[runtime] Fix snprintf usage (#4109)

runtime/src/main/cpp/KString.cpp

@@ -919,13 +919,14 @@ KInt Kotlin_StringBuilder_insertInt(KRef builder, KInt position, KInt value) {
   RuntimeAssert(toArray->count_ >= 11 + position, "must be true");
   char cstring[12];
   auto length = konan::snprintf(cstring, sizeof(cstring), "%d", value);
+  RuntimeAssert(length >= 0, "This should never happen"); // may be overkill
+  RuntimeAssert(length < sizeof(cstring), "Unexpectedly large value"); // Can't be, but this is what sNprintf for
   auto* from = &cstring[0];
   auto* to = CharArrayAddressOfElementAt(toArray, position);
-  auto* end = from + length;
-  while (from != end) {
+  while (*from) {
     *to++ = *from++;
   }
-  return length;
+  return from - cstring;
 }
 
 

runtime/src/main/cpp/Porting.cpp

@@ -110,16 +110,21 @@ void consolePrintf(const char* format, ...) {
   char buffer[1024];
   va_list args;
   va_start(args, format);
-  int rv = vsnprintf_impl(buffer, sizeof(buffer) - 1, format, args);
+  int rv = vsnprintf_impl(buffer, sizeof(buffer), format, args);
+  if (rv < 0) return; // TODO: this may be too much exotic, but should i try to print itoa(error) and terminate?
+  if (rv >= sizeof(buffer)) rv = sizeof(buffer) - 1;  // TODO: Consider realloc or report truncating.
   va_end(args);
   consoleWriteUtf8(buffer, rv);
 }
 
+// TODO: Avoid code duplication.
 void consoleErrorf(const char* format, ...) {
   char buffer[1024];
   va_list args;
   va_start(args, format);
-  int rv = vsnprintf_impl(buffer, sizeof(buffer) - 1, format, args);
+  int rv = vsnprintf_impl(buffer, sizeof(buffer), format, args);
+  if (rv < 0) return; // TODO: this may be too much exotic, but should i try to print itoa(error) and terminate?
+  if (rv >= sizeof(buffer)) rv = sizeof(buffer) - 1;  // TODO: Consider realloc or report truncating.
   va_end(args);
   consoleErrorUtf8(buffer, rv);
 }