Minimal project to demonstrate the problem with open file descriptors when tests are launched via ctest

README.md

Run the following commands:

mkdir build
cd build
cmake ..
make

Then, there are two different ways to run tests:

1. Via CTest:

ctest --verbose

1. Manually run test executable

./test_list_open_file_descriptors

In case of manual execution test passes successfully, but when it's launched via ctest command, it fails with the following error:

1: Expected equality of these values:
1:   actual_file_descriptors
1:     Which is: { 3, 2, 0, 1 }
1:   expected_file_descriptors
1:     Which is: { 2, 1, 0 }
1

CMakeLists.txt

cmake_minimum_required(VERSION 3.15)
project(ctest_file_descriptors_problem_demo)

set(CMAKE_CXX_STANDARD 17)

add_library(list_open_file_descriptors list_open_file_descriptors.cpp)

include(FetchContent)
FetchContent_Declare(
        googletest
        GIT_REPOSITORY https://github.com/google/googletest.git
        GIT_TAG release-1.10.0
)
FetchContent_MakeAvailable(googletest)

enable_testing()
include(GoogleTest)

add_executable(test_list_open_file_descriptors test_list_open_file_descriptors.cpp)
target_link_libraries(test_list_open_file_descriptors list_open_file_descriptors gtest_main)
gtest_discover_tests(test_list_open_file_descriptors)

list_open_file_descriptors.cpp

#include "list_open_file_descriptors.h"

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <dirent.h>
#include <unistd.h>

void DiscoverOpenFileDescriptors(std::unordered_set<int> *out_file_descriptors) {
  int dirfd = open("/dev/fd", O_RDONLY);
  DIR *dir = fdopendir(dirfd);

  for (dirent *entry = readdir(dir); entry; entry = readdir(dir)) {
    const std::string entry_name = entry->d_name;
    if (entry_name == "." || entry_name == "..") {
      continue;
    }
    const int fd = std::atoi(entry->d_name);
    if (fd != dirfd) {
      out_file_descriptors->insert(fd);
    }
  }
  
  closedir(dir);
  close(dirfd);
}

list_open_file_descriptors.h

#ifndef CTEST_FILE_DESCRIPTORS_PROBLEM_DEMO__LIST_OPEN_FILE_DESCRIPTORS_H_
#define CTEST_FILE_DESCRIPTORS_PROBLEM_DEMO__LIST_OPEN_FILE_DESCRIPTORS_H_

#include <unordered_set>

void DiscoverOpenFileDescriptors(std::unordered_set<int> *out_file_descriptors);

#endif //CTEST_FILE_DESCRIPTORS_PROBLEM_DEMO__LIST_OPEN_FILE_DESCRIPTORS_H_

test_list_open_file_descriptors.cpp

#include "list_open_file_descriptors.h"

#include <gtest/gtest.h>

TEST(OpenFileDescriptorsTest, OnlyStdinStdoutAndStderrShouldBeDetectedIfNoFilesAreOpen) {
  std::unordered_set<int> actual_file_descriptors;
  DiscoverOpenFileDescriptors(&actual_file_descriptors);

  std::unordered_set<int> expected_file_descriptors{0, 1, 2};

  ASSERT_EQ(actual_file_descriptors, expected_file_descriptors);
}