features: forbid checking control file or status of unexported features

Since only some features are exported we should prohibit accidentally
checking features that are not exported. This patch does just that.

Signed-off-by: Zygmunt Krynicki <me@zygoon.pl>

features/features.go

@@ -97,10 +97,16 @@ func (f SnapdFeature) IsExported() bool {
 // Snapd considers the feature enabled if the file is present.
 // The contents of the file are not important.
 func (f SnapdFeature) ControlFile() string {
+	if !f.IsExported() {
+		panic(fmt.Sprintf("cannot compute the control file of feature %q because that feature is not exported", f))
+	}
 	return filepath.Join(dirs.FeaturesDir, f.String())
 }
 
-// IsEnabled checks if a given snapd feature is enabled.
+// IsEnabled checks if a given exported snapd feature is enabled.
 func (f SnapdFeature) IsEnabled() bool {
+	if !f.IsExported() {
+		panic(fmt.Sprintf("cannot check if feature %q is enabled because that feature is not exported", f))
+	}
 	return osutil.FileExists(f.ControlFile())
 }

features/features_test.go

@@ -74,6 +74,9 @@ func (*featureSuite) TestIsEnabled(c *C) {
 	err = ioutil.WriteFile(filepath.Join(dirs.FeaturesDir, f.String()), nil, 0644)
 	c.Assert(err, IsNil)
 	c.Check(f.IsEnabled(), Equals, true)
+
+	// Features that are not exported cannot be queried.
+	c.Check(features.Layouts.IsEnabled, PanicMatches, `cannot check if feature "layouts" is enabled because that feature is not exported`)
 }
 
 func (*featureSuite) TestIsEnabledByDefault(c *C) {
@@ -85,5 +88,7 @@ func (*featureSuite) TestIsEnabledByDefault(c *C) {
 }
 
 func (*featureSuite) TestControlFile(c *C) {
-	c.Check(features.Layouts.ControlFile(), Equals, "/var/lib/snapd/features/layouts")
+	c.Check(features.PerUserMountNamespace.ControlFile(), Equals, "/var/lib/snapd/features/per-user-mount-namespace")
+	// Features that are not exported don't have a control file.
+	c.Check(features.Layouts.ControlFile, PanicMatches, `cannot compute the control file of feature "layouts" because that feature is not exported`)
 }