feat(master): registry digest lookup and polling loop (#9)

internal/master/registry/registry.go

@@ -0,0 +1,55 @@
+// Package registry looks up the digest a tag points at in a container
+// registry, using HEAD requests only.
+package registry
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/google/go-containerregistry/pkg/authn"
+	"github.com/google/go-containerregistry/pkg/name"
+	"github.com/google/go-containerregistry/pkg/v1/remote"
+
+	"github.com/shcizo/package-updater/internal/imageref"
+)
+
+// Client looks up tag digests with HEAD requests only. Docker Hub counts a
+// GET of a manifest as a pull against the rate limit and a HEAD as nothing,
+// so Client never issues a manifest GET.
+type Client struct {
+	keychain  authn.Keychain
+	userAgent string
+}
+
+// New returns a Client that finds credentials the way the Docker CLI does:
+// in $DOCKER_CONFIG/config.json, else ~/.docker/config.json. Registries
+// without an entry are accessed anonymously. userAgent is added to the
+// User-Agent header of every request.
+func New(userAgent string) *Client {
+	return &Client{keychain: authn.DefaultKeychain, userAgent: userAgent}
+}
+
+// Digest returns the digest that "repo:tag" points at, as the registry
+// reports it: for a multi-arch image that is the digest of the index, the
+// same one `docker inspect` shows in RepoDigests. A digest suffix in ref is
+// ignored. It sends HEAD /v2/<repo>/manifests/<tag> and nothing else to the
+// manifest endpoint; it does not fall back to GET when the registry omits
+// the digest header.
+func (c *Client) Digest(ctx context.Context, ref string) (string, error) {
+	p := imageref.Parse(ref)
+	if p.Tag == "" {
+		return "", fmt.Errorf("image %q has no tag", ref)
+	}
+	r, err := name.ParseReference(p.Repo + ":" + p.Tag)
+	if err != nil {
+		return "", fmt.Errorf("parse %s: %w", ref, err)
+	}
+	desc, err := remote.Head(r,
+		remote.WithContext(ctx),
+		remote.WithAuthFromKeychain(c.keychain),
+		remote.WithUserAgent(c.userAgent))
+	if err != nil {
+		return "", fmt.Errorf("head %s: %w", r, err)
+	}
+	return desc.Digest.String(), nil
+}