Fix OS::VSNPrint to make it behave similar to linux and macos versions.

runtime/vm/os_win.cc

Index: runtime/vm/os_win.cc
===================================================================
--- runtime/vm/os_win.cc	(revision 6377)
+++ runtime/vm/os_win.cc	(working copy)
@@ -161,10 +161,19 @@
 
 
 int OS::VSNPrint(char* str, size_t size, const char* format, va_list args) {
+  if (str == NULL || size == 0) {
+    return _vscprintf(format, args);
+  }
   int written =_vsnprintf(str, size, format, args);
+  if (written < 0) {
+    // _vsnprintf returns -1 if the number of characters to be written is
+    // larger than 'size', so we call _vscprintf which returns the number
+    // of characters that would have been written.
+    written = _vscprintf(format, args);
+  }
   // Make sure to zero-terminate the string if the output was
   // truncated or if there was an error.
-  if (written < 0 || written >= size) {
+  if (written >= size) {
     if (size > 0) {

[hausner, 2012/04/10 20:40:04]

The size > 0 check is no longer necessary since we handle size == 0 at the
beginning.

[siva, 2012/04/10 20:42:25]

Done.

       str[size - 1] = '\0';
     }