have cancellation function return invisibly for consistency with later()

src/fd.cpp

@@ -26,6 +26,7 @@ inline SEXP R_mkClosure(SEXP formals, SEXP body, SEXP env) {
 
 extern CallbackRegistryTable callbackRegistryTable;
 extern SEXP later_fdcancel;
+extern SEXP later_invisibleSymbol;
 
 #ifdef _WIN32
 #define POLL_FUNC WSAPoll
@@ -205,9 +206,8 @@ Rcpp::RObject execLater_fd(Rcpp::Function callback, Rcpp::IntegerVector readfds,
   tct_thrd_detach(thr);
 
   Rcpp::XPtr<std::shared_ptr<std::atomic<bool>>> xptr(new std::shared_ptr<std::atomic<bool>>(flag), true);
-
   SEXP body, func;
-  PROTECT(body = Rf_lcons(later_fdcancel, Rf_cons(xptr, R_NilValue)));
+  PROTECT(body = Rf_lang2(later_invisibleSymbol, Rf_lang2(later_fdcancel, xptr)));
   func = R_mkClosure(R_NilValue, body, R_BaseEnv);
   UNPROTECT(1);
 

src/init.c

@@ -5,8 +5,9 @@
 #include <stdint.h> // for uint64_t
 
 // Symbols and preserved objects defined here.
-SEXP later_laterSymbol;
 SEXP later_fdcancel;
+SEXP later_invisibleSymbol;
+SEXP later_laterSymbol;
 
 /* FIXME:
 Check these declarations against the C/Fortran source code.
@@ -63,6 +64,7 @@ uint64_t execLaterNative2(void (*func)(void*), void* data, double secs, int loop
 int apiVersion(void);
 
 void PreserveObjects(void) {
+  later_invisibleSymbol = Rf_install("invisible");
   later_laterSymbol = Rf_install("later");
   R_PreserveObject(later_fdcancel = Rf_lang3(R_TripleColonSymbol, later_laterSymbol, Rf_install("fd_cancel")));
 }

tests/testthat/test-later-fd.R

@@ -23,6 +23,7 @@ test_that("later_fd", {
   expect_true(cancel())
   Sys.sleep(0.25)
   expect_false(cancel())
+  expect_invisible(cancel())
   run_now()
   expect_null(result)