lnrpc: add GetState to stateservice (v1/state)

This commit adds a new GetState call to the state service in order to
make leader election integrate simply with k8s using a readinessProbe.

rpcperms/interceptor.go

@@ -82,6 +82,7 @@ var (
 		// The State service must be available at all times, even
 		// before we can check macaroons, so we whitelist it.
 		"/lnrpc.State/SubscribeState": {},
+		"/lnrpc.State/GetState":       {},
 	}
 )
 
@@ -193,6 +194,31 @@ func (r *InterceptorChain) SetRPCActive() {
 	_ = r.ntfnServer.SendUpdate(r.state)
 }
 
+// rpcStateToWalletState converts rpcState to lnrpc.WalletState. Returns
+// WAITING_TO_START and an error on conversion error.
+func rpcStateToWalletState(state rpcState) (lnrpc.WalletState, error) {
+	const defaultState = lnrpc.WalletState_WAITING_TO_START
+	var walletState lnrpc.WalletState
+
+	switch state {
+	case waitingToStart:
+		walletState = lnrpc.WalletState_WAITING_TO_START
+	case walletNotCreated:
+		walletState = lnrpc.WalletState_NON_EXISTING
+	case walletLocked:
+		walletState = lnrpc.WalletState_LOCKED
+	case walletUnlocked:
+		walletState = lnrpc.WalletState_UNLOCKED
+	case rpcActive:
+		walletState = lnrpc.WalletState_RPC_ACTIVE
+
+	default:
+		return defaultState, fmt.Errorf("unknown wallet state %v", state)
+	}
+
+	return walletState, nil
+}
+
 // SubscribeState subscribes to the state of the wallet. The current wallet
 // state will always be delivered immediately.
 //
@@ -201,24 +227,14 @@ func (r *InterceptorChain) SubscribeState(req *lnrpc.SubscribeStateRequest,
 	stream lnrpc.State_SubscribeStateServer) error {
 
 	sendStateUpdate := func(state rpcState) error {
-		resp := &lnrpc.SubscribeStateResponse{}
-		switch state {
-		case waitingToStart:
-			resp.State = lnrpc.WalletState_WAITING_TO_START
-		case walletNotCreated:
-			resp.State = lnrpc.WalletState_NON_EXISTING
-		case walletLocked:
-			resp.State = lnrpc.WalletState_LOCKED
-		case walletUnlocked:
-			resp.State = lnrpc.WalletState_UNLOCKED
-		case rpcActive:
-			resp.State = lnrpc.WalletState_RPC_ACTIVE
-
-		default:
-			return fmt.Errorf("unknown wallet state %v", state)
+		walletState, err := rpcStateToWalletState(state)
+		if err != nil {
+			return err
 		}
 
-		return stream.Send(resp)
+		return stream.Send(&lnrpc.SubscribeStateResponse{
+			State: walletState,
+		})
 	}
 
 	// Subscribe to state updates.
@@ -262,6 +278,24 @@ func (r *InterceptorChain) SubscribeState(req *lnrpc.SubscribeStateRequest,
 	}
 }
 
+// GetState returns he current wallet state.
+func (r *InterceptorChain) GetState(_ context.Context,
+	req *lnrpc.GetStateRequest) (*lnrpc.GetStateResponse, error) {
+
+	r.RLock()
+	state := r.state
+	r.RUnlock()
+
+	walletState, err := rpcStateToWalletState(state)
+	if err != nil {
+		return nil, err
+	}
+
+	return &lnrpc.GetStateResponse{
+		State: walletState,
+	}, nil
+}
+
 // AddMacaroonService adds a macaroon service to the interceptor. After this is
 // done every RPC call made will have to pass a valid macaroon to be accepted.
 func (r *InterceptorChain) AddMacaroonService(svc *macaroons.Service) {