Fix issue when using intermediate string returns

In MSVC the following can lead to segfault:
sass_copy_c_string(stream.str().c_str());
Reason is that the string returned by str() is disposed before
sass_copy_c_string is invoked. The string is actually a stack
object, so indeed nobody is holding on to it. So it seems
perfectly fair to release it right away. So the const char*
by c_str will point to invalid memory. I'm not sure if this is
the behavior for all compiler, but I'm pretty sure we would
have gotten more issues reported if that would be the case.
Wrapping it in a functions seems the cleanest approach as the
function must hold on to the stack variable until it's done.

Author: mgreter

src/sass.cpp

@@ -70,3 +70,24 @@ extern "C" {
   }
 
 }
+
+namespace Sass {
+
+  // helper to aid dreaded MSVC debug mode
+  char* sass_copy_string(std::string str)
+  {
+    // In MSVC the following can lead to segfault:
+    // sass_copy_c_string(stream.str().c_str());
+    // Reason is that the string returned by str() is disposed before
+    // sass_copy_c_string is invoked. The string is actually a stack
+    // object, so indeed nobody is holding on to it. So it seems
+    // perfectly fair to release it right away. So the const char*
+    // by c_str will point to invalid memory. I'm not sure if this is
+    // the behavior for all compiler, but I'm pretty sure we would
+    // have gotten more issues reported if that would be the case.
+    // Wrapping it in a functions seems the cleanest approach as the
+    // function must hold on to the stack variable until it's done.
+    return sass_copy_c_string(str.c_str());
+  }
+
+}

src/sass.hpp

@@ -45,6 +45,9 @@
 // include C-API header
 #include "sass/base.h"
 
+// For C++ helper
+#include <string>
+
 // output behaviours
 namespace Sass {
 
@@ -57,7 +60,11 @@ namespace Sass {
   const static Sass_Output_Style INSPECT = SASS_STYLE_INSPECT;
   const static Sass_Output_Style TO_SASS = SASS_STYLE_TO_SASS;
 
-};
+  // helper to aid dreaded MSVC debug mode
+  // see implementation for more details
+  char* sass_copy_string(std::string str);
+
+}
 
 // input behaviours
 enum Sass_Input_Style {

src/sass_context.cpp

@@ -17,6 +17,17 @@
 
 #define LFEED "\n"
 
+// C++ helper
+namespace Sass {
+  // see sass_copy_c_string(std::string str)
+  static inline JsonNode* json_mkstream(const std::stringstream& stream)
+  {
+    // hold on to string on stack!
+    std::string str(stream.str());
+    return json_mkstring(str.c_str());
+  }
+}
+
 extern "C" {
   using namespace Sass;
 
@@ -103,10 +114,9 @@ extern "C" {
       json_append_member(json_err, "line", json_mknumber((double)(e.pstate.line+1)));
       json_append_member(json_err, "column", json_mknumber((double)(e.pstate.column+1)));
       json_append_member(json_err, "message", json_mkstring(e.what()));
-      json_append_member(json_err, "formatted", json_mkstring(msg_stream.str().c_str()));
-
+      json_append_member(json_err, "formatted", json_mkstream(msg_stream));
       try { c_ctx->error_json = json_stringify(json_err, "  "); } catch(...) {}
-      c_ctx->error_message = sass_copy_c_string(msg_stream.str().c_str());
+      c_ctx->error_message = sass_copy_string(msg_stream.str());
       c_ctx->error_text = sass_copy_c_string(e.what());
       c_ctx->error_status = 1;
       c_ctx->error_file = sass_copy_c_string(e.pstate.path);
@@ -123,9 +133,9 @@ extern "C" {
       msg_stream << "Unable to allocate memory: " << ba.what() << std::endl;
       json_append_member(json_err, "status", json_mknumber(2));
       json_append_member(json_err, "message", json_mkstring(ba.what()));
-      json_append_member(json_err, "formatted", json_mkstring(msg_stream.str().c_str()));
+      json_append_member(json_err, "formatted", json_mkstream(msg_stream));
       try { c_ctx->error_json = json_stringify(json_err, "  "); } catch(...) {}
-      c_ctx->error_message = sass_copy_c_string(msg_stream.str().c_str());
+      c_ctx->error_message = sass_copy_string(msg_stream.str());
       c_ctx->error_text = sass_copy_c_string(ba.what());
       c_ctx->error_status = 2;
       c_ctx->output_string = 0;
@@ -138,9 +148,9 @@ extern "C" {
       msg_stream << "Internal Error: " << e.what() << std::endl;
       json_append_member(json_err, "status", json_mknumber(3));
       json_append_member(json_err, "message", json_mkstring(e.what()));
-      json_append_member(json_err, "formatted", json_mkstring(msg_stream.str().c_str()));
+      json_append_member(json_err, "formatted", json_mkstream(msg_stream));
       try { c_ctx->error_json = json_stringify(json_err, "  "); } catch(...) {}
-      c_ctx->error_message = sass_copy_c_string(msg_stream.str().c_str());
+      c_ctx->error_message = sass_copy_string(msg_stream.str());
       c_ctx->error_text = sass_copy_c_string(e.what());
       c_ctx->error_status = 3;
       c_ctx->output_string = 0;
@@ -153,9 +163,9 @@ extern "C" {
       msg_stream << "Internal Error: " << e << std::endl;
       json_append_member(json_err, "status", json_mknumber(4));
       json_append_member(json_err, "message", json_mkstring(e.c_str()));
-      json_append_member(json_err, "formatted", json_mkstring(msg_stream.str().c_str()));
+      json_append_member(json_err, "formatted", json_mkstream(msg_stream));
       try { c_ctx->error_json = json_stringify(json_err, "  "); } catch(...) {}
-      c_ctx->error_message = sass_copy_c_string(msg_stream.str().c_str());
+      c_ctx->error_message = sass_copy_string(msg_stream.str());
       c_ctx->error_text = sass_copy_c_string(e.c_str());
       c_ctx->error_status = 4;
       c_ctx->output_string = 0;
@@ -168,9 +178,9 @@ extern "C" {
       msg_stream << "Internal Error: " << e << std::endl;
       json_append_member(json_err, "status", json_mknumber(4));
       json_append_member(json_err, "message", json_mkstring(e));
-      json_append_member(json_err, "formatted", json_mkstring(msg_stream.str().c_str()));
+      json_append_member(json_err, "formatted", json_mkstream(msg_stream));
       try { c_ctx->error_json = json_stringify(json_err, "  "); } catch(...) {}
-      c_ctx->error_message = sass_copy_c_string(msg_stream.str().c_str());
+      c_ctx->error_message = sass_copy_string(msg_stream.str());
       c_ctx->error_text = sass_copy_c_string(e);
       c_ctx->error_status = 4;
       c_ctx->output_string = 0;
@@ -184,7 +194,7 @@ extern "C" {
       json_append_member(json_err, "status", json_mknumber(5));
       json_append_member(json_err, "message", json_mkstring("unknown"));
       try { c_ctx->error_json = json_stringify(json_err, "  "); } catch(...) {}
-      c_ctx->error_message = sass_copy_c_string(msg_stream.str().c_str());
+      c_ctx->error_message = sass_copy_string(msg_stream.str());
       c_ctx->error_text = sass_copy_c_string("unknown");
       c_ctx->error_status = 5;
       c_ctx->output_string = 0;

src/units.hpp

@@ -77,7 +77,9 @@ namespace Sass {
         ss << "Incompatible units: ";
         ss << "'" << unit_to_string(a) << "' and ";
         ss << "'" << unit_to_string(b) << "'";
-        msg = ss.str().c_str();
+        // hold on to string on stack!
+        std::string str(ss.str());
+        msg = str.c_str();
       }
       virtual const char* what() const throw()
       {