[Flags] ValidateCommandLineOption

[hari90]

ValidateCommandLineOption API to test a flag value. On success we can then set it using SetCommandLineOption.

bool
FlagRegistry::ValidateValueLocked(CommandLineFlag* flag, const char* value,
                                       string* err_msg) const {
  FlagValue* dummy_value = flag->current_->New();
  bool result = TryParseLocked(flag, dummy_value, value, err_msg);
  delete dummy_value;

  if (result) {
    // On success clear the message which says '<flag> set to <value>'.
    *err_msg = "";
  }
  return result;
}

// --------------------------------------------------------------------
// ValidateCommandLineOption()
//    Check if we can set a flag to a particular value. If this check
//    passes then the flag can be successfuly set to this value using
//    SetCommandLineOption.
// --------------------------------------------------------------------
bool ValidateCommandLineOption(const char* flagname, const char* value, string* err_msg) {
  FlagRegistry* const registry = FlagRegistry::GlobalRegistry();
  FlagRegistryLock frl(registry);
  CommandLineFlag* flag = registry->FindFlagLocked(flagname);
  if (!flag) {
    *err_msg = StringPrintf("Unknown command line flag '%s'", flagname);
    return false;
  }

  return registry->ValidateValueLocked(flag, value, err_msg);
}