code: private <ReqT, RespT> boolean authenticateCall(ServerCall<ReqT, RespT> call, Metadata headers) {
      // Fail validation for cancelled server calls.
      if (call.isCancelled()) {
        LOG.debug("Server call has been cancelled: %s",
            call.getMethodDescriptor().getFullMethodName());
        return false;
      }
  
      // Try to fetch channel Id from the metadata.
      UUID channelId = headers.get(ChannelIdInjector.S_CLIENT_ID_KEY);
      boolean callAuthenticated = false;
      if (channelId != null) {
        try {
          // Fetch authenticated username for this channel and set it.
          AuthenticatedUserInfo userInfo = mAuthenticationServer.getUserInfoForChannel(channelId);
          if (userInfo != null) {
            AuthenticatedClientUser.set(userInfo.getAuthorizedUserName());
            AuthenticatedClientUser.setConnectionUser(userInfo.getConnectionUserName());
            AuthenticatedClientUser.setAuthMethod(userInfo.getAuthMethod());
          } else {
            AuthenticatedClientUser.remove();
          }
          callAuthenticated = true;
        } catch (UnauthenticatedException e) {
          String message = String.format("Channel: %s is not authenticated for call: %s",
              channelId.toString(), call.getMethodDescriptor().getFullMethodName());
          call.close(Status.UNAUTHENTICATED.withDescription(message), headers);
        }
      } else {
        String message = String.format("Channel Id is missing for call: %s.",
            call.getMethodDescriptor().getFullMethodName());
        call.close(Status.UNAUTHENTICATED.withDescription(message), headers);
      }
      return callAuthenticated;
    }
docs: Authenticates given call against auth-server state.
  Fails the call if it's not originating from an authenticated client channel.
  It sets thread-local authentication information for the call with the user information
  that is kept on auth-server.