As noted in that review, the idea then was to reuse the check from _getdirentries(). Presumably the error case is not the one to optimize for, so detecting the error later is ok? That said, I think we don't call _getdirentries() in the unionfs case? Does the test fail without this change?

Also, if you add this back here, you need to remove the code that maps EINVAL from _getdirentries() to ENOTDIR as that mapping was added as part of the earlier commit.

In the unionfs case, we call openat() which presumably will also return ENOTDIR if the fd is not associated with a directory.

I was surprised to see that the errno mapping was needed, though. POSIX doesn't have getdirentries(), but it does have posix_getdents() which is roughly equivalent and is required to set errno to ENOTDIR if the file descriptor is not associated with a directory. I'll look into changing getdirentries().

I just discovered case where the test makes a difference. If the file descriptor is not associated with a directory, but also not open for reading, you get EBADF rather than ENOTDIR. This is the case when running the test in this review, because creat() only opens the file for writing. Rather than reinstate the check, though, I'll adjust the test.