pkg/ns: verify netns when initialized with GetNS

[steveej]

Fixes #221.

[dcbw]

I wouldn't set mounted:true for plain GetNS() calls. The reason is because only the creator of the netns should be disposing of it via ns.Close(), and things that call GetNS() are the creator. I think all you want to do here is return an error if the mountpoint is not an NSFS mount point.

[dcbw]

My take on this would be https://github.com/dcbw/cni/commits/namespace-validate ...

[dcbw]

Still need to close the FD here on error return.

[steveej]

Thanks!

[dcbw]

LGTM now, but how about some tests like:

        It("fails when the path is not a namespace", func() {
            tempFile, err := ioutil.TempFile("", "nstest")
            Expect(err).NotTo(HaveOccurred())
            defer tempFile.Close()
            defer os.Remove(tempFile.Name())

            _, err = ns.GetNS(tempFile.Name())
            Expect(err).To(MatchError(fmt.Sprintf("no network namespace mounted on file %q", tempFile.Name())))
        })

[steveej]

LGTM now, but how about some tests like:

Right, always good to have more tests! I fleshed your draft out and added a test case.

[dcbw]

Right, always good to have more tests! I fleshed your draft out and added a test case.

Thanks, test LGTM.