fs: add WithAllowXAttrErrors CopyOpt

[AkihiroSuda]

This option allows ignoring errors during copying xattr like security.selinux,
which is not always supported.

Reported in several issues including

• Buildkit on coreOS failed with xattr openfaas/openfaas-cloud#312
• [openfaas/openfaas-cloud#312] Buildkit on coreOS failed with xattr moby/buildkit#704
• [CoreOS] failed to set xattr "security.selinux" genuinetools/img#45
• https://bugzilla.redhat.com/show_bug.cgi?id=1596918

Signed-off-by: Akihiro Suda suda.akihiro@lab.ntt.co.jp

[AkihiroSuda]

cc @tonistiigi

[AkihiroSuda]

So the PR appeared to have nothing to do with the buildkit failure but should be still useful

[nixpanic]

Please consider this change on top of yours. It continues copying xattrs for a file, even if one of them fails (like security.selinux).

Feel free to incorporate the modification in an updated PR. Thanks!

[AkihiroSuda]

@nixpanic Thanks, cherry-picked your commit

[AkihiroSuda]

@dmcgowan @cpuguy83 PTAL?

[cpuguy83]

Oh, nice!
Also related: moby/moby#38155

[cpuguy83]

I think for this case, being able to pass in an error handler on file copy would be handy.

Something like:

func(err error) error

Where if the handler wants to ignore the error it would return nil.

[AkihiroSuda]

Added type XAttrErrorHandler func(dst, src, xattrKey string, err error) error. PTAL.

[nixpanic]

LGTM, thanks!

[nixpanic]

Hey guys, any progress with reviewing/merging? This PR works well for me and I'd like to see it included so that I can consume it in kubernetes-retired/external-storage#1013 and a future CSI driver.

Thanks!

[mikebrow]

see comment

[estesp]

LGTM