Fix an error where we get a JSON syntax error on server 404 or 50X.

We were testing to see if the cached metadata was nil, but we actually
set it to an empty data.Signed object, but didn't always set it to nil
if we failed to get local metadata.

Signed-off-by: Ying Li <ying.li@docker.com>

client/client_update_test.go

@@ -461,7 +461,6 @@ func TestUpdateRemoteRoot50XCannotUseLocalCache(t *testing.T) {
 // but is missing other data, then we propagate the ErrMetaNotFound.  Skipping
 // force check, because that only matters for root.
 func TestUpdateNonRootRemoteMissingMetadataNoLocalCache(t *testing.T) {
-	// TODO: fix json syntax error
 	for _, role := range append(data.BaseRoles, "targets/a", "targets/a/b") {
 		if role == data.CanonicalRootRole {
 			continue
@@ -531,7 +530,6 @@ func TestUpdateNonRootRemoteMissingMetadataCannotUseLocalCache(t *testing.T) {
 // If there is no local cache, we just update. If the server 50X's when getting
 // metadata, we propagate ErrServerUnavailable.
 func TestUpdateNonRootRemote50XNoLocalCache(t *testing.T) {
-	// TODO: fix json syntax error
 	for _, role := range append(data.BaseRoles, "targets/a", "targets/a/b") {
 		if role == data.CanonicalRootRole {
 			continue

tuf/client/client.go

@@ -247,19 +247,21 @@ func (c *Client) downloadTimestamp() error {
 	// We may not have a cached timestamp if this is the first time
 	// we're interacting with the repo. This will result in the
 	// version being 0
-	var download bool
-	old := &data.Signed{}
-	version := 0
+	var (
+		saveToCache bool
+		old         *data.Signed
+		version     = 0
+	)
 	cachedTS, err := c.cache.GetMeta(role, maxSize)
 	if err == nil {
-		err := json.Unmarshal(cachedTS, old)
+		cached := &data.Signed{}
+		err := json.Unmarshal(cachedTS, cached)
 		if err == nil {
-			ts, err := data.TimestampFromSigned(old)
+			ts, err := data.TimestampFromSigned(cached)
 			if err == nil {
 				version = ts.Signed.Version
 			}
-		} else {
-			old = nil
+			old = cached
 		}
 	}
 	// unlike root, targets and snapshot, always try and download timestamps
@@ -278,14 +280,14 @@ func (c *Client) downloadTimestamp() error {
 		logrus.Warn("Error while downloading remote metadata, using cached timestamp - this might not be the latest version available remotely")
 		s = old
 	} else {
-		download = true
+		saveToCache = true
 	}
 	err = signed.Verify(s, role, version, c.keysDB)
 	if err != nil {
 		return err
 	}
 	logrus.Debug("successfully verified timestamp")
-	if download {
+	if saveToCache {
 		c.cache.SetMeta(role, raw)
 	}
 	ts, err := data.TimestampFromSigned(s)