is_valid_fd() uses dup() or fstat() to check whether an fd is valid. Both operations are costly. 

fcntl() with F_GETFD returns the file descriptor flags (e.g. CLOEXEC) and -1 with errno set to EBADF when fd is not an open file descriptor. It's faster than duplicating + closing a fd or calling fstat(). The idea to use fcntl(fd, F_GETFD) is inspired by the patch [1]. According to Stackoverflow [2]:

> fcntl(fd, F_GETFD) is the canonical cheapest way to check that fd is a valid open file descriptor.
> F_GETFD is cheaper in principle since it only dereferences the (process-local) file descriptor in kernel space, not the underlying open file description (process-shared) which it refers to.

[1] https://github.com/singlestore-labs/cpython/commit/0364554615c79b9364a0acf3038147a999ea2219

[2] https://stackoverflow.com/questions/12340695/how-to-check-if-a-given-file-descriptor-stored-in-a-variable-is-still-valid

New changeset f87ea0350286837e9e96de03f8bfa215176c2928 by Christian Heimes in branch 'main':
bpo-45915: use fcntl(fd, F_GETFD) in is_valid_fd() (GH-29821)
https://github.com/python/cpython/commit/f87ea0350286837e9e96de03f8bfa215176c2928

I've created bpo-45919 with a suggested enhancement to use GetFileType() in Windows, since the Windows C runtime does not provide fcntl().

Thank you!

PR 29821 adds __APPLE__ to the platforms that use fcntl(fd, F_GETFD). Is this okay on macOS, given bpo-30225? Apparently fstat() fails if the other end of a pipe is closed.