lnrpc+rpcserver: add QoL flag for permission check

To make it even more obvious that by default the permissions to check
aren't taken from the full method provided, we add a new flag that does
that on request.

rpcserver.go

@@ -8794,6 +8794,20 @@ func (r *rpcServer) ListPermissions(_ context.Context,
 func (r *rpcServer) CheckMacaroonPermissions(ctx context.Context,
 	req *lnrpc.CheckMacPermRequest) (*lnrpc.CheckMacPermResponse, error) {
 
+	// Sanity-check the input parameters to eliminate impossible
+	// combinations.
+	switch {
+	case len(req.Permissions) > 0 && req.CheckDefaultPermsFromFullMethod:
+		return nil, fmt.Errorf("cannot check default permissions " +
+			"from full method and from provided permission list " +
+			"at the same time")
+
+	case len(req.FullMethod) == 0 && req.CheckDefaultPermsFromFullMethod:
+		return nil, fmt.Errorf("cannot check default permissions " +
+			"from full method without providing the full method " +
+			"name")
+	}
+
 	// Turn grpc macaroon permission into bakery.Op for the server to
 	// process.
 	permissions := make([]bakery.Op, len(req.Permissions))
@@ -8804,6 +8818,22 @@ func (r *rpcServer) CheckMacaroonPermissions(ctx context.Context,
 		}
 	}
 
+	// If the user wants to check the default permissions for the
+	// full method, then we'll use the interceptor chain to obtain the
+	// default permissions for the full method. This overwrites the
+	// user-provided permissions parsed above, but those are required to be
+	// empty anyway if the flag is turned on.
+	if req.CheckDefaultPermsFromFullMethod {
+		allPerms := r.interceptorChain.Permissions()
+		methodPerms, ok := allPerms[req.FullMethod]
+		if !ok {
+			return nil, fmt.Errorf("no permissions found for "+
+				"full method %s", req.FullMethod)
+		}
+
+		permissions = methodPerms
+	}
+
 	err := r.macService.CheckMacAuth(
 		ctx, req.Macaroon, permissions, req.FullMethod,
 	)