fix memory leak in Rcpp::warning under tryCatch handlers

The variadic Rcpp::warning() template formatted its message into a
std::string whose destructor would be skipped if Rf_warning() longjmp-ed
(e.g. via tryCatch(warning=...)), leaking the heap buffer. Copy the
formatted message into a stack buffer and let the std::string be
destroyed before Rf_warning() is called.

Closes #1474.

Author: kevinushey

inst/NEWS.Rd

@@ -32,6 +32,11 @@
       with current R versions (Dirk in \ghpr{1469} fixing \ghit{1468})
       \item The \code{Nullable::as()} exporter now uses an explicit cast to
       the templated type (Dirk in \ghpr{1471} fixing \ghit{1470})
+      \item A memory leak in the variadic \code{Rcpp::warning()} template
+      has been fixed by copying the formatted message into a stack buffer
+      before \code{Rf_warning()} is invoked, so the \code{std::string}
+      destructor runs even when an R warning handler triggers a
+      \code{longjmp} (Kevin fixing \ghit{1474})
     }
     \item Changes in Rcpp Documentation:
     \itemize{

inst/include/Rcpp/exceptions.h

@@ -24,6 +24,7 @@
 #define Rcpp__exceptions__h
 
 #include <Rversion.h>
+#include <cstdio>
 
 #ifndef RCPP_DEFAULT_INCLUDE_CALL
 #define RCPP_DEFAULT_INCLUDE_CALL true
@@ -186,7 +187,17 @@ struct LongjumpException {
 
     template <typename... Args>
     inline void warning(const char* fmt, Args&&... args ) {
-        Rf_warning("%s", tfm::format(fmt, std::forward<Args>(args)... ).c_str());
+        // Rf_warning() may longjmp out of this frame (e.g. when the caller
+        // installs a warning handler via tryCatch(warning=...)). A longjmp
+        // skips C++ destructors, so the std::string returned by tfm::format()
+        // would leak its heap buffer. Copy into a stack buffer and let the
+        // std::string be destroyed before Rf_warning() is invoked.
+        char buf[8192];
+        {
+            const std::string msg = tfm::format(fmt, std::forward<Args>(args)...);
+            std::snprintf(buf, sizeof(buf), "%s", msg.c_str());
+        }
+        Rf_warning("%s", buf);
     }                                                           // #nocov end
 
     template <typename... Args>

inst/tinytest/test_dataframe.R

@@ -117,3 +117,13 @@ expect_warning( DataFrame_PushZeroLength())
 ## issue #1232: push on empty data.frame
 df <- DataFrame_PushOnEmpty()
 expect_equal(ncol(df), 3L)
+
+## issue #1474: warning thrown from DataFrame::push_back used to leak the
+## formatted std::string when a tryCatch handler longjmp-ed past its dtor.
+## Loop a few iterations so valgrind makes any regression obvious.
+got_warning <- FALSE
+for (i in 1:5) {
+    tryCatch(DataFrame_PushWrongSize(),
+             warning = function(w) { got_warning <<- TRUE })
+}
+expect_true(got_warning)