WIP: Add fallback handler

OCI registries don't provide a way to retrieve metadata in the same call
as listing tags.

This means that you have to do a separate API request for each tag if
you want the metadata, which could be very slow for a large number of
tags.

As such, preferably use the selfhosted client as a fallback, but add a
fallback fallback to the oci handler for registries that are
incompatible with the selfhosted API implementation.

pkg/client/client.go

@@ -11,9 +11,9 @@ import (
 	"github.com/jetstack/version-checker/pkg/client/acr"
 	"github.com/jetstack/version-checker/pkg/client/docker"
 	"github.com/jetstack/version-checker/pkg/client/ecr"
+	"github.com/jetstack/version-checker/pkg/client/fallback"
 	"github.com/jetstack/version-checker/pkg/client/gcr"
 	"github.com/jetstack/version-checker/pkg/client/ghcr"
-	"github.com/jetstack/version-checker/pkg/client/oci"
 	"github.com/jetstack/version-checker/pkg/client/quay"
 	"github.com/jetstack/version-checker/pkg/client/selfhosted"
 )
@@ -76,7 +76,7 @@ func New(ctx context.Context, log *logrus.Entry, opts Options) (*Client, error)
 		selfhostedClients = append(selfhostedClients, sClient)
 	}
 
-	fallbackClient, err := oci.New()
+	fallbackClient, err := fallback.New(ctx, log)
 	if err != nil {
 		return nil, fmt.Errorf("failed to create fallback client: %s", err)
 	}

pkg/client/fallback/fallback.go

@@ -0,0 +1,51 @@
+package fallback
+
+import (
+	"context"
+
+	"github.com/jetstack/version-checker/pkg/api"
+	"github.com/jetstack/version-checker/pkg/client/oci"
+	"github.com/jetstack/version-checker/pkg/client/selfhosted"
+	"github.com/sirupsen/logrus"
+)
+
+type Client struct {
+	SelfHosted *selfhosted.Client
+	OCI        *oci.Client
+}
+
+func New(ctx context.Context, log *logrus.Entry) (*Client, error) {
+	sh, err := selfhosted.New(ctx, log, new(selfhosted.Options))
+	if err != nil {
+		return nil, err
+	}
+	oci, err := oci.New()
+	if err != nil {
+		return nil, err
+	}
+
+	return &Client{
+		SelfHosted: sh,
+		OCI:        oci,
+	}, nil
+}
+
+func (c *Client) Name() string {
+	return "fallback"
+}
+
+func (c *Client) Tags(ctx context.Context, host, repo, image string) ([]api.ImageTag, error) {
+	// TODO: Cache selfhosted/oci by host
+	if tags, err := c.SelfHosted.Tags(ctx, host, repo, image); err == nil {
+		return tags, err
+	}
+	return c.OCI.Tags(ctx, host, repo, image)
+}
+
+func (c *Client) IsHost(host string) bool {
+	return true
+}
+
+func (c *Client) RepoImageFromPath(path string) (string, string) {
+	return c.SelfHosted.RepoImageFromPath(path)
+}