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the posix/windows implementations include EINVAL and ENOTSUP style errors in the check.
plan9 also at least defines these (syscall.EINVAL and syscall.EPLAN9). Is there a reason those shouldn't bubble up to returning true in isErrnoNotSupported?

the posix/windows implementations include EINVAL and ENOTSUP style errors in the check.
plan9 also at least defines these (syscall.EINVAL and syscall.EPLAN9). Is there a reason those shouldn't bubble up to returning true in isErrnoNotSupported?

This function seems to be only called with an unwrapped os.PathError error returned by File.Sync. File.Sync can return os.ErrInvalid for nil receiver, but it's not wrapped in os.PathError. I don't think it ever returns syscall.EINVAL or syscall.EPLAN9. I can return true for those errors if the function is meant for general use, not just to check File.Sync errors.

I can return true for those errors if the function is meant for general use, not just to check File.Sync errors.

It seems like it shouldn't hurt, and hopefully reduces the potential for surprise in the future if it stays consistent with other platforms it case it gets used for something else

I've added syscall.EINVAL and syscall.EPLAN9. os.ErrInvalid is not being checked on unix or windows, so leaving that out.