Add String Formatting Util

(Merged from http://go/wvgerrit/160042.)

Since we don't have access to std::format yet, this patch adds a
function to wvutil to format text into a std::string.

Bug: 255466913
Test: x86-64
Test: raven
Change-Id: I28043da76af5b4772a29fa7e7241343caf9b54a1

libwvdrmengine/cdm/util/src/string_format.cpp

@@ -0,0 +1,40 @@
+// Copyright 2022 Google LLC. All Rights Reserved. This file and proprietary
+// source code may only be used and distributed under the Widevine License
+// Agreement.
+
+#include "string_format.h"
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <memory>
+
+namespace wvutil {
+
+bool FormatString(std::string* out, const char* fmt, ...) {
+  if (out == nullptr || fmt == nullptr) return false;
+
+  va_list ap1;
+  va_start(ap1, fmt);
+  const int desired_size = vsnprintf(nullptr, 0, fmt, ap1);
+  va_end(ap1);
+
+  if (desired_size < 0) return false;
+  const size_t buffer_size =
+      static_cast<size_t>(desired_size) + 1;  // +1 for null
+  std::unique_ptr<char[]> buffer(new char[buffer_size]);
+
+  va_list ap2;
+  va_start(ap2, fmt);
+  const int actual_size = vsnprintf(buffer.get(), buffer_size, fmt, ap2);
+  va_end(ap2);
+
+  if (actual_size != desired_size) return false;
+
+  out->assign(buffer.get(), actual_size);
+  return true;
+}
+
+}  // namespace wvutil