Fix remoteFromMirror with GCS bucket (#734)

* Fix remoteFromMirror with GCS bucket

The mirror was reconfigured, but the URL was never reparsed, so when
checking the scheme, there was a nil pointer exception. This fix simply
returns early for GCS buckets.

Signed-off-by: Hayden Blauzvern <hblauzvern@google.com>

pkg/tuf/client.go

@@ -698,7 +698,7 @@ func remoteFromMirror(mirror string) (client.RemoteStore, error) {
 	// This is for compatibility with specifying a GCS bucket remote.
 	u, parseErr := url.ParseRequestURI(mirror)
 	if parseErr != nil {
-		mirror = fmt.Sprintf("https://%s.storage.googleapis.com", mirror)
+		return client.HTTPRemoteStore(fmt.Sprintf("https://%s.storage.googleapis.com", mirror), nil, nil)
 	}
 	if u.Scheme != "file" {
 		return client.HTTPRemoteStore(mirror, nil, nil)

pkg/tuf/client_test.go

@@ -789,3 +789,28 @@ func TestTargetsSubfolder(t *testing.T) {
 	}
 	checkTargetsAndMeta(t, tuf, []string{newTarget})
 }
+
+func Test_remoteFromMirror(t *testing.T) {
+	// test GCS mirror
+	mirror := "test-bucket"
+	_, err := remoteFromMirror(mirror)
+	if err != nil {
+		t.Fatalf("unexpected error with GCS mirror: %v", err)
+	}
+
+	// test HTTP mirror
+	mirror = "https://tuf-root-staging.storage.googleapis.com"
+	_, err = remoteFromMirror(mirror)
+	if err != nil {
+		t.Fatalf("unexpected error with GCS mirror: %v", err)
+	}
+
+	// test local mirror
+	tufRoot := t.TempDir()
+	os.Mkdir(fmt.Sprintf("%s/targets", tufRoot), 0o0750)
+	mirror = fmt.Sprintf("file://%s", tufRoot)
+	_, err = remoteFromMirror(mirror)
+	if err != nil {
+		t.Fatalf("unexpected error with GCS mirror: %v", err)
+	}
+}