The following code is constant-time.

#include <errno.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

#define _GNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h>

typedef void(* issue_t)(uint8_t*, size_t);

#if defined(__linux__) && defined(SYS_getrandom)
void issue(uint8_t *out, size_t outlen) {
  while(outlen > 0) {
    ssize_t ret = syscall(SYS_getrandom, out, outlen, 0);
    if((ret == -1) && (errno == EINTR)) {
      continue;
    }
    else if(ret == -1) {
      fprintf(stderr, "Error while executing syscall getrandom\n");
      abort();
    }
    out += ret;
    outlen -= ret;
  }
}
#else
void issue(uint8_t *out, size_t outlen) {
  int fd = -1;
  while(fd == -1) {
    fd = open("/dev/urandom", O_RDONLY);
    if((fd == -1) && (errno == EINTR)) {
      continue;
    }
    else if(fd == -1) {
      fprintf(stderr, "Error while opening /dev/urandom\n");
      abort();
    }
  }

  while(outlen > 0) {
    ssize_t ret = read(fd, out, outlen);
    if((ret == -1) && (errno == EINTR)) {
      continue;
    }
    else if(ret == -1) {
      close(fd);
      fprintf(stderr, "Error while reading from /dev/urandom\n");
      abort();
    }

    out += ret;
    outlen -= ret;
  }
  close(fd);
}
#endif

int main(void) {
  uint8_t out[1] = {0};
  size_t size = 1;
  // this test has no secret variables from the start, but the randomness from 
  // /dev/urandom or SYS_getrandom may be the input to some function generating
  // a secret 

  // declarations and markup here

  volatile issue_t func = issue;
  func(out, size);

  return 0;
}

See the compilation results with different optimization levels on GodBolt.