ociarchive: Add new ArchiveFileNotFoundError

[cgwalters]

This is for containers/skopeo#2114
which is in turn a dependency of coreos/rpm-ostree#4598

Basically I want to map ENOENT to a clear error, because the build
tooling wants to treat "target image is not present" differently
from "DNS lookup failed" or "we got EPERM".

There's a bit of code motion here because we need to move
the os.Open() call before creating a temporary directory.

Signed-off-by: Colin Walters walters@verbum.org

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[cgwalters]

What's the right place to unit test this?

[cgwalters]

Or, to address containers/skopeo#2114 (comment) I guess we could create a new ImageStructureNotFound (or ImageContainerNotFound) error or so. But I'd still want to map that underneath OpenImageOptional in skopeo's API.

[mtrmac]

Or, to address containers/skopeo#2114 (comment) I guess we could create a new ImageStructureNotFound (or ImageContainerNotFound) error or so.

I’d prefer that; ImageNotFound is explicitly documented not to match this situation

But I'd still want to map that underneath OpenImageOptional in skopeo's API.

Sure, that works for me.

[mtrmac]

What's the right place to unit test this?

I’d place it in a new oci/archive/oci_src.go:TestNewImageSource. (TestReferenceNewImage or a new unit test for createUntarTempDir would also work, but *_transport.go attract a bit more attention due to signature/identity concerns, so limiting the scope of those files comes with less concern.)

[cgwalters]

OK, updated for latest review and added a unit test.

[mtrmac]

LGTM. Thanks!

[cgwalters]

(Hm, the "PRs must be targeting tip to merge" on this repo is a little surprising)