CMake: Replace ExternalProject_Add with FetchContent. (#290)

This reduces the amount of code for including/building dependencies.

Author: meastp

CMakeLists.txt

@@ -30,6 +30,7 @@ include(CTest) # Defines option BUILD_TESTING.
 enable_testing()
 
 list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake)
+
 include(OpenCensusDeps)
 
 include(OpenCensusHelpers)

cmake/FetchContent.cmake

@@ -0,0 +1,23 @@
+# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+# file Copyright.txt or https://cmake.org/licensing for details.
+
+cmake_minimum_required(VERSION ${CMAKE_VERSION})
+
+# We name the project and the target for the ExternalProject_Add() call
+# to something that will highlight to the user what we are working on if
+# something goes wrong and an error message is produced.
+
+project(${contentName}-populate NONE)
+
+include(ExternalProject)
+ExternalProject_Add(${contentName}-populate
+                    ${ARG_EXTRA}
+                    SOURCE_DIR          "${ARG_SOURCE_DIR}"
+                    BINARY_DIR          "${ARG_BINARY_DIR}"
+                    CONFIGURE_COMMAND   ""
+                    BUILD_COMMAND       ""
+                    INSTALL_COMMAND     ""
+                    TEST_COMMAND        ""
+                    USES_TERMINAL_DOWNLOAD  YES
+                    USES_TERMINAL_UPDATE    YES
+)

cmake/FetchContent/CMakeLists.cmake.in

@@ -12,10 +12,29 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+include(FetchContent)
+
+fetchcontent_declare(googletest
+                     GIT_REPOSITORY
+                     https://github.com/abseil/googletest
+                     GIT_TAG
+                     ed2fe122f8dc9aca844d724986d1d5cf5b65ea4e)
+fetchcontent_declare(abseil
+                     GIT_REPOSITORY
+                     https://github.com/abseil/abseil-cpp
+                     GIT_TAG
+                     master)
+fetchcontent_declare(prometheus
+                     GIT_REPOSITORY
+                     https://github.com/jupp0r/prometheus-cpp
+                     GIT_TAG
+                     master)
+
+fetchcontent_getproperties(googletest)
 if(BUILD_TESTING)
-  if(NOT TARGET gtest_main)
-    message(STATUS "Dependency: googletest (BUILD_TESTING=${BUILD_TESTING})")
+  message(STATUS "Dependency: googletest (BUILD_TESTING=${BUILD_TESTING})")
 
+  if(NOT googletest_POPULATED)
     if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
       # All the libraries in the build must use either /MD or /MT (runtime
       # library to link)
@@ -29,73 +48,27 @@ if(BUILD_TESTING)
         ON)
     endif()
 
-    configure_file(${CMAKE_CURRENT_SOURCE_DIR}/cmake/googletest.CMakeLists.txt
-                   ${CMAKE_BINARY_DIR}/googletest-download/CMakeLists.txt)
-    execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
-                    RESULT_VARIABLE result
-                    WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/googletest-download)
-    if(result)
-      message(FATAL_ERROR "CMake step failed: ${result}")
-    endif()
-    execute_process(COMMAND ${CMAKE_COMMAND} --build .
-                    RESULT_VARIABLE result
-                    WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/googletest-download)
-    if(result)
-      message(FATAL_ERROR "Build step failed: ${result}")
-    endif()
-
-    add_subdirectory(${CMAKE_BINARY_DIR}/googletest-src
-                     ${CMAKE_BINARY_DIR}/googletest-build EXCLUDE_FROM_ALL)
+    fetchcontent_populate(googletest)
+    add_subdirectory(${googletest_SOURCE_DIR} ${googletest_BINARY_DIR}
+                     EXCLUDE_FROM_ALL)
   endif()
 endif()
 
-# Load abseil second, it depends on googletest.
-if(NOT TARGET absl::base)
-  message(STATUS "Dependency: abseil")
-
-  configure_file(${CMAKE_CURRENT_SOURCE_DIR}/cmake/abseil.CMakeLists.txt
-                 ${CMAKE_BINARY_DIR}/abseil-download/CMakeLists.txt)
-  execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
-                  RESULT_VARIABLE result
-                  WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/abseil-download)
-  if(result)
-    message(FATAL_ERROR "CMake step failed: ${result}")
-  endif()
-  execute_process(COMMAND ${CMAKE_COMMAND} --build .
-                  RESULT_VARIABLE result
-                  WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/abseil-download)
-  if(result)
-    message(FATAL_ERROR "Build step failed: ${result}")
-  endif()
-
-  add_subdirectory(${CMAKE_BINARY_DIR}/abseil-src
-                   ${CMAKE_BINARY_DIR}/abseil-build EXCLUDE_FROM_ALL)
+fetchcontent_getproperties(abseil)
+if(NOT abseil_POPULATED)
+  fetchcontent_populate(abseil)
+  add_subdirectory(${abseil_SOURCE_DIR} ${abseil_BINARY_DIR} EXCLUDE_FROM_ALL)
 endif()
 
-if(NOT TARGET prometheus-cpp::core)
-  message(STATUS "Dependency: prometheus-cpp")
-
-  configure_file(${CMAKE_CURRENT_SOURCE_DIR}/cmake/prometheus-cpp.CMakeLists.txt
-                 ${CMAKE_BINARY_DIR}/prometheus-download/CMakeLists.txt)
-  execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
-                  RESULT_VARIABLE result
-                  WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/prometheus-download)
-  if(result)
-    message(FATAL_ERROR "CMake step failed: ${result}")
-  endif()
-  execute_process(COMMAND ${CMAKE_COMMAND} --build .
-                  RESULT_VARIABLE result
-                  WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/prometheus-download)
-  if(result)
-    message(FATAL_ERROR "Build step failed: ${result}")
-  endif()
-
+fetchcontent_getproperties(prometheus)
+if(NOT prometheus_POPULATED)
   set(ENABLE_PUSH OFF CACHE BOOL "Build prometheus-cpp push library" FORCE)
   set(ENABLE_PULL OFF CACHE BOOL "Build prometheus-cpp pull library" FORCE)
   set(ENABLE_COMPRESSION OFF
       CACHE BOOL "Enable gzip compression for prometheus-cpp"
       FORCE)
   set(ENABLE_TESTING OFF CACHE BOOL "Build test for prometheus-cpp" FORCE)
-  add_subdirectory(${CMAKE_BINARY_DIR}/prometheus-src
-                   ${CMAKE_BINARY_DIR}/prometheus-build EXCLUDE_FROM_ALL)
+  fetchcontent_populate(prometheus)
+  add_subdirectory(${prometheus_SOURCE_DIR} ${prometheus_BINARY_DIR}
+                   EXCLUDE_FROM_ALL)
 endif()