Fix segfault from readline(); avoid unnecessary string copy

NEWS.md

@@ -2,6 +2,9 @@
 
 - Fixed segfault encountered when running Python finalizer (#1663, #1664)
 
+- Fixed segfault encountered in RStudio when rapidly switching 
+  between R and Python chunks in a Quarto document (#1665).
+
 # reticulate 1.39.0
 
 - Python background threads can now run in parallel with the R session (#1641).

src/RcppExports.cpp

@@ -824,12 +824,12 @@ BEGIN_RCPP
 END_RCPP
 }
 // readline
-SEXP readline(const std::string& prompt);
+SEXP readline(const char* prompt);
 RcppExport SEXP _reticulate_readline(SEXP promptSEXP) {
 BEGIN_RCPP
     Rcpp::RObject rcpp_result_gen;
     Rcpp::RNGScope rcpp_rngScope_gen;
-    Rcpp::traits::input_parameter< const std::string& >::type prompt(promptSEXP);
+    Rcpp::traits::input_parameter< const char* >::type prompt(promptSEXP);
     rcpp_result_gen = Rcpp::wrap(readline(prompt));
     return rcpp_result_gen;
 END_RCPP

src/readline.cpp

@@ -1,31 +1,30 @@
-
 #include <R.h>
 #include <Rinternals.h>
 
-#include <string>
+#include <cstring>  // for strlen, strchr
 #define READLINE_BUFFER_SIZE (8192)
 extern "C" int R_ReadConsole(const char*, unsigned char*, int, int);
 
 // [[Rcpp::export]]
-SEXP readline(const std::string& prompt)
+SEXP readline(const char* prompt)
 {
   // read user input (ensure null termination)
   char buffer[READLINE_BUFFER_SIZE];
-  R_ReadConsole(prompt.c_str(), (unsigned char*) buffer, READLINE_BUFFER_SIZE, 1);
-  buffer[READLINE_BUFFER_SIZE - 1] = '\0';
+  if (R_ReadConsole(prompt, (unsigned char*) buffer, READLINE_BUFFER_SIZE, 1) == 0)
+    return R_NilValue;
 
-  // construct resulting string
-  std::string result(buffer, buffer + strlen(buffer));
+  buffer[READLINE_BUFFER_SIZE - 1] = '\0';  // Ensure null termination
 
-  // truncate to location of inserted newline. if not found, assume
-  // the user canceled input with e.g. R_EOF
-  std::string::size_type index = result.find('\n');
-  if (index == std::string::npos)
-    return R_NilValue;
+  // Find the location of the newline character, if any
+  char* newline_pos = strchr(buffer, '\n');
+  if (newline_pos == nullptr)
+    return R_NilValue;  // If no newline found, assume user canceled
+
+  // Determine length up to the newline (excluding the trailing newline)
+  size_t input_length = newline_pos - buffer;
 
-  // return result (leaving out trailing newline)
   SEXP resultSEXP = PROTECT(Rf_allocVector(STRSXP, 1));
-  SET_STRING_ELT(resultSEXP, 0, Rf_mkCharLen(buffer, index));
+  SET_STRING_ELT(resultSEXP, 0, Rf_mkCharLen(buffer, input_length));
   UNPROTECT(1);
   return resultSEXP;
 }