Move static local variable out of templated function (#299)

* Move `static` local variable out of templated function

* NEWS bullet

* Make `format_check` happy

---------

Co-authored-by: Romain François <romain@rstudio.com>

Author: DavisVaughan

Diff for: NEWS.md

@@ -1,5 +1,7 @@
 # cpp11 (development version)
 
+* Fixed a performance issue related to nested `unwind_protect()` calls (#298).
+
 * Minor performance improvements to the cpp11 protect code. (@kevinushey)
 
 * Silenced an unknown attribute warning specific to the Intel compiler

Diff for: inst/include/cpp11/protect.hpp

@@ -54,20 +54,34 @@ inline void set_option(SEXP name, SEXP value) {
   SETCAR(opt, value);
 }
 
-inline Rboolean& get_should_unwind_protect() {
+inline Rboolean* setup_should_unwind_protect() {
   SEXP should_unwind_protect_sym = Rf_install("cpp11_should_unwind_protect");
   SEXP should_unwind_protect_sexp = Rf_GetOption1(should_unwind_protect_sym);
+
   if (should_unwind_protect_sexp == R_NilValue) {
+    // Allocate and initialize once, then let R manage it.
+    // That makes this a shared global across all compilation units.
     should_unwind_protect_sexp = PROTECT(Rf_allocVector(LGLSXP, 1));
+    SET_LOGICAL_ELT(should_unwind_protect_sexp, 0, TRUE);
     detail::set_option(should_unwind_protect_sym, should_unwind_protect_sexp);
     UNPROTECT(1);
   }
 
-  Rboolean* should_unwind_protect =
-      reinterpret_cast<Rboolean*>(LOGICAL(should_unwind_protect_sexp));
-  should_unwind_protect[0] = TRUE;
+  return reinterpret_cast<Rboolean*>(LOGICAL(should_unwind_protect_sexp));
+}
+
+inline Rboolean* access_should_unwind_protect() {
+  // Setup is run once per compilation unit, but all compilation units
+  // share the same global option, so each compilation unit's static pointer
+  // will point to the same object.
+  static Rboolean* p_should_unwind_protect = setup_should_unwind_protect();
+  return p_should_unwind_protect;
+}
+
+inline Rboolean get_should_unwind_protect() { return *access_should_unwind_protect(); }
 
-  return should_unwind_protect[0];
+inline void set_should_unwind_protect(Rboolean should_unwind_protect) {
+  *access_should_unwind_protect() = should_unwind_protect;
 }
 
 }  // namespace detail
@@ -80,12 +94,11 @@ inline Rboolean& get_should_unwind_protect() {
 template <typename Fun, typename = typename std::enable_if<std::is_same<
                             decltype(std::declval<Fun&&>()()), SEXP>::value>::type>
 SEXP unwind_protect(Fun&& code) {
-  static auto should_unwind_protect = detail::get_should_unwind_protect();
-  if (should_unwind_protect == FALSE) {
+  if (detail::get_should_unwind_protect() == FALSE) {
     return std::forward<Fun>(code)();
   }
 
-  should_unwind_protect = FALSE;
+  detail::set_should_unwind_protect(FALSE);
 
   static SEXP token = [] {
     SEXP res = R_MakeUnwindCont();
@@ -95,7 +108,7 @@ SEXP unwind_protect(Fun&& code) {
 
   std::jmp_buf jmpbuf;
   if (setjmp(jmpbuf)) {
-    should_unwind_protect = TRUE;
+    detail::set_should_unwind_protect(TRUE);
     throw unwind_exception(token);
   }
 
@@ -120,7 +133,7 @@ SEXP unwind_protect(Fun&& code) {
   // unset it here before returning the value ourselves.
   SETCAR(token, R_NilValue);
 
-  should_unwind_protect = TRUE;
+  detail::set_should_unwind_protect(TRUE);
 
   return res;
 }