easy_init();
    curl_easy_setopt(http_handle, CURLOPT_URL, url);
    curl_easy_setopt(http_handle, CURLOPT_WRITEDATA, output);
    curl_multi_add_handle(multi_handle, http_handle);
  }

  /* we start some action by calling perform right away */
  int running_handles = 0; /* keep number of running handles */
  do {
    curl_multi_perform(multi_handle, &running_handles);
    int msgq = 0;
    do {
      CURLMsg *m = curl_multi_info_read(multi_handle, &msgq);
      if(m && (m->msg == CURLMSG_DONE)){
        long res_status;
        curl_easy_getinfo(m->easy_handle, CURLINFO_RESPONSE_CODE, &res_status);
        fprintf(stderr, "Req complete (HTTP %ld). Remaining: %d (max con: %d)\n", res_status, running_handles, MY_MAXCON);
        int expected_con = fmin(running_handles, MY_MAXCON);
        int num_read = print_multi_fd(multi_handle);
        if(num_read != expected_con){
          fprintf(stderr, "FAIL: read_fd_set does not have expected length %d!\n", expected_con); 
          ret = 1;
        }
        curl_multi_remove_handle(multi_handle, m->easy_handle);
        curl_easy_cleanup(m->easy_handle);
      }
    } while(msgq > 0);
  } while(running_handles > 0);

  curl_multi_cleanup(multi_handle);
  curl_global_cleanup();
  if(!ret)
    fprintf(stderr, "SUCCESS! All good!\n");
  return ret;
}