glibc dns timeout reproduction

curl_parallel.c

#include <curl/curl.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define NUM_THREADS 20

static pthread_mutex_t outlock = PTHREAD_MUTEX_INITIALIZER;
static int skip_dns = 0;

struct write_cb_data {
  char buf[11];
  size_t buf_written;
};

size_t write_cb(char *ptr, size_t elsize, size_t nmeb, void *ctxarg) {
  size_t sz = elsize * nmeb;
  struct write_cb_data *ctx = ctxarg;
  size_t to_write = sz;
  if (to_write > sizeof(ctx->buf) - 1 - ctx->buf_written) {
    to_write = sizeof(ctx->buf) - 1 - ctx->buf_written;
  }
  if (to_write > 0) {
    memcpy(ctx->buf + ctx->buf_written, ptr, to_write);
  }
  ctx->buf_written += to_write;
  return sz;
}



void *worker(void *ctx) {
  int worker_nr = *((int *)ctx);

  pthread_mutex_lock(&outlock);
  printf("[%d] starting\n", worker_nr);
  pthread_mutex_unlock(&outlock);

  CURL *curl = curl_easy_init();
  if (!curl) {
    pthread_mutex_lock(&outlock);
    printf("[%d] curl_easy_init failed\n", worker_nr);
    pthread_mutex_unlock(&outlock);
    return NULL;
  }

  char errbuf[CURL_ERROR_SIZE] = {0};
  struct write_cb_data writectx = {0};

  curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
  struct curl_slist *dns_cache = curl_slist_append(NULL, "example.com:80:93.184.216.34");
  if (skip_dns) {
    curl_easy_setopt(curl, CURLOPT_RESOLVE, dns_cache);
  }
  curl_easy_setopt(curl, CURLOPT_ERRORBUFFER, errbuf);
  curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_cb);
  curl_easy_setopt(curl, CURLOPT_WRITEDATA, &writectx);

  CURLcode res = curl_easy_perform(curl);
  curl_slist_free_all(dns_cache);
  if (res != CURLE_OK) {
    pthread_mutex_lock(&outlock);
    printf("[%d] curl_easy_perform failed: %d (%s)\n", worker_nr, res, errbuf);
    pthread_mutex_unlock(&outlock);
    return NULL;
  }

  pthread_mutex_lock(&outlock);
  printf("[%d] output: %s\n", worker_nr, writectx.buf);
  pthread_mutex_unlock(&outlock);
  return NULL;
}

int main(int argc, char **argv) {
  curl_global_init(CURL_GLOBAL_NOTHING);

  if (argc > 1 && strcmp(argv[1], "--skip-dns") == 0) {
    skip_dns = 1;
  }
  printf("skip_dns: %d\n", skip_dns);

  printf("PID is %d: hit enter to start", getpid());
  fflush(stdout);
  while (getchar() != '\n') {}



  pthread_t threads[NUM_THREADS] = {0};
  int ids[NUM_THREADS];
  for (int i = 0; i < NUM_THREADS; i++) {
    ids[i] = i;
    int r = pthread_create(&threads[i], NULL, worker, &ids[i]);
    if (r != 0) {
      pthread_mutex_lock(&outlock);
      char errbuf[256];
      printf("pthread_create %d failed: %s\n", i, strerror_r(r, errbuf, sizeof(errbuf)));
      pthread_mutex_unlock(&outlock);
      fflush(stdout);
      abort();
    }
  }
  for (int i = 0; i < NUM_THREADS; i++) {
    int r = pthread_join(threads[i], NULL);
    if (r != 0) {
      pthread_mutex_lock(&outlock);
      char errbuf[256];
      printf("pthread_join %d failed: %s\n", i, strerror_r(r, errbuf, sizeof(errbuf)));
      pthread_mutex_unlock(&outlock);
      fflush(stdout);
      abort();
    }
    pthread_mutex_lock(&outlock);
    printf("[%d] joined\n", i);
    pthread_mutex_unlock(&outlock);
  }
  printf("hit enter to exit");
  fflush(stdout);
  while (getchar() != '\n') {}

  return 0;
}