Update Backend interface to latest iteration

What will hopfully be the final version of the Backend interface. This
combines the MultiState interface into Backend since it will be required
to implement, and simplifies the interface because the Backend is no
longer responsible for tracking the current state.

backend/backend.go

@@ -26,23 +26,21 @@ type Backend interface {
 	// not be loaded locally: the proper APIs should be called on state.State
 	// to load the state. If the state.State is a state.Locker, it's up to the
 	// caller to call Lock and Unlock as needed.
-	State() (state.State, error)
-}
+	//
+	// If the named state doesn't exist it will be created. The "default" state
+	// is always assumed to exist.
+	State(name string) (state.State, error)
 
-// MultiState is an interface that a backend can implement to allow changing
-// between named states depending on the configured environment.
-type MultiState interface {
-	// States returns a list of configured named states and the current state.
-	States() ([]string, string, error)
-
-	// ChangeState changes to the named state. If the named state doesn't exist
-	// it will be created.
-	ChangeState(name string) error
-
-	// DeleteState removes the named state if it exists. If the current state is
-	// deleted, the backend should change to the default state. It is an error
+	// DeleteState removes the named state if it exists. It is an error
 	// to delete the default state.
+	//
+	// DeleteState does not prevent deleting a state that is in use. It is the
+	// responsibility of the caller to hold a Lock on the state when calling
+	// this method.
 	DeleteState(name string) error
+
+	// States returns a list of configured named states.
+	States() ([]string, error)
 }
 
 // Enhanced implements additional behavior on top of a normal backend.

backend/nil.go

@@ -25,16 +25,15 @@ func (Nil) Configure(*terraform.ResourceConfig) error {
 	return nil
 }
 
-func (Nil) State() (state.State, error) {
+func (Nil) State(string) (state.State, error) {
 	// We have to return a non-nil state to adhere to the interface
 	return &state.InmemState{}, nil
 }
 
-func (Nil) States() ([]string, string, error) {
-	// The default state always exists
-	return []string{DefaultStateName}, DefaultStateName, nil
-}
-
-func (Nil) ChangeState(string) error {
+func (Nil) DeleteState(string) error {
 	return nil
 }
+
+func (Nil) States() ([]string, error) {
+	return []string{DefaultStateName}, nil
+}