[MESOS-3141] Compiler warning when mocking function type has an enum return type.

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.23.0
• Fix Version/s: 0.24.0
• Labels: mesosphere
• Story Points: 3
• Assignee: haosdent
• Reporter: Michael Park
• Created: 24/Jul/15 01:24
• Updated: 26/Nov/18 12:20
• Resolved: 26/Nov/18 12:20

Description

The purpose of this ticket is to document a very cryptic error message (actually a warning that gets propagated by -Werror) that gets generated by clang-3.5 from gmock source code when trying to mock a perfectly innocent-looking function.

Problem

The following code is attempting to mock a MesosExecutorDriver:

class MockMesosExecutorDriver : public MesosExecutorDriver {
public:
  MockMesosExecutorDriver(mesos::Executor* executor)
    : MesosExecutorDriver(executor) {}

  MOCK_METHOD1(sendStatusUpdate, Status(const TaskStatus&));
};

The above code generates the following error message:

In file included from ../3rdparty/libprocess/3rdparty/gmock-1.6.0/include/gmock/gmock.h:58:
In file included from ../3rdparty/libprocess/3rdparty/gmock-1.6.0/include/gmock/gmock-actions.h:46:
../3rdparty/libprocess/3rdparty/gmock-1.6.0/include/gmock/internal/gmock-internal-utils.h:355:10: error: indirection of non-volatile null pointer will be deleted, not trap [-Werror,-Wnull-dereference]
  return *static_cast<typename remove_reference<T>::type*>(__null);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../3rdparty/libprocess/3rdparty/gmock-1.6.0/include/gmock/gmock-actions.h:78:22: note: in instantiation of function template specialization 'testing::internal::Invalid<mesos::Status>' requested here
    return internal::Invalid<T>();
                     ^
../3rdparty/libprocess/3rdparty/gmock-1.6.0/include/gmock/gmock-actions.h:190:43: note: in instantiation of member function 'testing::internal::BuiltInDefaultValue<mesos::Status>::Get' requested here
        internal::BuiltInDefaultValue<T>::Get() : *value_;
                                          ^
../3rdparty/libprocess/3rdparty/gmock-1.6.0/include/gmock/gmock-spec-builders.h:1435:34: note: in instantiation of member function 'testing::DefaultValue<mesos::Status>::Get' requested here
    return DefaultValue<Result>::Get();
                                 ^
../3rdparty/libprocess/3rdparty/gmock-1.6.0/include/gmock/gmock-spec-builders.h:1334:22: note: in instantiation of member function 'testing::internal::FunctionMockerBase<mesos::Status (const mesos::TaskStatus &)>::PerformDefaultAction' requested here
        func_mocker->PerformDefaultAction(args, call_description));
                     ^
../3rdparty/libprocess/3rdparty/gmock-1.6.0/include/gmock/gmock-spec-builders.h:1448:26: note: in instantiation of function template specialization 'testing::internal::ActionResultHolder<mesos::Status>::PerformDefaultAction<mesos::Status (const mesos::TaskStatus &)>' requested here
    return ResultHolder::PerformDefaultAction(this, args, call_description);
                         ^
../3rdparty/libprocess/3rdparty/gmock-1.6.0/include/gmock/gmock-generated-function-mockers.h:81:7: note: in instantiation of member function 'testing::internal::FunctionMockerBase<mesos::Status (const mesos::TaskStatus &)>::UntypedPerformDefaultAction' requested here
class FunctionMocker<R(A1)> : public
      ^
../3rdparty/libprocess/3rdparty/gmock-1.6.0/include/gmock/internal/gmock-internal-utils.h:355:10: note: consider using __builtin_trap() or qualifying pointer with 'volatile'
  return *static_cast<typename remove_reference<T>::type*>(__null);
         ^

The source of the issue here is that Status is an enum. In gmock-1.6.0/include/gmock/internal/gmock-internal-utils.h you can find the following function:

template <typename T>
T Invalid() {
  return *static_cast<typename remove_reference<T>::type*>(NULL);
}

This function gets called with the return type of a mocked function. In our case, the return type of the mocked function is Status.

Attempting to compile the following minimal example with clang-3.5 reproduces the error message:

#include <type_traits>

template <typename T>
T invalid() {
  return *static_cast<typename std::remove_reference<T>::type *>(nullptr);
}

enum E { A, B };

int main() {
  invalid<E>();
}

• See it online on GCC Explorer

Note that if the type is not an enum, the warning is not generated. This is why existing mocked functions that return non-enum types such as Future<void> does not encounter this issue.

Solutions

The simplest solution is to add -Wno-null-deference to mesos_tests_CPPFLAGS in src/Makefile.am.

mesos_tests_CPPFLAGS = $(MESOS_CPPFLAGS) -Wno-null-dereference

Another solution is to upgrade gmock from 1.6 to 1.7 because this problem is solved in the newer versions.

In gmock 1.7

template <typename T>
inline T Invalid() {
  return const_cast<typename remove_reference<T>::type&>(
      *static_cast<volatile typename remove_reference<T>::type*>(NULL));
}

Add volatile could avoid this warning. https://goo.gl/opCiLC

Issue Links

• is related to: MESOS-1073 Upgrade gmock from 1.6 to 1.7 and set C++11 support