interfaces/builtin: ensure we don't register interfaces twice

interfaces/builtin/all.go

@@ -20,29 +20,35 @@
 package builtin
 
 import (
+	"fmt"
 	"sort"
 
 	"github.com/snapcore/snapd/interfaces"
 )
 
 var (
-	allInterfaces []interfaces.Interface
-	sorted        bool
+	allInterfaces map[string]interfaces.Interface
 )
 
 // Interfaces returns all of the built-in interfaces.
 func Interfaces() []interfaces.Interface {
-	if !sorted {
-		sort.Sort(byIfaceName(allInterfaces))
-		sorted = true
+	ifaces := make([]interfaces.Interface, 0, len(allInterfaces))
+	for _, iface := range allInterfaces {
+		ifaces = append(ifaces, iface)
 	}
-	return allInterfaces
+	sort.Sort(byIfaceName(ifaces))
+	return ifaces
 }
 
 // registerIface appends the given interface into the list of all known interfaces.
 func registerIface(iface interfaces.Interface) {
-	allInterfaces = append(allInterfaces, iface)
-	sorted = false
+	if allInterfaces[iface.Name()] != nil {
+		panic(fmt.Errorf("cannot register duplicate interface %q", iface.Name()))
+	}
+	if allInterfaces == nil {
+		allInterfaces = make(map[string]interfaces.Interface)
+	}
+	allInterfaces[iface.Name()] = iface
 }
 
 type byIfaceName []interfaces.Interface

interfaces/builtin/all_test.go

@@ -26,6 +26,7 @@ import (
 	"github.com/snapcore/snapd/interfaces/apparmor"
 	"github.com/snapcore/snapd/interfaces/builtin"
 	"github.com/snapcore/snapd/interfaces/dbus"
+	"github.com/snapcore/snapd/interfaces/ifacetest"
 	"github.com/snapcore/snapd/interfaces/kmod"
 	"github.com/snapcore/snapd/interfaces/mount"
 	"github.com/snapcore/snapd/interfaces/seccomp"
@@ -293,3 +294,16 @@ func (s *AllSuite) TestNoInterfaceImplementsOldBackendMethods(c *C) {
 		}
 	}
 }
+
+func (s *AllSuite) TestRegisterIface(c *C) {
+	restore := builtin.MockInterfaces(nil)
+	defer restore()
+
+	// Registering an interface works correctly.
+	iface := &ifacetest.TestInterface{InterfaceName: "foo"}
+	builtin.RegisterIface(iface)
+	c.Assert(builtin.Interface("foo"), DeepEquals, iface)
+
+	// Duplicates are detected.
+	c.Assert(func() { builtin.RegisterIface(iface) }, PanicMatches, `cannot register duplicate interface "foo"`)
+}

interfaces/builtin/export_test.go

@@ -19,8 +19,24 @@
 
 package builtin
 
+import (
+	"github.com/snapcore/snapd/interfaces"
+)
+
 func MprisGetName(iface *MprisInterface, attribs map[string]interface{}) (string, error) {
 	return iface.getName(attribs)
 }
 
 var ResolveSpecialVariable = resolveSpecialVariable
+
+func MockInterfaces(ifaces map[string]interfaces.Interface) (restore func()) {
+	old := allInterfaces
+	allInterfaces = ifaces
+	return func() { allInterfaces = old }
+}
+
+func Interface(name string) interfaces.Interface {
+	return allInterfaces[name]
+}
+
+var RegisterIface = registerIface