c++ - Building mongo-cxx-driver using CMake ExternalProject_Add

Question

I am trying to build mongo-cxx-driver in a CMake based project. This project is supposed to build on Windows, macOS and in an ubuntu container and I want to ensure that my software on all these platforms will use the same driver version so I cannot afford installing the driver and its dependencies via apt-get, brew etc. So I am left with one option: ExternalProject_Add. But I am having difficulty making that work given how libmongoc is setup. Below is the CMake module I currently have.

include(ExternalProject)

set(libmongoc_CMAKE_ARGS
    "-DCMAKE_BUILD_TYPE:STRIING=${CMAKE_BUILD_TYPE}"
    "-DENABLE_TESTS:BOOL=OFF"
    "-DENABLE_STATIC:BOOL=OFF"
    "-DENABLE_EXAMPLES:BOOL=OFF"
    "-DENABLE_EXTRA_ALIGNMENT:BOOL=OFF"
)

set(mongocxx_CMAKE_ARGS
    "-DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}"
    "-DCMAKE_BUILD_TYPE:STRIING=${CMAKE_BUILD_TYPE}"
    "-DBUILD_SHARED_LIBS:BOOL=ON"
    "-DENABLE_TESTS:BOOL=OFF"
    "-DENABLE_EXAMPLES:BOOL=OFF"
    "-DBSONCXX_POLY_USE_BOOST:BOOL=ON"
    "-DBSONCXX_POLY_USE_MNMLSTC:BOOL=OFF"
    "-Dlibbson-1.0_DIR:PATH=${OTS_DEPDENDENCIES_DIR}/libmongoc/src/libbson"
)

if (NOT TARGET libmongoc)
    ExternalProject_Add(
        libmongoc
        GIT_REPOSITORY  "https://github.com/mongodb/mongo-c-driver.git"
        GIT_TAG         "1.12.0"
        SOURCE_DIR      "${OTS_DEPDENDENCIES_DIR}/libmongoc"
        BINARY_DIR      "${OTS_DEPDENDENCIES_DIR}/libmongoc"
        CMAKE_ARGS      "${libmongoc_CMAKE_ARGS}"
        INSTALL_COMMAND ""
    )
endif()

if (NOT TARGET mongocxx)
    ExternalProject_Add(
        mongocxx
        GIT_REPOSITORY  "https://github.com/mongodb/mongo-cxx-driver.git"
        GIT_TAG         "r3.3.1"
        SOURCE_DIR      "${OTS_DEPDENDENCIES_DIR}/mongocxx"
        BINARY_DIR      "${OTS_DEPDENDENCIES_DIR}/mongocxx"
        CMAKE_ARGS      "${mongocxx_CMAKE_ARGS}"
        INSTALL_COMMAND ""
        DEPENDS         libmongoc
    )
endif()

Note the CMAKE option libbson-1.0_DIR given as one of the CMAKE_ARGS for mongo-cxx-driver. I am skeptic about it and I believe that may be the culprit. With it I get the following error:

CMake Error at ${OTS_DEPENDENCIES_DIR}/libmongoc/src/libbson/libbson-1.0-config.cmake:30 (message):
    File or directory
    ${OTS_DEPENDENCIES_DIR}/include/libbson-1.0
    referenced by variable BSON_INCLUDE_DIRS does not exist !
Call Stack (most recent call first):
    ${OTS_DEPENDENCIES_DIR}/libmongoc/src/libbson/libbson-1.0-config.cmake:46 (set_and_check)
    src/bsoncxx/CMakeLists.txt:81 (find_package)

which kind of makes sense because src/bsoncxx/CMakeLists.txt:81 reads:

get_filename_component(PACKAGE_PREFIX_DIR "${CMAKE_CURRENT_LIST_DIR}/../../../" ABSOLUTE)
set_and_check (BSON_INCLUDE_DIRS "${PACKAGE_PREFIX_DIR}/include/libbson-1.0")

which makes CMake end up looking for libbson-1.0 in ${OTS_DEPDENDENCIES_DIR}/include that does not exist. If only, I could tell cmake, "hey don't run this find_package" I can give you the path to INCLUDE_DIR, LIBRARIES and DEFINITIONS myself.

If I remove this option, I get the following error:

CMake Error at src/bsoncxx/CMakeLists.txt:81 (find_package):
By not providing "Findlibbson-1.0.cmake" in CMAKE_MODULE_PATH this project
has asked CMake to find a package configuration file provided by
"libbson-1.0", but CMake did not find one.

Could not find a package configuration file provided by "libbson-1.0"
(requested version 1.10.0) with any of the following names:

libbson-1.0Config.cmake
libbson-1.0-config.cmake

Add the installation prefix of "libbson-1.0" to CMAKE_PREFIX_PATH or set
"libbson-1.0_DIR" to a directory containing one of the above files.  If
"libbson-1.0" provides a separate development package or SDK, be sure it
has been installed.

which is not very odd either because CMake tries to find_package libbson-1.0 but cannot figure out where it is installed.

Answer 1

preliminary remarks

While looking at the details, here are few preliminary comments:

• Use different directory for SOURCE_DIR and BINARY_DIR
• Instead of CMAKE_ARG, prefer CMAKE_CACHE_ARGS
• libbson-1.0_DIR should NOT be set to a source directory, instead if should be set to a build directory containing a config-file package (link below provide more details about this concept)
• Make sure to always specify the type of CMake argument (-DCMAKE_CXX_COMPILER:PATH=${CMAKE_CXX_COMPILER} instead of -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER})
• Do not set CMAKE_BUILD_TYPE for multi-configuration generator

Regarding this last point, this means that you should do the following:

set(EXTERNAL_PROJECT_OPTIONAL_CMAKE_CACHE_ARGS)
if(NOT DEFINED CMAKE_CONFIGURATION_TYPES)
  list(APPEND EXTERNAL_PROJECT_OPTIONAL_CMAKE_CACHE_ARGS
    -DCMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}
    )
endif()

In this post, you can learn how you could structure your project: Correct way to use third-party libraries in cmake project

working project allowing to compile the mongocxx "test.cpp"

Below is the content of CMakeLists.txt and test.cpp allowing to build an executable named <build-dir>/Test-build/test_mongocxx :

• CMakeLists.txt:

  cmake_minimum_required(VERSION 3.12)
  
  set(CMAKE_CXX_STANDARD 11) 
  
  project(Test)
  
  option(${PROJECT_NAME}_SUPERBUILD "Build ${PROJECT_NAME} and the projects it depends on." ON)
  
  if(${PROJECT_NAME}_SUPERBUILD)
  
      include(ExternalProject)
  
      set(common_cmake_cache_args
          -DCMAKE_CXX_COMPILER:PATH=${CMAKE_CXX_COMPILER}
      )
      if(NOT DEFINED CMAKE_CONFIGURATION_TYPES)
          list(APPEND common_cmake_cache_args
              -DCMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}
          )
      endif()
  
      ExternalProject_Add(libmongoc
          GIT_REPOSITORY  "https://github.com/mongodb/mongo-c-driver.git"
          GIT_TAG         "1.12.0"
          GIT_PROGRESS    1
          GIT_SHALLOW     1
          SOURCE_DIR      "${CMAKE_BINARY_DIR}/libmongoc"
          BINARY_DIR      "${CMAKE_BINARY_DIR}/libmongoc-build"
          INSTALL_DIR     "${CMAKE_BINARY_DIR}/libmongoc-install"
          CMAKE_CACHE_ARGS
              ${common_cmake_cache_args}
              -DCMAKE_INSTALL_PREFIX:PATH=${CMAKE_BINARY_DIR}/libmongoc-install
              -DENABLE_TESTS:BOOL=OFF
              -DENABLE_STATIC:BOOL=OFF
              -DENABLE_EXAMPLES:BOOL=OFF
              -DENABLE_EXTRA_ALIGNMENT:BOOL=OFF
          #INSTALL_COMMAND ""
      )
      set(libmongoc-1.0_DIR "${CMAKE_BINARY_DIR}/libmongoc-install/lib/cmake/libmongoc-1.0/")
      set(libbson-1.0_DIR "${CMAKE_BINARY_DIR}/libmongoc-install/lib/cmake/libbson-1.0/")
  
      ExternalProject_Add(libmongocxx
          GIT_REPOSITORY  "https://github.com/mongodb/mongo-cxx-driver.git"
          GIT_TAG         "r3.3.1"
          GIT_PROGRESS    1
          GIT_SHALLOW     1
          SOURCE_DIR      "${CMAKE_BINARY_DIR}/libmongocxx"
          BINARY_DIR      "${CMAKE_BINARY_DIR}/libmongocxx-build"
          INSTALL_DIR     "${CMAKE_BINARY_DIR}/libmongocxx-install"
          CMAKE_CACHE_ARGS
              ${common_cmake_cache_args}
              -DCMAKE_INSTALL_PREFIX:PATH=${CMAKE_BINARY_DIR}/libmongocxx-install
              -DBUILD_SHARED_LIBS:BOOL=ON
              -DENABLE_TESTS:BOOL=OFF
              -DENABLE_EXAMPLES:BOOL=OFF
              -DBSONCXX_POLY_USE_BOOST:BOOL=OFF
              -DBSONCXX_POLY_USE_MNMLSTC:BOOL=ON
              -DBSONCXX_POLY_USE_STD:BOOL=OFF
              -Dlibmongoc-1.0_DIR:PATH=${libmongoc-1.0_DIR}
              -Dlibbson-1.0_DIR:PATH=${libbson-1.0_DIR}
          DEPENDS
              libmongoc
      )
      set(libmongocxx_DIR "${CMAKE_BINARY_DIR}/libmongocxx-install/lib/cmake/libmongocxx-3.3.1/")
      set(libbsoncxx_DIR "${CMAKE_BINARY_DIR}/libmongocxx-install//lib/cmake/libbsoncxx-3.3.1/")
  
  
      function(ExternalProject_AlwaysConfigure proj)
        # This custom external project step forces the configure and later
        # steps to run.
        _ep_get_step_stampfile(${proj} "configure" stampfile)
        ExternalProject_Add_Step(${proj} forceconfigure
          COMMAND ${CMAKE_COMMAND} -E remove ${stampfile}
          COMMENT "Forcing configure step for '${proj}'"
          DEPENDEES build
          ALWAYS 1
          )
      endfunction()
  
      ExternalProject_Add(${PROJECT_NAME}
          SOURCE_DIR "${CMAKE_SOURCE_DIR}"
          BINARY_DIR "${CMAKE_BINARY_DIR}/${PROJECT_NAME}-build"
          DOWNLOAD_COMMAND ""
          UPDATE_COMMAND ""
          CMAKE_CACHE_ARGS
              ${common_cmake_cache_args}
              -D${PROJECT_NAME}_SUPERBUILD:BOOL=OFF
              -Dlibbsoncxx_DIR:PATH=${libbsoncxx_DIR}
              -Dlibmongocxx_DIR:PATH=${libmongocxx_DIR}
          INSTALL_COMMAND ""
          DEPENDS
              libmongocxx
      )
      ExternalProject_AlwaysConfigure(${PROJECT_NAME})
      return()
  endif()
  
  message(STATUS "Configuring inner-build")
  
  find_package(libmongocxx REQUIRED)
  
  add_executable(test_mongocxx test.cpp)
  target_link_libraries(test_mongocxx PUBLIC ${LIBMONGOCXX_LIBRARIES})
  target_include_directories(test_mongocxx PUBLIC ${LIBMONGOCXX_INCLUDE_DIRS})
  target_compile_definitions(test_mongocxx PUBLIC ${LIBMONGOCXX_DEFINITIONS})
  

• test.cpp (copied from https://mongodb.github.io/mongo-cxx-driver/mongocxx-v3/installation/#step-6-test-your-installation):

  #include <iostream>
  
  #include <bsoncxx/builder/stream/document.hpp>
  #include <bsoncxx/json.hpp>
  
  #include <mongocxx/client.hpp>
  #include <mongocxx/instance.hpp>
  
  int main(int, char**) {
      mongocxx::instance inst{};
      mongocxx::client conn{mongocxx::uri{}};
  
      bsoncxx::builder::stream::document document{};
  
      auto collection = conn["testdb"]["testcollection"];
      document << "hello" << "world";
  
      collection.insert_one(document.view());
      auto cursor = collection.find({});
  
      for (auto&& doc : cursor) {
          std::cout << bsoncxx::to_json(doc) << std::endl;
      }
  }