The actual behavior varies across platforms, and due to the inherent
race, we can't do anything better (other than to always return 0).

Fixes #12710.

Change-Id: Icb52f0f1f0a267e0f9f70767cae427f3f0239965
Reviewed-on: https://go-review.googlesource.com/14881Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>