Resolve ambiguity on registry v2 ping

v2 ping now checks for a Docker-Distribution-API-Version
header that identifies the endpoint as "registry/2.0"

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

docs/endpoint.go

@@ -227,6 +227,21 @@ func (e *Endpoint) pingV2() (RegistryInfo, error) {
 	}
 	defer resp.Body.Close()
 
+	// The endpoint may have multiple supported versions.
+	// Ensure it supports the v2 Registry API.
+	var supportsV2 bool
+
+	for _, versionName := range resp.Header[http.CanonicalHeaderKey("Docker-Distribution-API-Version")] {
+		if versionName == "registry/2.0" {
+			supportsV2 = true
+			break
+		}
+	}
+
+	if !supportsV2 {
+		return RegistryInfo{}, fmt.Errorf("%s does not appear to be a v2 registry endpoint", e)
+	}
+
 	if resp.StatusCode == http.StatusOK {
 		// It would seem that no authentication/authorization is required.
 		// So we don't need to parse/add any authorization schemes.

docs/endpoint_test.go

@@ -1,6 +1,11 @@
 package registry
 
-import "testing"
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+)
 
 func TestEndpointParse(t *testing.T) {
 	testData := []struct {
@@ -27,3 +32,59 @@ func TestEndpointParse(t *testing.T) {
 		}
 	}
 }
+
+// Ensure that a registry endpoint that responds with a 401 only is determined
+// to be a v1 registry unless it includes a valid v2 API header.
+func TestValidateEndpointAmbiguousAPIVersion(t *testing.T) {
+	requireBasicAuthHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Add("WWW-Authenticate", `Basic realm="localhost"`)
+		w.WriteHeader(http.StatusUnauthorized)
+	})
+
+	requireBasicAuthHandlerV2 := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Add("Docker-Distribution-API-Version", "registry/2.0")
+		requireBasicAuthHandler.ServeHTTP(w, r)
+	})
+
+	// Make a test server which should validate as a v1 server.
+	testServer := httptest.NewServer(requireBasicAuthHandler)
+	defer testServer.Close()
+
+	testServerURL, err := url.Parse(testServer.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testEndpoint := Endpoint{
+		URL:     testServerURL,
+		Version: APIVersionUnknown,
+	}
+
+	if err = validateEndpoint(&testEndpoint); err != nil {
+		t.Fatal(err)
+	}
+
+	if testEndpoint.Version != APIVersion1 {
+		t.Fatalf("expected endpoint to validate to %s, got %s", APIVersion1, testEndpoint.Version)
+	}
+
+	// Make a test server which should validate as a v2 server.
+	testServer = httptest.NewServer(requireBasicAuthHandlerV2)
+	defer testServer.Close()
+
+	testServerURL, err = url.Parse(testServer.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testEndpoint.URL = testServerURL
+	testEndpoint.Version = APIVersionUnknown
+
+	if err = validateEndpoint(&testEndpoint); err != nil {
+		t.Fatal(err)
+	}
+
+	if testEndpoint.Version != APIVersion2 {
+		t.Fatalf("expected endpoint to validate to %s, got %s", APIVersion2, testEndpoint.Version)
+	}
+}