compat API: allow enforcing short-names resolution to Docker Hub

[vrothberg]

The Docker-compatible REST API has historically behaved just as the rest
of Podman and Buildah (and the atomic Docker in older RHEL/Fedora) where
containers-registries.conf is centrally controlling which registries
a short name may resolve to during pull or local image lookups. Please
refer to a blog for more details [1].

Docker, however, is only resolving short names to docker.io which has
been reported (see #12320) to break certain clients who rely on this
behavior. In order to support this scenario, containers.conf(5)
received a new option to control whether Podman's compat API resolves
to docker.io only or behaves as before.

Most endpoints allow for directly normalizing parameters that represent
an image. If set in containers.conf, Podman will then normalize the
references directly to docker.io. The build endpoint is an outlier
since images are also referenced in Dockerfiles. The Buildah API,
however, supports specifying a custom types.SystemContext in which
we can set a field that enforces short-name resolution to docker.io
in c/image/pkg/shortnames.

Notice that this a "hybrid" approach of doing the normalization directly
in the compat endpoints and in pkg/shortnames by passing a system
context. Doing such a hybrid approach is neccessary since the compat
and the libpod endpoints share the same libimage.Runtime which makes
a global enforcement via the libimage.Runtime.systemContext
impossible. Having two separate runtimes for the compat and the libpod
endpoints seems risky and not generally applicable to all endpoints.

[1] https://www.redhat.com/sysadmin/container-image-short-names

Fixes: #12320
Signed-off-by: Valentin Rothberg rothberg@redhat.com

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[vrothberg]

@edsantiago, I'd appreciate your eyes on the new tests

[mheon]

Question: does this affect lookup of localhost/ prefixed locally-built images? I think that is OK if so, given Podman 4.0.

[mtrmac]

Just a thought (I didn’t review the PR otherwise at all!)

Several (but by no means all) instances might be a tiny bit shorter with a

if err := utils.NormalizeOverwriteToDockerHub(r, &body.Config.Image); err != nil { … }

with the obvious wrapper

func NormalizeOverwriteToDockerHub(r …, inPlace *string) error {
   v, err := NormalizeToDockerHub(r, *inPlace)
   if err != nil { return err }
   *inPlace = v
   return nil
}

---

(*grumble* about Go’s error handling ergonomics)

[vrothberg]

Question: does this affect lookup of localhost/ prefixed locally-built images? I think that is OK if so, given Podman 4.0.

Yes. localhost/prefixing will be disabled, just as with Docker.

[mheon]

Alright. That's a breaking change (at least when the containers.conf flag is flipped on), so we'll need to document it. I threw on the correct label for that.

[vrothberg]

Alright. That's a breaking change (at least when the containers.conf flag is flipped on), so we'll need to document it. I threw on the correct label for that.

Oh absolutely. It's changing the entire short-name resolution of the compat API, so it goes beyond the localhost/ tagging convention.

[vrothberg]

Updated now with c/image@main. Ready for reviewing/merging.

[vrothberg]

@rhatdan @baude @jwhonce PTAL

[edsantiago]

Really thorough test suite, thank you. One nit, one question.

[baude]

LGTM and may i never work with images again ...

[rhatdan]

/lgtm