Fix build on Plan 9

[fhs]

No description provided.

[welcome]

Thank you for submitting this PR!
A maintainer will be here shortly to review it.
We are super grateful, but we are also overloaded! Help us by making sure that:

• The context for this PR is clear, with relevant discussion, decisions
  and stakeholders linked/mentioned.

• Your contribution itself is clear (code comments, self-review for the
  rest) and in its best form. Follow the code contribution
  guidelines
  if they apply.

Getting other community members to do a review would be great help too on complex PRs (you can ask in the chats/forums). If you are unsure about something, just leave us a comment.
Next steps:

• A maintainer will triage and assign priority to this PR, commenting on
  any missing things and potentially assigning a reviewer for high
  priority items.

• The PR gets reviews, discussed and approvals as needed.

• The PR is merged by maintainers when it has been approved and comments addressed.

We currently aim to provide initial feedback/triaging within two business days. Please keep an eye on any labelling actions, as these will indicate priorities and status of your contribution.
We are very grateful for your contribution!

[willscott]

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?

[fhs]

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.

[willscott]

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

[fhs]

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