Issue with /dev/zero fcntl/F_SETFL

[Jeremy Evans]

The following program:

#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

int main(int argc, char **argv) {
  int fd;
  char c;

  if ((fd = open("/dev/zero", 0)) < 0) {
    err(1, NULL);
  }
  printf("F_GETFL before: %i\n", fcntl(fd, F_GETFL));
  if ((fcntl(fd,F_SETFL, O_NONBLOCK)) == -1) {
    printf("F_GETFL after: %i\n", fcntl(fd, F_GETFL));
    err(1, NULL);
  }
  if ((read(fd, &c, 1)) < 0) {
    err(1, NULL);
  }

  return c;
}

has the following output:

F_GETFL before: 0
F_GETFL after: 4
test_zero_nonblock: Operation not supported by device

I think this may be a bug.  I'm guessing either:

1) fcntl(fd,F_SETFL, O_NONBLOCK) shouldn't fail, because
   open("/dev/zero", O_NONBLOCK) doesn't fail; or

2) fcntl should not modify the flags for the file descriptor if it does
   fail.

or both.

After more testing, it appears any fcntl/F_SETFL call on /dev/zero
fails, even if you are setting the flags to the same value.

One of ruby's specs fails due to this issue.

Thanks,
Jeremy