Catalog for V2 API Implementation

This change adds a basic catalog endpoint to the API, which returns a list,
or partial list, of all of the repositories contained in the registry.  Calls
to this endpoint are somewhat expensive, as every call requires walking a
large part of the registry.

Instead, to maintain a list of repositories, you would first call the catalog
endpoint to get an initial list, and then use the events API to maintain
any future repositories.

Signed-off-by: Patrick Devine <patrick.devine@docker.com>

docs/storage/catalog.go

@@ -0,0 +1,62 @@
+package storage
+
+import (
+	"path"
+	"sort"
+	"strings"
+
+	log "github.com/Sirupsen/logrus"
+	"github.com/docker/distribution"
+	"github.com/docker/distribution/context"
+	storageDriver "github.com/docker/distribution/registry/storage/driver"
+)
+
+type catalogSvc struct {
+	ctx    context.Context
+	driver storageDriver.StorageDriver
+}
+
+var _ distribution.CatalogService = &catalogSvc{}
+
+// Get returns a list, or partial list, of repositories in the registry.
+// Because it's a quite expensive operation, it should only be used when building up
+// an initial set of repositories.
+func (c *catalogSvc) Get(maxEntries int, lastEntry string) ([]string, bool, error) {
+	log.Infof("Retrieving up to %d entries of the catalog starting with '%s'", maxEntries, lastEntry)
+	var repos []string
+
+	root, err := defaultPathMapper.path(repositoriesRootPathSpec{})
+	if err != nil {
+		return repos, false, err
+	}
+
+	Walk(c.ctx, c.driver, root, func(fileInfo storageDriver.FileInfo) error {
+		filePath := fileInfo.Path()
+
+		// lop the base path off
+		repoPath := filePath[len(root)+1:]
+
+		_, file := path.Split(repoPath)
+		if file == "_layers" {
+			repoPath = strings.TrimSuffix(repoPath, "/_layers")
+			if repoPath > lastEntry {
+				repos = append(repos, repoPath)
+			}
+			return ErrSkipDir
+		} else if strings.HasPrefix(file, "_") {
+			return ErrSkipDir
+		}
+
+		return nil
+	})
+
+	sort.Strings(repos)
+
+	moreEntries := false
+	if len(repos) > maxEntries {
+		moreEntries = true
+		repos = repos[0:maxEntries]
+	}
+
+	return repos, moreEntries, nil
+}