rpc_proxy+subservers: error out of GetRemoteConn if not ready

Due to the rpcProxy being started early now, it could be the case that a
call is made to `GetRemoteConn` before the remote connection has
actually been set up. This commit catches this case so that an error can
be returned and a panic avoided.

Author: ellemouton

rpc_proxy.go

@@ -304,7 +304,12 @@ func (p *rpcProxy) makeDirector(allowLitRPC bool) func(ctx context.Context,
 		// since it must either be an lnd call or something that'll be
 		// handled by the integrated daemons that are hooking into lnd's
 		// gRPC server.
-		handled, conn := p.subServerMgr.GetRemoteConn(requestURI)
+		handled, conn, err := p.subServerMgr.GetRemoteConn(requestURI)
+		if err != nil {
+			return outCtx, nil, status.Errorf(
+				codes.Unavailable, err.Error(),
+			)
+		}
 		if handled {
 			return outCtx, conn, nil
 		}

subservers/manager.go

@@ -153,7 +153,7 @@ func (s *Manager) RegisterRestServices(ctx context.Context,
 // and if so, the remote connection to that sub-server is returned. The bool
 // return value indicates if the uri is managed by one of the sub-servers
 // running in remote mode.
-func (s *Manager) GetRemoteConn(uri string) (bool, *grpc.ClientConn) {
+func (s *Manager) GetRemoteConn(uri string) (bool, *grpc.ClientConn, error) {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
 
@@ -163,13 +163,18 @@ func (s *Manager) GetRemoteConn(uri string) (bool, *grpc.ClientConn) {
 		}
 
 		if !ss.Remote() {
-			return false, nil
+			return false, nil, nil
+		}
+
+		if ss.remoteConn == nil {
+			return true, nil, fmt.Errorf("not yet connected to "+
+				"remote sub-server(%s)", ss.Name())
 		}
 
-		return true, ss.remoteConn
+		return true, ss.remoteConn, nil
 	}
 
-	return false, nil
+	return false, nil, nil
 }
 
 // ValidateMacaroon checks if any of the manager's sub-servers owns the given