FD_CLOEXEC not set for all systems

[rittneje]

Currently this library only opens the database file with close-on-exec set if the kernel supports O_CLOEXEC (namely, modern versions of Linux).

libmaxminddb/src/maxminddb.c

Lines 413 to 416 in 5842352

	#ifdef O_CLOEXEC
	flags |= O_CLOEXEC;
	#endif
	int fd = open(mmdb->filename, flags);

However, some Unix systems, such as macos, do not support O_CLOEXEC. In such cases, this library needs to fall back to setting the flag via fcntl. While not as airtight as the atomic syscall in a multithreaded program, it is better than nothing.

Something like this should work.

#ifdef FD_CLOEXEC
int flags = fcntl(fd, F_GETFD);
if (flags >= 0) {
    fcntl(fd, F_SETFD, flags | FD_CLOEXEC);
}
#endif