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/handlers/catalog.go

@@ -0,0 +1,82 @@
+package handlers
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/url"
+	"strconv"
+
+	"github.com/docker/distribution/registry/api/errcode"
+	"github.com/gorilla/handlers"
+)
+
+const maximumReturnedEntries = 100
+
+func catalogDispatcher(ctx *Context, r *http.Request) http.Handler {
+	catalogHandler := &catalogHandler{
+		Context: ctx,
+	}
+
+	return handlers.MethodHandler{
+		"GET": http.HandlerFunc(catalogHandler.GetCatalog),
+	}
+}
+
+type catalogHandler struct {
+	*Context
+}
+
+type catalogAPIResponse struct {
+	Repositories []string `json:"repositories"`
+}
+
+func (ch *catalogHandler) GetCatalog(w http.ResponseWriter, r *http.Request) {
+	q := r.URL.Query()
+	lastEntry := q.Get("last")
+	maxEntries, err := strconv.Atoi(q.Get("n"))
+	if err != nil || maxEntries < 0 {
+		maxEntries = maximumReturnedEntries
+	}
+
+	repos, moreEntries, err := ch.Catalog.Get(maxEntries, lastEntry)
+	if err != nil {
+		ch.Errors = append(ch.Errors, errcode.ErrorCodeUnknown.WithDetail(err))
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+
+	// Add a link header if there are more entries to retrieve
+	if moreEntries {
+		urlStr, err := createLinkEntry(r.URL.String(), maxEntries, repos)
+		if err != nil {
+			ch.Errors = append(ch.Errors, errcode.ErrorCodeUnknown.WithDetail(err))
+			return
+		}
+		w.Header().Set("Link", urlStr)
+	}
+
+	enc := json.NewEncoder(w)
+	if err := enc.Encode(catalogAPIResponse{
+		Repositories: repos,
+	}); err != nil {
+		ch.Errors = append(ch.Errors, errcode.ErrorCodeUnknown.WithDetail(err))
+		return
+	}
+}
+
+// Use the original URL from the request to create a new URL for
+// the link header
+func createLinkEntry(origURL string, maxEntries int, repos []string) (string, error) {
+	calledURL, err := url.Parse(origURL)
+	if err != nil {
+		return "", err
+	}
+
+	calledURL.RawQuery = fmt.Sprintf("n=%d&last=%s", maxEntries, repos[len(repos)-1])
+	calledURL.Fragment = ""
+	urlStr := fmt.Sprintf("<%s>; rel=\"next\"", calledURL.String())
+
+	return urlStr, nil
+}